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O nosso conhecimento sobra uma dieta saudável está baseada em conhecimentos desactualizados.
Partindo de um mercado de rua no Brasil e passando por um supermercado em Boston são reveladas algumas verdades surpreendentes.

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O nosso conhecimento sobra uma dieta saudável está baseada em conhecimentos desactualizados. Partindo de um mercado de rua no Brasil e passando por um supermercado em Boston são reveladas algumas verdades surpreendentes.

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What would happen if you didn’t drink water? - Mia Nacamulli

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What would happen if you didn’t drink water? - Mia Nacamulli

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What would happen if you didn’t drink water? - Mia Nacamulli

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PROLONGED EXERCISE IN THE HEAT– Written by Julien Périard, Qatar

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PROLONGED EXERCISE IN THE HEAT– Written by Julien Périard, Qatar

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Prolonged exercise in the heat

- Written by Julien Périard, Qatar

EXERCISE IN THE HEAT

It is well established that prolonged exercise performance is impaired when undertaken in hot climatic conditions. As exercise becomes protracted, body core temperature increases and is accompanied by an elevation in cardiovascular strain (e.g. a rise in heart rate) and perceived exertion. Performance consequently deteriorates as the ability to maintain a given power output or running velocity progressively declines.

Typically, resting internal body temperature is regulated around 37°C by thermoregulatory reflexes. The initiation of these thermolytic (cutaneous vasoconstriction and shivering) and thermogenetic (cutaneous vasodilation and sweating) reflexes allows for heat to be conserved or dissipated to the environment by way of conduction, convection, radiation and evaporation. The onset threshold and intensity of the reflexes will depend on the severity of the thermal stimulus and the thermal gradient between the skin and the environment. When exercise is performed in cool conditions the gradient is negative as the temperature of the skin is lower than that of the environment. This allows for the efficient dissipation of metabolically generated heat. As a result, body temperature rises to a safe steady state. However, heat dissipation pathways are less effective in hot environments due to a reduction in the thermal gradient, whereby skin temperature approaches or surpasses environmental temperature. The rise in body core temperature is thus exacerbated, especially if relative humidity is high and sweat evaporation is limited by a low vapour pressure gradient between the air and the skin.

Interestingly, the rise in body core temperature during exercise is directly proportional to workload and relative intensity. Given that mechanical efficiency varies from 20 to 25%, most of the metabolic energy converted to produce movement is released as heat. For example, exercise performed at 80 to 90% of maximal aerobic capacity (VO ₂ max) could increase core temperature by 1°C every 5 to 8 minutes if heat was not dissipated. As such, the rate of rise in body temperature during exercise is related to both the human body's ability to release heat to the environment and the intensity of exercise.

One of the ultimate challenges of human endurance is that of the marathon (42 km). During a 2 hour 10 minute marathon the calculated rate of heat production is approximately 1400 kcal.h⁻¹. Depending on the climatic conditions (i.e. temperature and humidity), performance may be severely impaired if the rate of heat loss does not offset the rate of heat gain. Table 1 illustrates the influence of hot and humid climatic conditions on Olympic and International Association of Athletics Federation World Championship marathon performances from 1983 to 2012. Greater decrements in performance, represented by slower winning times and greater percentages of athletes not finishing the race, occur when temperature and humidity are high at the start of the race and progressively increase (e.g. Tokyo 1991, Athens 1997, Osaka 2007). Conversely, the top 10 marathons of all time, ranging from 2:03:38 to 2:04:50, have all been run on occasions when the temperature at the start of the race was below 13°C. In fact, optimum temperature for competitive endurance performance is around 11 to 12°C. A notable exception is the performance of Kenyan runner Sammy Wanjiru at the 2008 Beijing Olympics. Despite a starting ambient temperature of 26.5°C and a relative humidity of 62%, he ran a blistering 2:06:32. This undoubtedly reflected his tremendous fitness, acclimatisation state and mental strength.

