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Great outline and understanding of who says what
good splitting of workload even with changes to group dynamic
nice segways

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When researching next week gather stats and sources to back up what you think

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Statistics and data
mental health tied back to corona virus
talk about compassion throughout with being repetitive
why should you care/ personal story

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What are your personal basic needs or what do you consider an important necessity?

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What are your personal basic needs or what do you consider an important necessity?

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Nutrients are essential for the body

Nutrient roles

Our bodies are built of and powered by solely what we eat and drink. Food is the source of all of the energy needed Why do humans have to eat? Is it just because we get hungry? That is not all. People get energy from food. We need energy all the time, when we run, jump, sing, and even when we sleep. We create all the energy we need by eating. The structures components that make up the human body, such as muscles, organs, and bones, are also composed of the nutrients contained in food, meaning that it is. This is why eating and taking in the nutrients that provide energy and become the components of our bodily structures is essential to for sustaining human life.

Editorial supervision: Yuki Sugishima (National Registered Dietician/ Certified Sports Dietician*), Assistant Professor, Shigakkan University, Faculty of Wellness, Department of Nutrition

Joint certified by The Japan Dietetic Association and The Japan Sport Association

The roles of the five major nutrients

Nutrients are the substances found in food which drive biological activity, and are essential for the human body. They are categorized as proteins, fats, carbohydrates (sugars, dietary fiber), vitamins, and minerals, and perform the following vital functions.

1. 1Building all parts of the body such as muscle, bone, teeth, and blood

2. 2Producing energy (power and heat)

3. 3Keeping the body in good working order

Proteins:
the building blocks of the body

Protein is the main constituent of the body, making up the muscles, internal organs, skin, and blood, etc. If compared to a car, it would be the material from which the body and engine are made. Just like a car cannot be driven fast without a solid frame, without protein the human body cannot be kept in a healthy state. There are 20 kinds of amino acids that make up proteins, 9 of which are not synthesized in the body, and are therefore called essential amino acids. These essential amino acids must be supplemented from meals.

Fats/carbohydrates (sugars):
the body's energy (power and heat) source

Although consuming too much fat can lead to obesity, small amounts can provide a highly efficient energy source (9kcal per gram). Carbohydrates can be broken down further into the two categories of sugars and dietary fiber. Sugars are the carbohydrates which can be used as an energy source to move the body (such as during exercise) and are stored in the liver and muscles as glycogen. Sugars are also the main source of energy for the brain. Dietary fiber, on the other hand, is also known as the sixth major nutrient and is the indigestible portion of food that is not broken down by human digestive enzymes. It helps to increase the number of good bacteria in the digestive tract, maintain the balance of bacteria in the intestines, and as such is very beneficial for the body. Fats can be found in abundance in the fatty portion of meats and in cooking oils, and sugars can be found in ample quantities in grains, potatoes, sugar, and fruit.

Vitamins/minerals:
keeping the body in good working order

Vitamins and minerals are not used as energy, but instead aid in breaking down and building up proteins, fats, and sugars, and are an essential nutrient for keeping the body healthy and in good working order. "Vitamin" is the general term for the organic compounds (compounds containing carbon) which are mostly impossible for the body to synthesize, and a lack of these vitamins can possibly lead to various deficiency-related diseases. Vitamins can be broken down into two categories: water-soluble and fat-soluble. Fat-soluble vitamins (A, D, E, K) dissolve easily in fats and can lead to a vitamin overdose if consumed in large amounts. Water-soluble vitamins (B vitamins, C) dissolve easily in water and thus carry very little risk of overdose. Vitamins can be found in large amounts in fruit and vegetables.
"Mineral," also referred to as inorganic matter, is a general term for the chemical elements which make up the human body excluding oxygen, carbon, hydrogen, and nitrogen. There are 16 different minerals which are necessary for the human body, including calcium, iron, and sodium, and they can be found in abundance in fruit, vegetables, seaweed, and milk/dairy products.

Other Focus Areas

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The role of nutrients and nutrient consumption | Otsuka Pharmaceutical Co., Ltd.

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Our bodies are built of and powered by solely what we eat and drink. Food is the source of all of the energy needed Why do humans have to eat? Is it just because we get hungry? That is not all. People get energy from food.

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Boston housing rates keep accelerating to an extend that many family struggle to afford rent. 

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Boston housing rates keep accelerating to an extend that many family struggle to afford rent. 

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Boston Mayor Marty Walsh in his State of the City message on Tuesday put it this way: "Housing is the biggest economic challenge our residents face . . . We know we have been making progress, but rents and home prices are still too high for too many people."

He pledged an additional $500 million over the next five years, doubling the city's current contribution to affordable housing. But to do that will require approval of a home rule petition on Beacon Hill that would allow the city to collect an additional 2 percent fee on the transfer of real estate valued at more than $2 million. Beacon Hill hasn't always been eager to allow communities that kind of leeway: Similar proposals from Somerville and Nantucket were left to languish during the last legislative session.

Meanwhile, the housing market in Boston, Somerville, Newton, and Quincy, keeps getting hotter. For example, the value of Boston's taxable property hit a record high of $164 billion in the last fiscal year, according to the latest study by the Boston Municipal Research Bureau. While most of the tax burden fell on businesses, that's up 78 percent from 2013. During that same period the unemployment rate in the city dropped from 6.1 percent to 3 percent and nearly 40,000 people moved into the city.

Sure, many of the newcomers are now in those gleaming new towers in the Seaport District, but just about anything within the city limits is in greater demand. The resulting bidding wars put housing largely out of reach to those not making a six-figure salary.

Creating a pool of money in communities like Boston to build more housing affordable for the families who grew up here and want to stay - as well as newcomers without lucrative jobs - is essential not only to a city's growth, but to its soul.

One vehicle for raising revenue is the home rule petition Walsh signed recently, which city officials estimate could have raised $169 million a year over the prosperous past decade. There are no guarantees of similar boom years ahead, but surely that $100 million per year figure the mayor is looking for isn't out of the question.

The prospects for Boston's proposal are unclear, but there are signs that the Legislature might be more attuned to the housing crisis than in the past. During this abbreviated year (formal sessions are due to end July 31), House lawmakers are giving some indications that housing is definitely on their agenda, already advancing two bills on the House floor aimed at encouraging multifamily housing and "smart growth" around transportation hubs.

Some legislators are also eyeing transfer taxes like the one Walsh wants - but their effort simply goes too far. A bill that would allow any community to pass a transfer tax to fund new affordable housing was introduced last week with the support of lawmakers from Somerville, Concord, Brookline, and the Cape and Islands in addition to Boston and a host of housing advocates.

That legislation provides communities with the option to impose the kind of targeted tax Boston has in mind - but it also creates opportunities for municipalities to overreach. Its chief flaw is that it would allow towns and cities to set the bar for a transfer fee far too low. By allowing for a threshold as low as the statewide median home price, it could hit property selling for as low as $400,000. That means it could raise housing prices for exactly the kind of first-time home buyers who are already having trouble finding property they can afford. The truth is Massachusetts needs more affordable housing that's subsidized by the government, but it also needs more housing that's affordable. Making market-rate housing for middle-class homeowners more expensive to pay for subsidized housing for the poor doesn't make sense. It should be levied only on truly high-end properties, as Boston's bill proposes.

While Beacon Hill should let communities get creative to solve their housing problems, proponents of the statewide local option transfer tax need a reality check. Legislators should strike a balance, finding a way to let towns and cities generate revenue for affordable housing that doesn't send home prices spiraling upward.

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Housing crisis needs all hands on deck - The Boston Globe

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Boston Mayor Marty Walsh in his State of the City message on Tuesday put it this way: "Housing is the biggest economic challenge our residents face . . . We know we have been making progress, but rents and home prices are still too high for too many people."

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Water importance and consequences if lack of.

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Water importance and consequences if lack of.

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ORIGINAL RESEARCH ARTICLE

First published:

Abstract

Body water homeostasis is critical for optimal physiological and cognitive function for humans. The majority of research has illustrated the negative biological consequences of failing to meet water needs. The human body has several mechanisms for detecting, regulating, and correcting body water deficits and excesses. However, variation exists in total water intake and how people meet those water needs as well as thirst thresholds and how well people tolerate water restriction. An evolutionary and developmental framework provides an underexplored perspective into human water needs by examining how adaptations, early life experiences and environments, as well as life course changes in health states and behaviors may shape these critical factors in body water homeostasis. This article first reviews biological and behavioral adaptations to water scarcity among animals and humans. It then examines human variation in water intake in a mostly water secure environment through the analysis of National Health and Nutrition Examination Survey dietary data and the link between water intake patterns and hydration biomarkers. Next, it reviews existing evidence of how maternal water restriction in utero and during lactation shape vasopressin release, thirst thresholds, drinking patterns, and body water homeostasis for the infant. Early life water restriction appears to have implications for hydration status, body size, and cardiovascular health. Finally, it examines how life course changes in health states and behaviors, including obesity, sleep, and parasitic infection, affect body water homeostasis. This article poses new questions about the plasticity and shaping of human water needs, thirst, and hydration behaviors.

1 INTRODUCTION

Human water needs are most commonly explained through physiology, including respiration, metabolism, thermoregulation, and waste disposal, as well as behavioral responses to the environment (Sawka, Cheuvront, & Carter, 2005). Individual variation in body size, physical activity level, health states, and for women, pregnancy and lactating status also drive water needs (Food and Nutrition Board and Institute of Medicine, 2004; Sawka et al., 2005). However, environments and water resources are dynamic. A critical gap exists in examining human water needs through an evolutionary framework (Wheeler, 1993). Using an evolutionary perspective allows for a deeper examination of how adaptations, early life experiences and environments, as well as life course changes in health states and behaviors may shape water intake and body water homeostasis.

Throughout human evolution, water needs have played an important selective pressure affecting survival. Changes in body size and proportion between Australopithecus and Homo erectus significantly affected water needs through thermoregulation by changing exposure to wind and solar radiation, which reduced sweat rates and water needs by 15% to 18%. And these changes in water efficiency may have allowed for longer periods of foraging (Wheeler, 1993). Another key example of morphological adaptations related to water needs among Homo are sweat glands, primarily eccrine glands, which allow for loss of body water in response to thermoregulation to keep body temperatures stable (Jablonski, 2004). Additionally, the external nose was an important adaptation for Homo to dry environments by providing the ability to retrieve moisture from exhaled air and then using this moisture to humidify the next breath to help conserve and reduce body water loss (Franciscus & Trinkaus, 1988; Lieberman, 2015). These adaptations demonstrate how important and integrative responses to water availability in the environment were to human evolution on a deep timescale.

The process of hydration and body water homeostasis is important to understand because it is acutely connected to physical and cognitive health as well as performance (Armstrong et al., 2012; Ganio et al., 2011). Hydration is a process that is constantly in flux as the body gains and loses water (Perrier et al., 2014). However, even acute dehydration, or ~2% of body weight lost through water, in the short‐term may lead to cognitive ability decline (Benton, Jenkins, Watkins, & Young, 2016; Goodman, Moreland, & Marino, 2019) and increased irritability and fatigue (Armstrong et al., 2012). Physical performance as seen via running times (Murray, 2007) and cycling performances (Adams et al., 2019) also deteriorate with acute dehydration. However, in elite endurance competitions athletes frequently lose much more than 2% of body mass, including the winners of the competitions (Beis, Wright‐Whyte, Fudge, Noakes, & Pitsiladis, 2012). Chronic acute dehydration is also associated with increased risk of urinary tract infections (Hooton et al., 2018), urolithiasis or kidney stones, and impaired kidney function (Clark et al., 2016), while dehydration beyond 10% of body weight can result in death (Popkin, D'Anci, & Rosenberg, 2010).

The Institute of Medicine, now the National Academy of Medicine, released adequate intake (AI) recommendations meant to meet the water needs of moderately active individuals in a healthy population (Food and Nutrition Board and Institute of Medicine, 2004). The AI for men is 3.7 L per day, for nonpregnant, nonlactating women it is 2.7 L, for pregnant women it is 3.0 L, and for lactating women it is 3.8 L. The European Food Safety Authority released lower AI recommendations: 2.5 L for men, 2.0 L for nonpregnant, nonlactating women, 2.3 L for pregnant women, and 2.7 L for lactating women (Jequier & Constant, 2010). These recommendations are based on nutritional surveys and water turnover studies.

However, variation has been documented in total water intake between and within populations across different environments ranging from 2 L per day among Greek adults to almost 10 L per day among Ecuadorian Shuar men (Braun et al., 2019; Christopher et al., 2019; Rosinger & Tanner, 2015; Tani et al., 2015). Additionally, the dietary constituents that make up total water intake, that is the percentage of water consumed from foods vs from beverages, vary. It is largely assumed that approximately 20% to 30% of total water intake comes from food sources with the remaining 70% to 80% from liquids (Jequier & Constant, 2010). However, at least two studies have demonstrated population variation in hydration strategies as both Japanese and Bolivian Tsimane' adults consumed roughly 50% of their total water intake from food sources (Rosinger & Tanner, 2015; Tani et al., 2015).

Since water balance is intimately tied to health and physiological functioning, several systems exist, including thirst, to return the body to homeostasis (Oka, Ye, & Zuker, 2015). The body and brain detect water deficits and excesses thereby releasing or restricting the anti‐diuretic hormone vasopressin and conserving body water or excreting it with the signals getting stronger depending on the level of imbalance (Armstrong & Johnson, 2018).

It is therefore critical to understand adaptations and biobehavioral responses to different environments when examining variation in human water needs and body water homeostasis. In attempting to explain why a biobehavioral approach is important, Leslie and Little explain (page 6) (Leslie & Little, 1999): "Human behavior is not explicable simply within the closed boundaries of society or of culture. Much adaptation ... deals also with the natural world in which society and culture are set: that is, with the biological constraints and capacities of humans themselves and with the biological effects and opportunities of the surrounding environment." Human water intake perfectly exemplifies this sentiment since it is a function of evolved physiology interacting with specific individual characteristics, physical access to water and foods, cultural milieus that provide a range of acceptable dietary items to drink and eat, but also highly adaptable to disparate environments.

Several evolutionary questions surrounding human water intake remain. What is the range of adaptations derived to conserve water? How does population variation in water intake arise in the first place? Aside from the known correlates of water needs, why do certain individuals consume more or less water? Do early life and life course exposures create a mismatch in water needs and water intake? An evolutionary perspective provides a unique way to understand how water restriction at different time‐points in an organism's lifecycle may affect form, function, and disease risk (Gluckman & Hanson, 2004a; Kuzawa & Quinn, 2009).

This article addresses four key questions in sections 2 to 5 to help explain variation in human water needs and body water homeostasis from an evolutionary perspective using examples from animal models and human biology:

1. What are the roles of adaptations to water scarce environments, biological and behavioral as well as phenotypic plasticity (or the extent to which the environment modifies the phenotype), in shaping water needs?

2. What does population variation in human water intake look like in a water‐secure location and how does this relate to hydration biomarkers?

3. How might water restriction during early life experiences shape water needs and what are the implications for body water homeostasis and the resulting health consequences?

4. Finally, how do changes in health states and behaviors throughout the life course affect water intake and the match between water intake and body water homeostasis?

2 ADAPTATIONS TO WATER‐SCARCE ENVIRONMENTS

In this section, I explore the question of biological adaptation vs phenotypic plasticity and behavioral modifications to different environments by examining urine concentrating ability, kidney function and morphology, dietary changes, and water turnover in the context of different examples from the animal kingdom and human evolution.