PERFORMANCE LIMITATIONS

Although field and laboratory-based studies have both demonstrated that aerobic exercise is impaired in the heat, contention remains as to the cause of the impairment. Therefore, a question within the field of environmental exercise physiology is whether performance is impaired via cardiovascular or central nervous system limitations. This question has led to the development of various experimental models attempting to reveal the mechanism(s) limiting prolonged exercise performance under heat stress. Currently, two mechanisms are proposed to explain the potential pathway by which fatigue develops. These hypotheses include:

Cardiovascular limitations impairing systemic oxygen delivery and local oxygen uptake in exercising muscles and

The attainment of a specific hyperthermic state, reducing central neural drive to exercising muscles.

Cardiovascular strain

It is well known that a rise in thermal strain (i.e. core and skin temperature) during prolonged moderate intensity exercise in the heat leads to a reflex increase in skin blood flow (Figure 1). This thermoregulatory mediated redistribution of blood towards cutaneous vascular beds increases skin temperature and narrows the core-to-skin temperature gradient.

In turn, this increases the skin blood flow requirements for heat dissipation, especially via convection. Consequently, competition for blood flow develops between thermoregulatory (skin blood flow) and metabolic (muscle blood flow) processes. During exercise at a fixed workload, this competition is manifested as a decrease in central blood volume, which is accompanied by a reduction in mean arterial pressure and a progressive increase in heart rate towards maximum (Figure 2).

The rise in heart rate decreases ventricular filling time and end-diastolic volume, ultimately reducing stroke volume and compromising the maintenance of cardiac output. When exercise is protracted and performed to exhaustion, these circulatory adjustments result in the attainment of maximum heart rate at submaximal workloads. This leads to a reduction in maximum cardiac output, and concomitantly VO ₂ max, which has been shown to decrease in proportion to the rise in thermal strain ². Thus, the large displacement of blood flow toward the skin during constant load aerobic exercise in the heat appears to play a key role in the development of cardiovascular strain and fatigue.

It was recently demonstrated that a progressive rise in cardiovascular strain was similarly associated with reductions in performance during self-paced exercise in the heat ³. During a 40 km cycling time trial in hot and cool conditions, it was noted that the development of thermal strain during the hot trial resulted in a sustained elevation in heart rate. It was also noted that mean arterial pressure, stroke volume and cardiac output decreased throughout the hot exercise time trial. This occurred despite the maintenance of a lower power output.

During the final maximal effort from 39 to 40 km it was found that peak oxygen uptake declined significantly more in the hot (18%) than in the cool condition when compared with baseline VO ₂ max measurements. The reduction was associated with an inability to increase mean arterial pressure and cardiac output, despite the attainment of maximum heart rate. These alterations in cardiovascular function contribute to the premise that exercise in the heat, whether performed at a constant rate or self-paced, is significantly influenced by circulatory adjustments.

Central nervous system alterations

The premise that central nervous system function impairs prolonged exercise performance in the heat originates from observations that fatigue consistently occurs at a specifically elevated core temperature. The attainment of this 'critical' core temperature during exercise-induced hyperthermia has been suggested to elicit a reduction in central neural drive to exercising muscles (central fatigue). It is proposed that alterations in the cerebral motor cortex act to reduce voluntary muscle activation in order to limit the accumulation of heat and prevent thermal injury.

To explain this mechanism, researchers have evaluated voluntary activation levels using electrical stimulation during sustained maximal voluntary isometric contractions. Such contractions were performed after cycling to exhaustion in the heat, as well as following submaximal steady state exercise in cool conditions for a comparable time period ⁴. It was shown that greater losses of force production occurred after exercise in the heat because of a decline in central neural drive to the active musculature. However, it has recently been demonstrated that the loss of force production capacity is not entirely attributable to central fatigue following fixed and self-paced exercise in the heat 5,6. In fact, only 20 to 40% of the additional loss of force could be linked to central factors. The remainder was related to alterations occurring at the peripheral level in the muscle. As such, the precise contribution of hyperthermia-induced central fatigue to the loss of force production capacity requires further investigation.

Interestingly, increases in core and skin temperature are associated with reductions in cerebral blood flow and cerebral oxygen delivery during moderate-intensity exercise to exhaustion in the heat. It is suggested that these reductions, along with an increase in cerebral temperature, may mediate the development of central fatigue. However, reductions in cerebral perfusion prior to exhaustion are associated with an increase in oxygen extraction, which enhances global brain metabolism and central nervous system drive ⁷. This indicates that a significant oxygen reserve is maintained by the brain at exhaustion, which may act to protect it against declines in perfusion. Furthermore, it is important to note that although exhaustion often occurs at an elevated core temperature, it also occurs in conjunction with a significant level of cardiovascular strain. A recent comparison of the thermal and cardiovascular responses of trained and untrained subjects exercising to exhaustion at moderate and high intensities in the heat demonstrated this phenomenon ⁸. It was shown that both groups fatigued earlier at the higher intensity, attaining a 0.5°C lower core temperature at exhaustion. However, the level of cardiovascular strain reached at exhaustion was similar between exercise intensities and fitness groups.

COUNTERMEASURES AND STRATEGIES TO IMPROVE PERFORMANCE IN THE HEAT

Although aerobic exercise performed in the heat is impaired relative to when it is conducted in cool conditions, various strategies can be utilised to minimise the impairment.

Aerobic fitness in itself improves the capacity to exercise in the heat and to tolerate greater levels of hyperthermia. This benefit stems from the physiological adaptations that occur in response to regular exposure to high core temperatures during exercise. Well-trained athletes have a larger plasma volume and may display better central venous pressure and cardiovascular stability under heat stress than their untrained counterparts. Trained individuals also exhibit larger net gains in body core temperature during heat-stress exercise, mostly due to their lower starting core temperature. Other proposed adaptations include a greater sensitivity in the thermal effector responses for cutaneous vasodilation and sweating. This corresponds to an earlier onset of heat dissipation, which attenuates the rise in body core temperature.

Heat acclimatisation is the process by which we develop a tolerance to the heat. When an individual is first exposed to unusually hot environmental conditions these may seem very stressful. However, following a few days the conditions will appear more tolerable. Initially, acclimatisation can be viewed as a behavioural adaptation whereby activity level is reduced and a change occurs in diet and the selection of clothing. The greatest adaptations, however, are those that occur physiologically. During acclimatisation, blood volume increases, resting core temperature decreases, as does the concentration of sodium in sweat and urine. From a sweating perspective, acclimatisation can be viewed as a training of the sweat glands to produce more sweat. Maximal sweat rate increases and a given submaximal rate of sweating is achieved at lower core and skin temperatures. As a result, core temperature and cardiovascular strain are reduced for given levels of exertion because evaporative heat loss capacity is enhanced. These adaptations can develop naturally when exposed to the elements in an outdoor environment (acclimatisation), or artificially when performing a laboratory-based passive or exercise heat-exposure regimen (acclimation).

Hydration and cardiovascular strain

During hyperthermia, cardiovascular function is greatly influenced by hydration status. In fact, gradual dehydration has been shown to exacerbate the rise in thermal and cardiovascular strain during heat-stress exercise in proportion to the extent of body water loss ². This accelerates the reductions in cardiac filling and stroke volume, impairing the maintenance of cardiac output and systemic vascular conductance. It leads to a decrease in systemic, skin and muscle blood flow. It also leads to a greater reliance on muscle glycogen as fuel, and a reduction in exercising muscle oxygen uptake at exhaustion.

Therefore, it is important that the extent of dehydration or body water loss be minimised in order to maintain performance. Currently, it is suggested to prevent per cent body mass losses beyond 2% when exercising in the heat to maintain performance. However, these recommendations are mostly based on laboratory experiments in which exercise intensity and workload are controlled. In field settings, athletes have been shown to have reached high core temperatures and lose significant amounts of body mass via sweating, while incurring minimal reductions in performance. However, the ability to perform in the heat may relate to various factors including pre-exercise hydration status, acclimatisation state, exercise intensity and aerobic fitness. Thus, it is better to err on the side of caution and undertake exercise in a well-hydrated state and limit the loss in total body water by hydrating properly during sustained exercise.

In an effort to reduce the starting core and skin temperature of athletes competing in the heat, researchers have developed various ways in which the body can be pre-cooled. This strategy is used to provide a wider range in which core temperature can rise, thus delaying the increase in cardiovascular strain. Several strategies can be employed to pre-cool athletes prior to competition including cold-water immersion, wearing ice vests, draping with cold towels and ingesting cold slurry drinks. Pre-cooling has been reviewed in a previous edition of this journal and the reader is referred to the article by Laursen ⁹ for a comprehensive and practical examination of current practices.