Water, as an essential nutrient, serves as a critical stressor to which humans must adapt (Haas & Harrison, 1977). Adaptation is a process in which an organism adjusts to a suite of stressors to better match their environment (Frisancho, 1993). Adaptations can last varying degrees of time, involve multiple timescales, and occur in individuals and populations. The developmental origins of health and disease (DOHaD) paradigm examines how in‐utero nutrition as well as intergenerational changes in nutrition and ecological stressors affect human adaptability and resulting health outcomes (Gluckman & Hanson, 2004a; Kuzawa, 2005). These phenotypic changes, such as smaller birthweights, attempt to better fit the organism to the environment it is receiving cues about (Fusco & Minelli, 2010). However, before discussing how changes in phenotype affect health outcomes, it is important to describe the range of adaptations to water scarcity to understand how these adaptations may lead to observed differences in water intake across population.

To understand human adaptation to differential water availability, species that live in niches with severe water scarcity provide for an analogy of their adaptations to these environments. Adaptations to water scarce, or arid, environments have been examined in species such as rats, birds, rabbits, and antelope to understand how these species adapt to extreme environments (Williams, Ostrowski, Bedin, & Ismail, 2001). Oryctolagus cuniculus rabbits living on different South African islands with variation in aridity show phenotypic plasticity as seen by renal morphological adaptations, that is higher relative medullary thickness (Tate & Marco, 2014). These adaptations resulted in an increased ability to concentrate urine and conserve body water. Similarly, birds have a general adaptation to water scarcity in that their basal metabolic rate (BMR) and total evaporative water loss decrease with increasing aridity (Tieleman, Williams, Buschur, & Brown, 2003). These adaptations would be favorable in water scarce environments because they would reduce dietary requirements and thermogenesis while decreased BMR would also reduce insensible water loss by reducing respiration rates. In hot‐dry environments, generating less heat and having a lower metabolic rate would reduce energetic expenditures and water turnover. However, an aridity gradient was not significantly associated with water turnover rate and urine osmolality, a biomarker of hydration, for the black‐tailed Tree Rat Thallomys , suggesting a genetic basis and evolutionary adaptation to dry environments (Coleman & Downs, 2009).

Birds present an interesting example in examining biological vs behavioral adaptations to water. While birds have general adaptations to water scarcity, they are also subject to dehydration and have behavioral adaptations to avoid water imbalance during migrations, such as flying at altitudes with more wind, not flying during the hottest parts of the day, seeking shade, and seeking water at stopover sites (Klaassen, 2004). Dietary preferences are also affected by exogenous water availability. In one study, water availability affected blackcaps dietary preference as experimentally‐water restricted birds switched to eating more water‐rich insects than fruit with lower water content (Tsurim et al., 2008). The finding that during periods of water stress, birds will choose to eat a diet filled with foods that are more water‐rich than under normal conditions indicates an underlying attempt to reach water balance through dietary behaviors.

Additionally, home water environments can have long‐term effects on water intake patterns. For example, water availability during migration at stopover sites affected how much food desert‐adapted blackcap birds ( Sylvia warblers and Turdidae chats) consumed depending on their winter habitat. If they were accustomed to extreme aridity, water availability at the stopover site was not as important to food consumption as for a bird that lived in a winter habitat with regular water availability (Sapir, Abramsky, Shochat, & Izhaki, 2004; Tsurim et al., 2008). The implications of this finding are in line with DOHaD theory in that water availability in an ecological habitat during ontogeny and preceding generations influences dietary behaviors of offspring that may lead to success of completing long migrations as well as reproductive success. Therefore, the amount of available water in a species' habitat may directly influence dietary behaviors and program them to need or want certain amounts of water.

Humans also use behavioral and biological adaptations to deal with water scarcity, one being voluntary dehydration , where workers and athletes do not consume enough water to replenish lost fluids during work or exercise. This allows humans to work longer in the heat without severe consequences and rehydrate later. For example, humans have flexibility in body water loss as documented in extreme cases like endurance hunts (Noakes, 2010) as well as elite endurance competitions, where marathon runners have been shown to lose up to 11.7% of body weight (Beis et al., 2012). However, the average gut size (~500 mL) and the gastric emptying rate of humans means that humans can only consume ~half a liter of water every 15 to 20 minutes and have to wait for this to empty before consuming more (Noakes, 2010).

As mammals, humans only have kidneys to osmoregulate, or keep homeostasis of water and salt balance. Many other vertebrates, including reptiles, birds, and fish, have other extrarenal functions to remove salt and other excreta without water loss (Beuchat, 1990a). Yet, for mammals, the kidney is able to meet substantial variation in dietary and environmental demands through phenotypic plasticity resulting in structural and functional modifications (Beuchat, 1990a). The loop of Henle in the nephron is what is responsible for increasing the concentration of urine in the kidney through countercurrent multiplication, where the urine becomes more concentrated as it moves deeper into the medulla of the kidney (Beuchat, 1990b). The ability to concentrate urine allows humans, like other primates (Wessling, Kühl, Mundry, Deschner, & Pruetz, 2018), to conserve body water in response to water deficits and seasonal stress.

A negative allometric, or scaling, relationship exists between body size and relative medullary thickness and maximum concentrated urine, such that the smallest mice and rodents produce the most concentrated urine (Beuchat, 1990b; Beuchat, 1996). This adaptation allows desert mice living in arid environments to persist for weeks with only dry food and no externally available water (MacMillen & Lee, 1967), whereas larger mammals like humans are more dependent on exogenous water sources. The human kidney, which is a metabolically expensive organ, is only slightly larger than expected based on other higher primates (Aiello & Wheeler, 1995). Additionally, humans fall close to the predicted maximum urine concentrating ability based on body size (Beuchat, 1990a). The environment, aridity, diet, metabolism, and activity levels all appear to affect this mammalian relationship between kidney morphology and urine concentration ability (Beuchat, 1996).

Therefore, evidence from the animal kingdom and humans suggests that genetic adaptations exist which have set up the ability to moderate water intake in different environments while phenotypic plasticity and behavioral modifications allow for variation in water intake in response to changing circumstances or environments.

3 POPULATION VARIATION IN HUMAN WATER NEEDS

Next, I discuss population variation in human water intake and the relationship to hydration biomarkers. I first present data from US adults to illustrate the wide variation in total water intake in a mostly water‐secure environment. I then present a hypothetical population water intake curve and discuss evidence indicating tight links between water intake and hydration biomarkers. Finally, I present data from the literature that interventions manipulating water intake affect these biomarkers meaning that they are modifiable, but that during postintervention ad libitum conditions, people revert to their usual place on this curve demonstrating relative stability on this curve once in adulthood.

3.1 Variation in water intake

The kernel density distribution of total water intake for US women and men illustrates that the majority of women consumed between 1 and 4 L on a given day and the majority of men consumed between 1.5 and 5 L (Figure 1). The 99th percentile of total water intake was 7.9 L for men and 6.3 L for women, which is similar to the top range measured via DLW at 7.7 L for men (Raman et al., 2004). However, the distribution extends to more than 10 L for less than 0.1% of adults. These data demonstrate that even in a relatively water‐secure environment, the amount of total water consumed from foods and beverages in the diet is widely variable to meet people's water needs.

While the 24‐hour automated multiple pass dietary recall has been validated for nutrient and energy intake against DLW for men and women (Blanton, Moshfegh, Baer, & Kretsch, 2006; Conway, Ingwersen, & Moshfegh, 2004), energy intake is generally under‐reported. With water intake, it is possible that some adults over‐estimate their water intake but most likely it is under‐reported as dietary recalls have been found to miss up to two drinking events compared to fluid intake diaries (Gandy, 2015). Nevertheless, a gap exists in comparing measured water intake from dietary recall methods vs DLW (Raman et al., 2004).

3.2 Association of water intake and hydration biomarkers

Using a hypothetical population curve describing water intake, the population breaks down into three primary groups: (a) chronically underhydrated individuals (the low intake group), who consume less than what is recommended; (b) the majority of people who are in line with water recommendations (the adequate amount) and consume closer to the population median; and (c) chronically overhydrated individuals (the high intake group), who consume more water than what is recommended (Figure 2).