SUMMARY

Prolonged exercise performance is impaired in hot environments. Although heat acclimatisation and strategies such as pre-cooling can improve performance and exercise-heat tolerance, the deleterious effects of hyperthermia are likely to develop when exercise is undertaken in hot and/or humid conditions. The mechanisms potentially mediating this performance decrement remain hotly debated. At present, fatigue cannot be attributed to a single causative factor as exercise in the heat is a multi-factorial endeavour influenced by alterations in circulatory, metabolic and neuromuscular function. The interplay of psychological factors further influences performance and our understanding of fatigue by modifying behavioural responses. However, it appears that in motivated individuals, a thermoregulatory-mediated rise in cardiovascular strain may play a primary role in mediating fatigue during exercise in the heat.

1. Martin DE. Strategies for optimising marathon performance in the heat. Sports Med 2007; 37:324-327.

2. Sawka MN, Leon LR, Montain SJ, Sonna LA. Integrated physiological mechanisms of exercise performance, adaptation, and maladaptation to heat stress. Compr Physiol 2011; 1:1883-1928.

3. Périard JD, Cramer MN, Chapman PG, Caillaud C, Thompson MW. Cardiovascular strain impairs prolonged self-paced exercise in the heat. Exp Physiol 2011; 96:134-144.

4. Nybo L, Nielsen B. Hyperthermia and central fatigue during prolonged exercise in humans. J Appl Physiol 2001; 91:1055-1060.

5. Périard JD, Caillaud C, Thompson MW. Central and peripheral fatigue during passive and exercise-induced hyperthermia. Med Sci Sports Exerc 2011; 43:1657-1665.

6. Périard JD, Cramer MN, Chapman PG, Caillaud C, Thompson MW. Neuromuscular function following prolonged intense self-paced exercise in hot climatic conditions. Eur J Appl Physiol 2011; 111:1561-1569.

7. Crandall CG, González-Alonso J. Cardiovascular function in the heat stressed human. Acta Physiol 2010; 199:407-423.

8. Périard JD, Caillaud C, Thompson MW. The role of aerobic fitness and exercise intensity on endurance performance in uncompensable heat stress conditions. Eur J Appl Physiol 2011; 112:1989-1999.

9. Laursen PB. Improving football performance in the heat: Practical pre-cooling options for the practitioner. Aspetar Sports Med J 2012; 1:13-17.

10. Périard JD, Thompson MW, Caillaud C, Quaresima V. Influence of heat stress and exercise intensity on vastus lateralis muscle and prefrontal cortex oxygenation. Eur J Appl Physiol 2012; DOI:10.1007/s00421-012-2427-4.

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Aspetar Sports Medicine Journal - Prolonged exercise in the heat

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Written by Julien Périard, Qatar EXERCISE IN THE HEAT It is well established that prolonged exercise performance is impaired when undertaken in hot climatic conditions. As exercise becomes protracted, body core temperature increases and is accompanied by an elevation in cardiovascular strain (e.g. a rise in heart rate) and perceived exertion.

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Qual é o valor das vitaminas? — Ginnie Trinh Nguyen

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Qual é o valor das vitaminas? — Ginnie Trinh Nguyen

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How do vitamins work? - Ginnie Trinh Nguyen

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FLUIDS: FACTS & FADS

– Written by Louise M Burke, Australia

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Fluids: Facts & Fads

- Written by Louise M Burke, Australia

If we judge nutrients by the length of time it takes for a lack of intake to cause problems, or by the seriousness of the symptoms caused by a deficiency, then water is a clear 'Number One'. It makes up half our body mass and enables many body functions to occur. In contrast, dehydration interferes with physiological functions and when extreme or coupled with other issues it can lead to death.

Humans have evolved with systems in place to maintain remarkably good fluid balance over the long-term, with thirst being the principal driver of fluid intake and urine production being the principle regulator of fluid loss. Of course, there are exceptions to the rule in terms of populations (e.g. children and the elderly), individuals (there is a genetic predisposition to be a good or a reluctant drinker) and situations (sudden changes in fluid loss/need to which it can take several days to adjust). This review will cover the myths and furious debates between sports scientists/physicians surrounding the fluid needs of athletes.

MYTH No. 1

Over the past decades, the world has undergone some major cultural changes. Even in regions where tap water is clean and pleasant tasting, the market for bottled water is lucrative. Not only are we willing to pay above the odds for water, but many of us appear to have a water bottle welded to our hands as we go about our daily activities. This is not intrinsically good or bad, but reflects a shift from times when we drank mostly at meals and scheduled snacks. At least one of the drivers is the mantra that we should drink eight glasses of water each day for our health. It's hard to trace the origins of this 'wise-sounding' advice, but scientific scrutiny shows that it is neither evidence-based nor necessary ¹.

A major criticism of the '8 glasses rule' is that it overlooks the substantial amount of fluid provided by food or prepared dishes (milk, yoghurt, soup, fruit, vegetables etc). For some people, these sources may be perfectly adequate to meet daily fluid needs without constant need for the tap or water bottle. The best advice is: each of us should develop everyday drinking patterns that suit our level of fluid loss and be able to scale it up or down as fluid losses change. Athletes are a special group with wide-ranging fluid losses. They may benefit from special attention to address the acute needs of exercise as well as daily rehydration.

MYTH No. 2

The '8 glasses rule' is often appended with the advice that for every cup of coffee, we should add another glass of water to our day. The good news is that the bad publicity about caffeine has also recently been refuted ². Contrary to popular belief and the warnings included in in-flight magazines on airplanes, caffeine has a marginal effect as a diuretic, particularly in people who regularly consume it. In fact, tea, coffee and cola drinks contribute a substantial source of fluid in the diet of the average adult. Although there are other aspects of these caffeine-containing fluids to be enjoyed (e.g. polyphenol content, social ambience) or cautioned against (e.g. effect on sleep, accompanying sugar content), they needn't be harshly judged on the basis of hydration. It's possible that suddenly cutting people off from their habitual intake of caffeine-containing drinks might actually impair hydration rather than improve it, since many people will not replace fluids they like with an equal volume of something that doesn't appeal to them.

Why drink during exercise?

Evaporation of sweat provides the major opportunity to dissipate the body heat generated as a by-product of exercise or absorbed from a hot environment. Sweat rates during exercise vary according to factors such as the intensity of exercise, environmental conditions (heat, humidity, airflow) and the athlete's size, clothing and state of fitness/acclimatisation. Typical sweat rates during sport vary from 0.5 to 2.0 l/hour and can cause substantial losses of fluid and electrolytes. In events lasting longer than 30 minutes, there may be advantages to drinking fluids during the session to offset these losses.

Opportunities to drink during sport vary according to rules, environment and culture.

In team games and racket sports, competitors can consume fluids during breaks in play such as substitutions, half-time or changes of end.

In continuous events such as marathons, cycling races and triathlon, the athlete must drink 'on the move'.

Intake is also determined by access to fluids. Limiting characteristics include rules that forbid drinks on the arena during play (e.g. football) or require the athlete to carry their own supplies (e.g. adventure races), while supportive situations include those in which drinks can be provided by trainers/handlers or from a network of aid stations/feed zones.

Even if drinks are available, athletes may need to factor in the time lost in slowing down to obtain and consume the drink as well as the risk of gastrointestinal discomfort.

Some degree of dehydration is inevitable in most sports because of the mismatch between the athlete's sweat losses and his or her capacity to replace fluids during the event. This is particularly the case in most high-level sports where exercise intensities ensure high sweat rates and the athlete may be reluctant to sacrifice the costs of drinking. Studies across a range of sports find that athletes typically drink at a rate that is around 50 to 70% of their sweat losses. A more recent phenomenon, however, is the observation that some athletes or exercisers actually drink more than they sweat.

Fluid mismatches during exercise can be aided or impaired by the athlete's hydration status at the start of the session. Some athletes begin the workout or event with a pre-existing fluid deficit, due to chronic dehydration in a hot environment or to inadequate opportunity to restore fluid balance following an earlier bout of exercise. Some athletes may even deliberately dehydrate to 'make weight' for competition in a weigh-division sport. On the other hand, some athletes deliberately over-hydrate in the hours or days before a challenging event and may be in a positive fluid balance as they approach the starting line or start of play.

DEBATE No. 1

How well hydrated should athletes aim to be?