Previous research has demonstrated that how much water people normally consume is tightly linked to their urine concentration biomarkers. While water intake changes from day to day, average intake stays fairly consistent without external factors changing requirements (Perrier et al., 2013). Studies have examined variation of hydration biomarkers in the water intake categories, that is, the high and low drinkers, finding that urine osmolality, urine specific gravity, and vasopressin (biomarkers of hydration), are all higher among the low intake group compared to the high intake group (Perrier et al., 2013; Perrier et al., 2013; Perrier, Rondeau, et al., 2013). These findings indicate that the amount of water individuals normally consume is strongly tied to urine concentration and hydration biomarkers.

Those in the low water intake group have low urine volume which serves as a physiological adaptation to conserve total body water and keep homeostasis. Nevertheless, low water intake may have long‐term implications for kidney health. For example, chronic water restriction among female rats induced kidney damage as evidenced by damaged renal tubules and glomerular shrinkage (Daily, Zhang, Wu, & Park, 2019), while low water intake in humans reduces the capacity to excrete sodium (Choukroun, Schmitt, Martinez, Drüeke, & Bankir, 1997). The largest longitudinal study among humans which has examined water intake and total urine output on kidney function found that adults with higher urine volume were significantly less likely to demonstrate decline of eGFR over 6 years than those with lower urine volume (Clark et al., 2011). A review of the literature shows that repeated chronic dehydration may be a key factor in chronic kidney disease (CKD) of unknown origin (Clark et al., 2016). However, a recent randomized control study among those who already had stage 3 CKD found that those who received coaching to drink more water (an extra liter a day) did not have significantly slower declines in kidney function (Clark et al., 2018). Therefore, daily water intake among those without kidney damage likely provides the most benefits to kidney health (Armstrong & Johnson, 2018; Clark et al., 2016).

3.3 Interventions of water intake reverse hydration biomarkers

When intake returned to ad libitum drinking, the effects disappear and water intake and urine concentration biomarkers returned to what they were at baseline. Therefore, where a person is in terms of their daily water intake manifests in biomarkers of hydration status-essentially the body adapts to the level of water an individual regularly consumes-and this has implications for kidney health. However, what is less clear is what drives where a person begins on this curve of water intake.

4 EARLY LIFE AND DEVELOPMENT INFLUENCES ON WATER NEEDS

Is there evidence that where a person ends up on the distribution of water intake is influenced by development? What happens when a drought or severe water shortage occurs to women who are pregnant or lactating and what might that do to the water needs of their offspring? When mismatches exist between the fetal environment and later life environments, these changes can increase risk of different metabolic diseases (Gluckman & Hanson, 2004b; Kuzawa, 2005). While the long‐term and intergenerational effects of food restriction during pregnancy are more well‐understood (Roseboom, de Rooij, & Painter, 2006), a gap exists in the DOHaD literature regarding the explicit effects of water restriction during early life since the emphasis is often on fetal energy availability and allocation (Gluckman & Hanson, 2004a) and water is calorically neutral.

In this section, I review evidence for the effects of early life water restriction on water need set points and health outcomes using animal studies and observational data from humans. I focus on two key periods in early life water restriction: pregnancy and lactation to examine how they affect body water balance for the fetus and infants as well as long‐term health implications.

4.1 Water restriction during pregnancy

4.2 Water restriction during lactation

Breast milk contains critical biochemical messages including those aimed at aiding immunological defenses, cytokines, and hormones like cortisol (Bernt & Walker, 1999), therefore, it is also possible that breast milk may contain signals about the availability of water in the environment to their infants.

External conditions related to water insecurity, defined as inadequate access to clean, safe water to live a healthy, culturally appropriate lifestyle (Jepson, Wutich, Colllins, Boateng, & Young, 2017), among lactating women may make it harder for them to meet their water needs and create excess stress. Research indicates that lactating women in hot‐humid environments without access to clean water had significantly higher urine specific gravity and odds of being dehydrated than nonlactating women (Rosinger, 2015). Moreover, poor water supply and quality in Kenya led to higher reported stress among lactating women (Collins et al., 2019). These lactating women stated they feared they did not have adequate breast milk supply because of the lack of clean water. Physiologically, stressful stimuli may impair the milk ejection reflex thereby affecting breast milk production, at least temporarily, through reduced oxytocin production (Dewey, 2001).

The timing of exposure to different ecological conditions in utero and during lactation can have long‐standing health and nutritional implications related to cardiovascular function and plasma osmolality, set points to thirst sensitivity and vasopressin secretion, growth, and body size. It is therefore reasonable to hypothesize that these developmental and early life experiences related to water insecurity and water restriction affect where on the population curve people begin as these conditions may affect biological and behavioral variation in water needs (Figure 3). Interestingly, when pregnant rat dams experienced water restriction that extended into lactation, this water restriction caused growth reduction of the adult offspring without catch‐up growth thereby further affecting water needs through reduced body size (Ross & Desai, 2005).

This fetal programming, influenced by ecological conditions, indicates that extreme water restriction can contribute to phenotypic plasticity during development as these conditions may send a signal to the fetus and infant about potential water availability (Kuzawa, 2005). An important future direction for understanding the effects of water restriction in early life on shaping water intake behaviors and human water needs necessitates a closer examination of hydration status and water‐related interventions from a DOHaD perspective.

5 LIFE COURSE CHANGES AND INTERACTIONS WITH HEALTH CONDITIONS AND BEHAVIORS

Many changes to health occur throughout the life course through disease, physiological stressors, and behaviors which may create homeostatic imbalances in body water. Going beyond lactation and pregnancy for women and more temporary daily changes that modify water needs, in this section I focus on three examples, a chronic health condition (obesity), an infectious disease (schistosomiasis), and a health behavior (sleep). I focus on these examples because they are potentially modifiable through interventions and affect a significant proportion of the global population. Additionally, they exemplify life course changes that affect body water homeostasis with chronic health implications (Ben‐Shlomo & Kuh, 2002).

5.1 Obesity and water imbalances

Previous research has found attenuated thermoregulatory responses among those with obesity (Kanikowska et al., 2013). One of the primary mechanisms driving this relationship is that adults with obesity may have an altered distribution of body fluid with a higher volume of extracellular relative to intracellular fluid (Stookey, Barclay, Arieff, & Popkin, 2006). This altered distribution of body fluid may affect homeostatic mechanisms regulating electrolyte balance within and between cells (Thornton, 2010). The consequences of this have been documented through higher incidences of heat illness reported among army recruits with obesity compared to those without obesity (Nutong et al., 2018). Moreover, obesity is linked to increased oxidative stress which may lead to chronic kidney disease (Sharma, 2014).

5.2 Parasitic infections and dehydration

A poignant example of a disturbance of body water homeostasis relates to Schistosoma species, which migrate to different organs. Several studies have documented edema and elevated total body water among those infected with Schistosoma (S.) mansoni (Andrade et al., 1971; De Lorenzo et al., 1997). When comparing the urine specific gravity (Usg) of women infected with S. haematobium , which infects the bladder, and S. mansoni , which infects the GI tract, to noninfected women in Tanzania, those that were infected with S. haematobium , but not S. mansoni had significantly elevated Usg and higher odds of dehydration than noninfected women (Rosinger et al., 2018). These findings suggest that the kidneys may face a tradeoff between fighting off the infection and filtering out waste thereby resulting in water imbalances.

Neglected tropical diseases, including the suite of parasites such as Schistosomiasis, Cryptosporidium, Giardiasis, amoebas, and others have major implications for hydration status and will be continually important to monitor in relation to kidney health and long‐term risk factors for chronic kidney disease (Jha et al., 2013).

5.3 Sleep and dehydration

While infections and health conditions directly interact with body water homeostasis, health behaviors are also tied to body water balance. Sleep duration is an important health behavior with large interindividual variation, especially cross‐culturally. In the US, average adult sleep duration is slightly less than 7 hours with variation between and within groups, while Himba agropastoralist adults sleep less than 6 hours, and Italian adults report 8 hours (Prall, Yetish, Scelza, & Siegel, 2018). However, there's an increasing trend of short sleep in the US across all race/ethnicities, but this is an even bigger problem among non‐Hispanic black adults who more than 40% report sleeping 6 or fewer hours (Sheehan, Frochen, Walsemann, & Ailshire, 2018). The recommendation is 7 to 9 hours for optimal health (Consensus Conference, 2015). Previous research has shown that shorter sleep reduced kidney function and filtration rate 10 years later (Petrov et al., 2014). However, few studies have examined how sleep duration is associated with hydration status beyond anecdotal reports of sleep‐restricted participants reporting trouble with blood draws due to dehydration (Opstad, 1992).