Guidelines for fluid intake during a session of exercise are based on the goal of avoiding a fluid deficit that could impair performance or health. However, defining this fluid deficit has become a major source of disagreement between scientists. There are many studies on the effects of different levels of fluid loss on exercise capacity (how long someone can keep exercising at a given pace or output) or performance (how well or how quickly an athlete can complete an exercise task). The outcomes vary according to characteristics of the research design including the type of exercise, the environment in which it is undertaken and whether the fluid deficit is incurred over the course of the session or was present at the start of exercise.

The majority view is that:

Impairments of aerobic and intermittent high-intensity exercise can typically be detected in both physical outcomes (power outputs, work rates etc) and mental characteristics (skills/concentration etc) when dehydration reaches ~2% of body mass.

Dehydration also increases the perception of effort of an exercise task meaning that performance might be sustained, but at a greater mental and physical cost.

Effects of dehydration seem clear when exercise is carried out in warm to hot conditions and increase with the size of the fluid deficit. Effects are smaller in cooler conditions and may be harder to detect.

It's unclear whether mild dehydration affects single efforts involving power or strength, although strength endurance is impaired.

Dehydration adds to the thermal strain of exercise and may exacerbate problems with body temperature control in hot weather.

Dehydration also increases the risk of gastrointestinal upsets during exercise, which can directly impair performance as well as interfere with the
effectiveness of hydration strategies.

Critics of this view argue that it is almost entirely based on laboratory studies which can't incorporate the cooling effects of wind and air resistance or the motivating effects of the competition atmosphere. As such, the presently available studies might overestimate the true effect of dehydration on performance in real life conditions. But a counter-argument is that the statistical interpretation of most studies is weighted against finding small but potentially important effects of dehydration on performance. Combining probability statistics with small sample sizes generally means that sports research is only capable of detecting substantial reductions or differences in performance. Such analyses may fail to recognise performance changes that would affect the outcomes of real-life sport, where events are decided by milliseconds and millimetres. Indeed, in studies where the same exercise is undertaken in a hot environment with incrementally increasing fluid deficits from 0 to 4% body mass, there is a parallel increase in thermoregulatory strain, cardiovascular drift and perception of effort. Intuitively, a similar subtle decrease in performance and/or increase in the effort required to perform is also happening, but we find it hard to draw the line in the sand where we consider it important. Some newer studies report impairment of performance in field conditions with mild fluid deficits.

DEBATE No. 2

How should we educate athletes to drink during sport?

Hydration guidelines have evolved since the 1970s thanks to increasing knowledge but also some justifiable criticisms of previous ideas and education messages. The first guidelines focussed on distance running with recommendations that were prescriptive and impractical to apply to other sports. In addition, they failed to recognise and incorporate the additional benefits of consuming carbohydrate during exercise of >1 hour duration. Although many athletes like the simplicity of prescriptive advice, even apparently simple guidelines often fail the test of scrutiny.

Early guidelines encouraged distance runners to drink 100 to 200 ml of water at aid stations provided every 2 to 3 km in a race. This might sound reasonable but, if taken literally, could mean an intake of 330 ml/hour by a slow runner through to 2 l/hour by a fast runner. Whether this would be inadequate or suitable for slower runners depends on their actual sweat rates. However, the high rate would be impossible to achieve when running at fast speeds and any runner who tried it would be likely to develop gastrointestinal discomfort and to lose a substantial amount of time at
aid stations.

Example 2: The same guideline of drinking 100 to 200 ml of fluid every 10 to 15 minutes in a football match can't be achieved because the rules prevent drinks from being taken onto the pitch during each 45 minute half.

Current guidelines by groups such as the American College of Sports Medicine ³ have tried to accommodate the different hydration characteristics of a range of sports, as well as the unique needs of each participant. They recommend that each athlete develop an individualised fluid plan based on an appreciation of his or her likely sweat rates and knowledge of opportunities to drink during the exercise session. Such personalised fluid plans should aim to keep fluid deficit below 2% of body mass, especially if the activities are undertaken in a hot environment, but also prevent over-drinking.

There is a strong criticism of these recommendations from some sports scientists on the basis that they are unnecessary or complicated. Instead, it has been argued that athletes should simply drink according to their thirst ⁴.

DEBATE No. 3

Are we doing enough to prevent over-hydration?

Observations from some sporting events over the past decade show that some individuals are overzealous with hydration tactics and drink at a rate that substantially exceeds their sweat losses and their ability to excrete fluid via urine. Excessive fluid intake, recognised by a substantial gain in body mass over the session, leads to hyponatremia: low plasma sodium concentrations. Among the risk factors are:

Being female: due to smaller body size (lower sweat rates) and willingness to adhere to advice (which may have emphasised the importance of drinking during exercise).

Being a slow competitor in an endurance or ultra-endurance race: combining a low rate of sweat loss with opportunities to drink multiple servings of the fluids provided at aid stations.

Inappropriate secretion of vasopressin: leading to a failure to excrete urine according to plasma osmolarity.

Mild hyponatremia may also occur as a result of large salt losses in individuals who excrete sweat that is salty or simply high in volume. However, overdrinking underpins the development of severe hyponatremia and ensuing encephalopathies which have caused several unfortunate and preventable deaths among athletes and military personnel. As a result, although it is recognised that it occurs in a minority of athletes especially during high-intensity sports with substantial sweat rates, new fluid guidelines are clear in their warnings against over-drinking during exercise. Nevertheless, there are claims that such guidelines are not strong enough and even that they have been hijacked by commercial interests in sports beverages ⁴.

Finding some common ground

The discourse on fluid intake during sport has become heated and emotive 4,5 and there is a need to find common ground. The opinion of this author comes from more than 30 years of working with elite athletes and a lifelong commitment to recreational sporting pursuits:

1. Thirst or ad libitum intake of fluid provides a reasonable starting point for developing a fluid plan for exercise.

2. There are several types of athletes or situations, however, where there may be benefits from building a more calculated approach than just drinking if you are thirsty.

In sports where opportunities for fluid intake are limited. The athlete may need to drink at the available opportunities early in an event (i.e. 'ahead of their thirst') to better pace total fluid intake over the session.

In sports where performance can benefit from the intakes of other nutrients that can be delivered in fluids, in particular, carbohydrate. Targets are regular tastes of carbohydrate for high-intensity events of 45 to 75 minutes, intakes of 30 to 60 g/hour for activities of >60 minutes of moderate or intermittent high-intensity exercise, and intakes of up to ~80 g/hour for ultra-endurance events of >3 to 4 hours. As long as it doesn't require an excessive fluid intake, athletes may develop a plan to consume beverages such as sports drinks to contribute to refuelling targets.

For individuals who are unable to respond appropriately to thirst or hunger. Most of us live in a food environment with continually upsized food portions which has clouded our ability to judge what a suitable amount to consume is. Upper and lower limits to fluid intake can be useful for some athletes (Figure 1).

These factors justify the benefits of developing a more sophisticated plan for intake during some sporting scenarios, particularly for competition. Such plans might include fluid, carbohydrate and other factors such as caffeine and beverage temperature. Athletes should do a periodic assessment of their hydration and fuelling practices during exercise, using a cost-benefit analysis to identify practical strategies that optimise performance with minimal risk of gastrointestinal discomfort or time lost in drinking. Updating and fine-tuning will allow the athlete to accommodate changing circumstances such as environmental conditions or the activity requirements of their sport.

Head of Sports Nutrition

Australian Institute of Sport

Contact: Louise.Burke@ausport.gov.au ; www.ausport.got.au/ais/nutrition

References

1. Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? Am J Physiol Regul Integr Comp Physiol 2002; 283:R993-1004.

2. Armstrong LE. Caffeine, body fluid-electrolyte balance, and exercise performance. Int J Sport Nutr Exerc Metab 2002; 12:189-206.

3. American College of Sports Medicine, Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc 2007; 39:377-390.

4. Noakes TD. Changes in body mass alone explain almost all of the variance in the serum sodium concentrations during prolonged exercise. Has commercial influence impeded scientific endeavour? Br J Sports Med 2011; 45:475-477.

5. Shephard RJ. Suppression of information on the prevalence and prevention of exercise-associated hyponatraemia. Br J Sports Med 2011; 45:1238-1242.

Image via Katherine Lopez

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Aspetar Sports Medicine Journal - Fluids: Facts & Fads

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Written by Louise M Burke, Australia If we judge nutrients by the length of time it takes for a lack of intake to cause problems, or by the seriousness of the symptoms caused by a deficiency, then water is a clear 'Number One'. It makes up half our body mass and enables many body functions to occur.