Sleep is linked with many homeostatic systems that are tied to hydration, including hunger and satiety endocrine‐signaling systems (Taheri, Lin, Austin, Young, & Mignot, 2004), cardiovascular function (Buxton & Marcelli, 2010), and kidney function (Li et al., 2017). While the literature on the relationship between sleep and hydration is scarce, a recent study found that sleeping 6 hours a night was associated with significantly elevated odds of inadequate hydration in both US and Chinese adults drawing on a cross‐cultural sample of over 26 000 adults (Rosinger et al., 2019). Interestingly, a cross‐over study examined the opposite relationship of hydration status on sleep and did not find that dehydration reduced sleep time, but there was a reduction in time spent in REM sleep phase (Aristotelous et al., 2019). Hydration and sleep may therefore be part of a cycle.

Overall, these health conditions, physiological stressors, and behaviors may create shifts in the match between where a person is on the curve during the life course and whether water intake level is sufficient (Figure 4). While the body adapts to the level of water intake adjusted for the new water needs, for example by releasing more vasopressin and having more concentrated urine, long‐term mismatches may have negative health effects on the kidney (Clark et al., 2016). Many of these conditions, like chronic infection with schistosomiasis may create irreversible kidney damage, which may affect body water homeostasis through the inability to concentrate urine and filter out toxins. Therefore, it is critical to identify interventions to reduce the conditions which lead to inadequate water intake.

6 CONCLUSION

This article discussed how adaptations and behaviors contribute to human water intake variation and body water homeostasis and how hydration biomarkers track variation in water intake. Additionally, this article explored how early life influences through water restriction in utero and during lactation may affect water intake setpoints and water needs with implications for cardiovascular and metabolic health. As an oft‐overlooked nutrient, this topic could be a new direction for the developmental origins of health and disease paradigm. Moreover, where a person begins on a distribution curve of water intake matching water needs may also be modified throughout their life as certain health conditions and behaviors may dysregulate body water homeostasis like obesity, parasitic exposures, and sleep duration. These factors may create a mismatch in people's set points of where they are on this curve in detecting body water deficits. Future work should utilize natural experiments to further understand and test how water restriction and dehydration during pregnancy and lactation may affect long‐term water needs and chronic health outcomes. Furthermore, these studies should attempt to disaggregate the effects of food restriction from water restriction.

ACKNOWLEDGMENTS

I thank Dr. Susan Tanner who years ago advised me to dive deeper in the literature to understand water‐related adaptations. Thanks to my whippet Athens for joining me on long runs and my daughter Maya for keeping me up at night when these ideas gelled in my head. I thank the Yale Biological Colloquium series for helpful discussions on an earlier version of this article. An earlier version of this article was presented in the Plenary on the Human Biology of Water at the Human Biology Association's annual meeting in Cleveland, Ohio on March 27, 2019.

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Abstract

Body water homeostasis is critical for optimal physiological and cognitive function for humans. The majority of research has illustrated the negative biological consequences of failing to meet water needs. The human body has several mechanisms for detecting, regulating, and correcting body water deficits and excesses. However, variation exists in total water intake and how people meet those water needs as well as thirst thresholds and how well people tolerate water restriction. An evolutionary and developmental framework provides an underexplored perspective into human water needs by examining how adaptations, early life experiences and environments, as well as life course changes in health states and behaviors may shape these critical factors in body water homeostasis. This article first reviews biological and behavioral adaptations to water scarcity among animals and humans. It then examines human variation in water intake in a mostly water secure environment through the analysis of National Health and Nutrition Examination Survey dietary data and the link between water intake patterns and hydration biomarkers. Next, it reviews existing evidence of how maternal water restriction in utero and during lactation shape vasopressin release, thirst thresholds, drinking patterns, and body water homeostasis for the infant. Early life water restriction appears to have implications for hydration status, body size, and cardiovascular health. Finally, it examines how life course changes in health states and behaviors, including obesity, sleep, and parasitic infection, affect body water homeostasis. This article poses new questions about the plasticity and shaping of human water needs, thirst, and hydration behaviors.

1 INTRODUCTION

Human water needs are most commonly explained through physiology, including respiration, metabolism, thermoregulation, and waste disposal, as well as behavioral responses to the environment (Sawka, Cheuvront, & Carter, ). Individual variation in body size, physical activity level, health states, and for women, pregnancy and lactating status also drive water needs (Food and Nutrition Board and Institute of Medicine, ; Sawka et al., ). However, environments and water resources are dynamic. A critical gap exists in examining human water needs through an evolutionary framework (Wheeler, ). Using an evolutionary perspective allows for a deeper examination of how adaptations, early life experiences and environments, as well as life course changes in health states and behaviors may shape water intake and body water homeostasis.

Throughout human evolution, water needs have played an important selective pressure affecting survival. Changes in body size and proportion between Australopithecus and Homo erectus significantly affected water needs through thermoregulation by changing exposure to wind and solar radiation, which reduced sweat rates and water needs by 15% to 18%. And these changes in water efficiency may have allowed for longer periods of foraging (Wheeler, ). Another key example of morphological adaptations related to water needs among Homo are sweat glands, primarily eccrine glands, which allow for loss of body water in response to thermoregulation to keep body temperatures stable (Jablonski, ). Additionally, the external nose was an important adaptation for Homo to dry environments by providing the ability to retrieve moisture from exhaled air and then using this moisture to humidify the next breath to help conserve and reduce body water loss (Franciscus & Trinkaus, ; Lieberman, ). These adaptations demonstrate how important and integrative responses to water availability in the environment were to human evolution on a deep timescale.

The process of hydration and body water homeostasis is important to understand because it is acutely connected to physical and cognitive health as well as performance (Armstrong et al., ; Ganio et al., ). Hydration is a process that is constantly in flux as the body gains and loses water (Perrier et al., ). However, even acute dehydration, or ~2% of body weight lost through water, in the short‐term may lead to cognitive ability decline (Benton, Jenkins, Watkins, & Young, ; Goodman, Moreland, & Marino, ) and increased irritability and fatigue (Armstrong et al., ). Physical performance as seen via running times (Murray, ) and cycling performances (Adams et al., ) also deteriorate with acute dehydration. However, in elite endurance competitions athletes frequently lose much more than 2% of body mass, including the winners of the competitions (Beis, Wright‐Whyte, Fudge, Noakes, & Pitsiladis, ). Chronic acute dehydration is also associated with increased risk of urinary tract infections (Hooton et al., ), urolithiasis or kidney stones, and impaired kidney function (Clark et al., ), while dehydration beyond 10% of body weight can result in death (Popkin, D'Anci, & Rosenberg, ).

The Institute of Medicine, now the National Academy of Medicine, released adequate intake (AI) recommendations meant to meet the water needs of moderately active individuals in a healthy population (Food and Nutrition Board and Institute of Medicine, ). The AI for men is 3.7 L per day, for nonpregnant, nonlactating women it is 2.7 L, for pregnant women it is 3.0 L, and for lactating women it is 3.8 L. The European Food Safety Authority released lower AI recommendations: 2.5 L for men, 2.0 L for nonpregnant, nonlactating women, 2.3 L for pregnant women, and 2.7 L for lactating women (Jequier & Constant, ). These recommendations are based on nutritional surveys and water turnover studies.

However, variation has been documented in total water intake between and within populations across different environments ranging from 2 L per day among Greek adults to almost 10 L per day among Ecuadorian Shuar men (Braun et al., ; Christopher et al., ; Rosinger & Tanner, ; Tani et al., ). Additionally, the dietary constituents that make up total water intake, that is the percentage of water consumed from foods vs from beverages, vary. It is largely assumed that approximately 20% to 30% of total water intake comes from food sources with the remaining 70% to 80% from liquids (Jequier & Constant, ). However, at least two studies have demonstrated population variation in hydration strategies as both Japanese and Bolivian Tsimane' adults consumed roughly 50% of their total water intake from food sources (Rosinger & Tanner, ; Tani et al., ).

Since water balance is intimately tied to health and physiological functioning, several systems exist, including thirst, to return the body to homeostasis (Oka, Ye, & Zuker, ). The body and brain detect water deficits and excesses thereby releasing or restricting the anti‐diuretic hormone vasopressin and conserving body water or excreting it with the signals getting stronger depending on the level of imbalance (Armstrong & Johnson, ).

It is therefore critical to understand adaptations and biobehavioral responses to different environments when examining variation in human water needs and body water homeostasis. In attempting to explain why a biobehavioral approach is important, Leslie and Little explain (page 6) (Leslie & Little, ): "Human behavior is not explicable simply within the closed boundaries of society or of culture. Much adaptation ... deals also with the natural world in which society and culture are set: that is, with the biological constraints and capacities of humans themselves and with the biological effects and opportunities of the surrounding environment." Human water intake perfectly exemplifies this sentiment since it is a function of evolved physiology interacting with specific individual characteristics, physical access to water and foods, cultural milieus that provide a range of acceptable dietary items to drink and eat, but also highly adaptable to disparate environments.

Several evolutionary questions surrounding human water intake remain. What is the range of adaptations derived to conserve water? How does population variation in water intake arise in the first place? Aside from the known correlates of water needs, why do certain individuals consume more or less water? Do early life and life course exposures create a mismatch in water needs and water intake? An evolutionary perspective provides a unique way to understand how water restriction at different time‐points in an organism's lifecycle may affect form, function, and disease risk (Gluckman & Hanson, ; Kuzawa & Quinn, ).

This article addresses four key questions in sections 2 to 5 to help explain variation in human water needs and body water homeostasis from an evolutionary perspective using examples from animal models and human biology:

1. What are the roles of adaptations to water scarce environments, biological and behavioral as well as phenotypic plasticity (or the extent to which the environment modifies the phenotype), in shaping water needs?

2. What does population variation in human water intake look like in a water‐secure location and how does this relate to hydration biomarkers?

3. How might water restriction during early life experiences shape water needs and what are the implications for body water homeostasis and the resulting health consequences?

4. Finally, how do changes in health states and behaviors throughout the life course affect water intake and the match between water intake and body water homeostasis?

2 ADAPTATIONS TO WATER‐SCARCE ENVIRONMENTS

In this section, I explore the question of biological adaptation vs phenotypic plasticity and behavioral modifications to different environments by examining urine concentrating ability, kidney function and morphology, dietary changes, and water turnover in the context of different examples from the animal kingdom and human evolution.

Water, as an essential nutrient, serves as a critical stressor to which humans must adapt (Haas & Harrison, ). Adaptation is a process in which an organism adjusts to a suite of stressors to better match their environment (Frisancho, ). Adaptations can last varying degrees of time, involve multiple timescales, and occur in individuals and populations. The developmental origins of health and disease (DOHaD) paradigm examines how in‐utero nutrition as well as intergenerational changes in nutrition and ecological stressors affect human adaptability and resulting health outcomes (Gluckman & Hanson, ; Kuzawa, ). These phenotypic changes, such as smaller birthweights, attempt to better fit the organism to the environment it is receiving cues about (Fusco & Minelli, ). However, before discussing how changes in phenotype affect health outcomes, it is important to describe the range of adaptations to water scarcity to understand how these adaptations may lead to observed differences in water intake across population.

To understand human adaptation to differential water availability, species that live in niches with severe water scarcity provide for an analogy of their adaptations to these environments. Adaptations to water scarce, or arid, environments have been examined in species such as rats, birds, rabbits, and antelope to understand how these species adapt to extreme environments (Williams, Ostrowski, Bedin, & Ismail, ). Oryctolagus cuniculus rabbits living on different South African islands with variation in aridity show phenotypic plasticity as seen by renal morphological adaptations, that is higher relative medullary thickness (Tate & Marco, ). These adaptations resulted in an increased ability to concentrate urine and conserve body water. Similarly, birds have a general adaptation to water scarcity in that their basal metabolic rate (BMR) and total evaporative water loss decrease with increasing aridity (Tieleman, Williams, Buschur, & Brown, ). These adaptations would be favorable in water scarce environments because they would reduce dietary requirements and thermogenesis while decreased BMR would also reduce insensible water loss by reducing respiration rates. In hot‐dry environments, generating less heat and having a lower metabolic rate would reduce energetic expenditures and water turnover. However, an aridity gradient was not significantly associated with water turnover rate and urine osmolality, a biomarker of hydration, for the black‐tailed Tree Rat Thallomys , suggesting a genetic basis and evolutionary adaptation to dry environments (Coleman & Downs, ).

Birds present an interesting example in examining biological vs behavioral adaptations to water. While birds have general adaptations to water scarcity, they are also subject to dehydration and have behavioral adaptations to avoid water imbalance during migrations, such as flying at altitudes with more wind, not flying during the hottest parts of the day, seeking shade, and seeking water at stopover sites (Klaassen, ). Dietary preferences are also affected by exogenous water availability. In one study, water availability affected blackcaps dietary preference as experimentally‐water restricted birds switched to eating more water‐rich insects than fruit with lower water content (Tsurim et al., ). The finding that during periods of water stress, birds will choose to eat a diet filled with foods that are more water‐rich than under normal conditions indicates an underlying attempt to reach water balance through dietary behaviors.

Additionally, home water environments can have long‐term effects on water intake patterns. For example, water availability during migration at stopover sites affected how much food desert‐adapted blackcap birds ( Sylvia warblers and Turdidae chats) consumed depending on their winter habitat. If they were accustomed to extreme aridity, water availability at the stopover site was not as important to food consumption as for a bird that lived in a winter habitat with regular water availability (Sapir, Abramsky, Shochat, & Izhaki, ; Tsurim et al., ). The implications of this finding are in line with DOHaD theory in that water availability in an ecological habitat during ontogeny and preceding generations influences dietary behaviors of offspring that may lead to success of completing long migrations as well as reproductive success. Therefore, the amount of available water in a species' habitat may directly influence dietary behaviors and program them to need or want certain amounts of water.

Humans also use behavioral and biological adaptations to deal with water scarcity, one being voluntary dehydration , where workers and athletes do not consume enough water to replenish lost fluids during work or exercise. This allows humans to work longer in the heat without severe consequences and rehydrate later. For example, humans have flexibility in body water loss as documented in extreme cases like endurance hunts (Noakes, ) as well as elite endurance competitions, where marathon runners have been shown to lose up to 11.7% of body weight (Beis et al., ). However, the average gut size (~500 mL) and the gastric emptying rate of humans means that humans can only consume ~half a liter of water every 15 to 20 minutes and have to wait for this to empty before consuming more (Noakes, ).

As mammals, humans only have kidneys to osmoregulate, or keep homeostasis of water and salt balance. Many other vertebrates, including reptiles, birds, and fish, have other extrarenal functions to remove salt and other excreta without water loss (Beuchat, ). Yet, for mammals, the kidney is able to meet substantial variation in dietary and environmental demands through phenotypic plasticity resulting in structural and functional modifications (Beuchat, ). The loop of Henle in the nephron is what is responsible for increasing the concentration of urine in the kidney through countercurrent multiplication, where the urine becomes more concentrated as it moves deeper into the medulla of the kidney (Beuchat, ). The ability to concentrate urine allows humans, like other primates (Wessling, Kühl, Mundry, Deschner, & Pruetz, ), to conserve body water in response to water deficits and seasonal stress.

A negative allometric, or scaling, relationship exists between body size and relative medullary thickness and maximum concentrated urine, such that the smallest mice and rodents produce the most concentrated urine (Beuchat, ; Beuchat, ). This adaptation allows desert mice living in arid environments to persist for weeks with only dry food and no externally available water (MacMillen & Lee, ), whereas larger mammals like humans are more dependent on exogenous water sources. The human kidney, which is a metabolically expensive organ, is only slightly larger than expected based on other higher primates (Aiello & Wheeler, ). Additionally, humans fall close to the predicted maximum urine concentrating ability based on body size (Beuchat, ). The environment, aridity, diet, metabolism, and activity levels all appear to affect this mammalian relationship between kidney morphology and urine concentration ability (Beuchat, ).

Therefore, evidence from the animal kingdom and humans suggests that genetic adaptations exist which have set up the ability to moderate water intake in different environments while phenotypic plasticity and behavioral modifications allow for variation in water intake in response to changing circumstances or environments.

3 POPULATION VARIATION IN HUMAN WATER NEEDS

Next, I discuss population variation in human water intake and the relationship to hydration biomarkers. I first present data from US adults to illustrate the wide variation in total water intake in a mostly water‐secure environment. I then present a hypothetical population water intake curve and discuss evidence indicating tight links between water intake and hydration biomarkers. Finally, I present data from the literature that interventions manipulating water intake affect these biomarkers meaning that they are modifiable, but that during postintervention ad libitum conditions, people revert to their usual place on this curve demonstrating relative stability on this curve once in adulthood.

3.1 Variation in water intake

The kernel density distribution of total water intake for US women and men illustrates that the majority of women consumed between 1 and 4 L on a given day and the majority of men consumed between 1.5 and 5 L (Figure 1). The 99th percentile of total water intake was 7.9 L for men and 6.3 L for women, which is similar to the top range measured via DLW at 7.7 L for men (Raman et al., ). However, the distribution extends to more than 10 L for less than 0.1% of adults. These data demonstrate that even in a relatively water‐secure environment, the amount of total water consumed from foods and beverages in the diet is widely variable to meet people's water needs.

While the 24‐hour automated multiple pass dietary recall has been validated for nutrient and energy intake against DLW for men and women (Blanton, Moshfegh, Baer, & Kretsch, ; Conway, Ingwersen, & Moshfegh, ), energy intake is generally under‐reported. With water intake, it is possible that some adults over‐estimate their water intake but most likely it is under‐reported as dietary recalls have been found to miss up to two drinking events compared to fluid intake diaries (Gandy, ). Nevertheless, a gap exists in comparing measured water intake from dietary recall methods vs DLW (Raman et al., ).

3.2 Association of water intake and hydration biomarkers

Using a hypothetical population curve describing water intake, the population breaks down into three primary groups: (a) chronically underhydrated individuals (the low intake group), who consume less than what is recommended; (b) the majority of people who are in line with water recommendations (the adequate amount) and consume closer to the population median; and (c) chronically overhydrated individuals (the high intake group), who consume more water than what is recommended (Figure 2).

Previous research has demonstrated that how much water people normally consume is tightly linked to their urine concentration biomarkers. While water intake changes from day to day, average intake stays fairly consistent without external factors changing requirements (Perrier et al., ). Studies have examined variation of hydration biomarkers in the water intake categories, that is, the high and low drinkers, finding that urine osmolality, urine specific gravity, and vasopressin (biomarkers of hydration), are all higher among the low intake group compared to the high intake group (Perrier et al., ; Perrier et al., ; Perrier, Rondeau, et al., ). These findings indicate that the amount of water individuals normally consume is strongly tied to urine concentration and hydration biomarkers.

Those in the low water intake group have low urine volume which serves as a physiological adaptation to conserve total body water and keep homeostasis. Nevertheless, low water intake may have long‐term implications for kidney health. For example, chronic water restriction among female rats induced kidney damage as evidenced by damaged renal tubules and glomerular shrinkage (Daily, Zhang, Wu, & Park, ), while low water intake in humans reduces the capacity to excrete sodium (Choukroun, Schmitt, Martinez, Drüeke, & Bankir, ). The largest longitudinal study among humans which has examined water intake and total urine output on kidney function found that adults with higher urine volume were significantly less likely to demonstrate decline of eGFR over 6 years than those with lower urine volume (Clark et al., ). A review of the literature shows that repeated chronic dehydration may be a key factor in chronic kidney disease (CKD) of unknown origin (Clark et al., ). However, a recent randomized control study among those who already had stage 3 CKD found that those who received coaching to drink more water (an extra liter a day) did not have significantly slower declines in kidney function (Clark et al., ). Therefore, daily water intake among those without kidney damage likely provides the most benefits to kidney health (Armstrong & Johnson, ; Clark et al., ).

3.3 Interventions of water intake reverse hydration biomarkers

When intake returned to ad libitum drinking, the effects disappear and water intake and urine concentration biomarkers returned to what they were at baseline. Therefore, where a person is in terms of their daily water intake manifests in biomarkers of hydration status-essentially the body adapts to the level of water an individual regularly consumes-and this has implications for kidney health. However, what is less clear is what drives where a person begins on this curve of water intake.

4 EARLY LIFE AND DEVELOPMENT INFLUENCES ON WATER NEEDS

Is there evidence that where a person ends up on the distribution of water intake is influenced by development? What happens when a drought or severe water shortage occurs to women who are pregnant or lactating and what might that do to the water needs of their offspring? When mismatches exist between the fetal environment and later life environments, these changes can increase risk of different metabolic diseases (Gluckman & Hanson, ; Kuzawa, ). While the long‐term and intergenerational effects of food restriction during pregnancy are more well‐understood (Roseboom, de Rooij, & Painter, ), a gap exists in the DOHaD literature regarding the explicit effects of water restriction during early life since the emphasis is often on fetal energy availability and allocation (Gluckman & Hanson, ) and water is calorically neutral.

In this section, I review evidence for the effects of early life water restriction on water need set points and health outcomes using animal studies and observational data from humans. I focus on two key periods in early life water restriction: pregnancy and lactation to examine how they affect body water balance for the fetus and infants as well as long‐term health implications.

4.1 Water restriction during pregnancy

4.2 Water restriction during lactation

Breast milk contains critical biochemical messages including those aimed at aiding immunological defenses, cytokines, and hormones like cortisol (Bernt & Walker, ), therefore, it is also possible that breast milk may contain signals about the availability of water in the environment to their infants.

External conditions related to water insecurity, defined as inadequate access to clean, safe water to live a healthy, culturally appropriate lifestyle (Jepson, Wutich, Colllins, Boateng, & Young, ), among lactating women may make it harder for them to meet their water needs and create excess stress. Research indicates that lactating women in hot‐humid environments without access to clean water had significantly higher urine specific gravity and odds of being dehydrated than nonlactating women (Rosinger, ). Moreover, poor water supply and quality in Kenya led to higher reported stress among lactating women (Collins et al., ). These lactating women stated they feared they did not have adequate breast milk supply because of the lack of clean water. Physiologically, stressful stimuli may impair the milk ejection reflex thereby affecting breast milk production, at least temporarily, through reduced oxytocin production (Dewey, ).

The timing of exposure to different ecological conditions in utero and during lactation can have long‐standing health and nutritional implications related to cardiovascular function and plasma osmolality, set points to thirst sensitivity and vasopressin secretion, growth, and body size. It is therefore reasonable to hypothesize that these developmental and early life experiences related to water insecurity and water restriction affect where on the population curve people begin as these conditions may affect biological and behavioral variation in water needs (Figure 3). Interestingly, when pregnant rat dams experienced water restriction that extended into lactation, this water restriction caused growth reduction of the adult offspring without catch‐up growth thereby further affecting water needs through reduced body size (Ross & Desai, ).

This fetal programming, influenced by ecological conditions, indicates that extreme water restriction can contribute to phenotypic plasticity during development as these conditions may send a signal to the fetus and infant about potential water availability (Kuzawa, ). An important future direction for understanding the effects of water restriction in early life on shaping water intake behaviors and human water needs necessitates a closer examination of hydration status and water‐related interventions from a DOHaD perspective.

5 LIFE COURSE CHANGES AND INTERACTIONS WITH HEALTH CONDITIONS AND BEHAVIORS

Many changes to health occur throughout the life course through disease, physiological stressors, and behaviors which may create homeostatic imbalances in body water. Going beyond lactation and pregnancy for women and more temporary daily changes that modify water needs, in this section I focus on three examples, a chronic health condition (obesity), an infectious disease (schistosomiasis), and a health behavior (sleep). I focus on these examples because they are potentially modifiable through interventions and affect a significant proportion of the global population. Additionally, they exemplify life course changes that affect body water homeostasis with chronic health implications (Ben‐Shlomo & Kuh, ).

5.1 Obesity and water imbalances

Previous research has found attenuated thermoregulatory responses among those with obesity (Kanikowska et al., ). One of the primary mechanisms driving this relationship is that adults with obesity may have an altered distribution of body fluid with a higher volume of extracellular relative to intracellular fluid (Stookey, Barclay, Arieff, & Popkin, ). This altered distribution of body fluid may affect homeostatic mechanisms regulating electrolyte balance within and between cells (Thornton, ). The consequences of this have been documented through higher incidences of heat illness reported among army recruits with obesity compared to those without obesity (Nutong et al., ). Moreover, obesity is linked to increased oxidative stress which may lead to chronic kidney disease (Sharma, ).

5.2 Parasitic infections and dehydration

A poignant example of a disturbance of body water homeostasis relates to Schistosoma species, which migrate to different organs. Several studies have documented edema and elevated total body water among those infected with Schistosoma (S.) mansoni (Andrade et al., ; De Lorenzo et al., ). When comparing the urine specific gravity (Usg) of women infected with S. haematobium , which infects the bladder, and S. mansoni , which infects the GI tract, to noninfected women in Tanzania, those that were infected with S. haematobium , but not S. mansoni had significantly elevated Usg and higher odds of dehydration than noninfected women (Rosinger et al., ). These findings suggest that the kidneys may face a tradeoff between fighting off the infection and filtering out waste thereby resulting in water imbalances.

Neglected tropical diseases, including the suite of parasites such as Schistosomiasis, Cryptosporidium, Giardiasis, amoebas, and others have major implications for hydration status and will be continually important to monitor in relation to kidney health and long‐term risk factors for chronic kidney disease (Jha et al., ).

5.3 Sleep and dehydration

While infections and health conditions directly interact with body water homeostasis, health behaviors are also tied to body water balance. Sleep duration is an important health behavior with large interindividual variation, especially cross‐culturally. In the US, average adult sleep duration is slightly less than 7 hours with variation between and within groups, while Himba agropastoralist adults sleep less than 6 hours, and Italian adults report 8 hours (Prall, Yetish, Scelza, & Siegel, ). However, there's an increasing trend of short sleep in the US across all race/ethnicities, but this is an even bigger problem among non‐Hispanic black adults who more than 40% report sleeping 6 or fewer hours (Sheehan, Frochen, Walsemann, & Ailshire, ). The recommendation is 7 to 9 hours for optimal health (Consensus Conference, ). Previous research has shown that shorter sleep reduced kidney function and filtration rate 10 years later (Petrov et al., ). However, few studies have examined how sleep duration is associated with hydration status beyond anecdotal reports of sleep‐restricted participants reporting trouble with blood draws due to dehydration (Opstad, ).

Sleep is linked with many homeostatic systems that are tied to hydration, including hunger and satiety endocrine‐signaling systems (Taheri, Lin, Austin, Young, & Mignot, ), cardiovascular function (Buxton & Marcelli, ), and kidney function (Li et al., ). While the literature on the relationship between sleep and hydration is scarce, a recent study found that sleeping 6 hours a night was associated with significantly elevated odds of inadequate hydration in both US and Chinese adults drawing on a cross‐cultural sample of over 26 000 adults (Rosinger et al., ). Interestingly, a cross‐over study examined the opposite relationship of hydration status on sleep and did not find that dehydration reduced sleep time, but there was a reduction in time spent in REM sleep phase (Aristotelous et al., ). Hydration and sleep may therefore be part of a cycle.

Overall, these health conditions, physiological stressors, and behaviors may create shifts in the match between where a person is on the curve during the life course and whether water intake level is sufficient (Figure 4). While the body adapts to the level of water intake adjusted for the new water needs, for example by releasing more vasopressin and having more concentrated urine, long‐term mismatches may have negative health effects on the kidney (Clark et al., ). Many of these conditions, like chronic infection with schistosomiasis may create irreversible kidney damage, which may affect body water homeostasis through the inability to concentrate urine and filter out toxins. Therefore, it is critical to identify interventions to reduce the conditions which lead to inadequate water intake.

6 CONCLUSION

This article discussed how adaptations and behaviors contribute to human water intake variation and body water homeostasis and how hydration biomarkers track variation in water intake. Additionally, this article explored how early life influences through water restriction in utero and during lactation may affect water intake setpoints and water needs with implications for cardiovascular and metabolic health. As an oft‐overlooked nutrient, this topic could be a new direction for the developmental origins of health and disease paradigm. Moreover, where a person begins on a distribution curve of water intake matching water needs may also be modified throughout their life as certain health conditions and behaviors may dysregulate body water homeostasis like obesity, parasitic exposures, and sleep duration. These factors may create a mismatch in people's set points of where they are on this curve in detecting body water deficits. Future work should utilize natural experiments to further understand and test how water restriction and dehydration during pregnancy and lactation may affect long‐term water needs and chronic health outcomes. Furthermore, these studies should attempt to disaggregate the effects of food restriction from water restriction.

ACKNOWLEDGMENTS

I thank Dr. Susan Tanner who years ago advised me to dive deeper in the literature to understand water‐related adaptations. Thanks to my whippet Athens for joining me on long runs and my daughter Maya for keeping me up at night when these ideas gelled in my head. I thank the Yale Biological Colloquium series for helpful discussions on an earlier version of this article. An earlier version of this article was presented in the Plenary on the Human Biology of Water at the Human Biology Association's annual meeting in Cleveland, Ohio on March 27, 2019.

Number of times cited according to CrossRef: 7

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# data[].attributes.attachment_link.title
Biobehavioral variation in human water needs: How adaptations, early life environments, and the life course affect body water homeostasis

# data[].attributes.attachment_link.description
ORIGINAL RESEARCH ARTICLE First published: Body water homeostasis is critical for optimal physiological and cognitive function for humans. The majority of research has illustrated the negative biological consequences of failing to meet water needs. The human body has several mechanisms for detecting, regulating, and correcting body water deficits and excesses.

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people may think irrationally without proper mental stability

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Boston low incomes resources are not available to everyone that needs it. For example many people are denied government help because of their citizenship statue or just because the person assign the case have a bias opinion about them. This is a problem because not only it affects the families economically but it affects the household lifestyle and mentally structure.

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In today's society, money is a key factor in gaining physical needs.

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Why mental needs are important for human survival?

# data[].attributes.subject
Why mental needs are important for human survival?

# data[].attributes.body
They are important because without a healthy mind the chances of surviving alone in the world decreases plus they would show physical deterioration as they fail to get out of that "state"

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We are group number five, and the pyramid has five layers. We could do a clever play on words with that.

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How are people being restricted of there basics needs?

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How are people being restricted of there basics needs?

# data[].attributes.headline
Why emotional needs are important for human survival? 

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Why emotional needs are important for human survival? 

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I believe they are important because if they lack to show emotion or compassion the person would have a high chance of suffering from mental disorders or shown to be unstable a person.

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Khatazja's Minions (?)

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- Compassion
- Love
- Socialization

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- Recreation
- Reference/Reality (Having some clear and defined rules on which one can base judgement)
- Truth
- Emotional Needs

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Centers for Disease Control and Prevention

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Mental health is important at every stage of life, from childhood and adolescence through adulthood. It is an important part of overall health and well-being and includes our emotional, psychological, and social well-being. It affects how we think, feel, and act.

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- Food / Water
-Shelter
- Access to a water source (for cleaning / bathing)
- Access to communication technology
- Enough Money to access said resources

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Basic needs are the minimum requirements a person needs to survive nowadays whether is food/ water or companionship and love.