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HYDRATION STRATEGIES FOR FOOTBALL

– Written by Ron Maughan and Susan Shirreffs, UK

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Hydration strategies for football

- Written by Ron Maughan and Susan Shirreffs, UK

KEY MESSAGE

Maintaining adequate hydration is important for performance. Fluid intake before, during (where appropriate) and after exercise is important, especially in hot climates. When sweat losses are high, foods and drinks consumed must contain sufficient water and salt to replace these losses.

Players know that training and match play are much harder in hot weather than cool. High humidity is also a concern and endurance performance generally falls as humidity levels increase. High levels of fitness help players cope better with heat but performance is still impaired.

Many different factors contribute to the increased fatigue that is experienced when exercising in difficult environments. When players work hard, they sweat. In a typical training session or in a game on a hot day, sweat losses may reach 3 L, though losses of 1 to 2 L are more typical. With many football players undertaking two training sessions a day in pre-season, such high sweat losses must be replaced on a daily basis. Along with water, various salts, especially sodium, will also be lost. Some players may lose 10 to 15 g of salt in their sweat in a single day, while others will lose very little, even at high sweat rates. On a cold day though, some players will lose very little sweat. Every player's hydration needs are therefore different and will vary with the intensity of training and games and with the weather over the season. Just as general training and competition strategies should be tailored for individual players in accordance with their unique needs and preferences, so too should their drinking and eating choices during training and match play. Players, coaches and trainers should 'fine tune' these recommendations to identify their own winning formula.

It is important that support staff recognise that two levels of strategy are required. The first is the team strategy that sets out general principles that will apply to all players and will take account of whether a game is at home or away, an early or late kick off, and all the other factors that will affect the drinking requirements and behaviours. At the second level each player needs a personal strategy that takes account of individual needs.

HOW MUCH AND WHEN TO DRINK?

Players should limit dehydration during training and matches by drinking water or a sports drink. Obvious opportunities to drink during a match include warm-up, during the last minute preparations before kick-off and at half time. During training, the coach or manager should organise drink breaks according to the weather and intensity of the session. At the final of the 2008 Olympic football competition in Beijing, the weather was hot and humid. Because of these conditions, the rules of the competition was relaxed and the referee was allowed to referee to stop play for 2 minutes at the halfway point of each half of the game. This short break allowed players to take extra drinks. Having established the precedent, similar breaks might be useful in the extreme playing conditions that are sometimes encountered in the Gulf region.

Training allows opportunities for players to get a feel for their own individual sweat rates and fluid needs so that drinking practices can be adjusted according to their individual needs (Table 1). It is not necessary for players to drink enough to match sweat loss, but the amount of dehydration should normally be limited to a loss of less than about 2% of body weight (i.e. a reduction in body weight of 1.0 kg for 50 kg person, 1.5 kg for a 75 kg person and 2 kg for a 100 kg person).

The negative effects of dehydration on high-intensity performance are greater in warm environments, so drinking practices in these conditions should be upgraded to reduce the overall fluid deficit. This may include drinking at the side line when match play is interrupted or having extra drink breaks during training sessions. In cooler conditions it may not be necessary to drink at all.

There should never be a need to drink more than sweat loss so that weight is gained during exercise. This will not help performance and is likely to cause gut discomfort; players who feel uncomfortable will not be focused on the game. It is often stated that the best advice is simply to drink when thirsty but the evidence for this advice is not entirely convincing.

WHEN DO YOU NEED MORE THAN WATER?

Depletion of muscle glycogen stores can be an issue for football players in games played at a high tempo, especially for players in mobile positions or with a running game style. High carbohydrate strategies - fuelling up in the days and hours before the game and consuming extra carbohydrate during the match - have been shown to enhance performance in such players.

Better intake of fluid and fuel during a game may not only keep players running further and faster in the second half of a match, but it can also help to maintain skills and judgement when players would otherwise become fatigued. Games are often won and lost in the last minutes of the match and fatigued players are at increased risk of injury.

The use of commercial sports drinks with a carbohydrate content of about 4 to 8% (4 to 8 g/100 ml) allows carbohydrate and fluid needs to be met simultaneously in most events. The intake of carbohydrate that is generally associated with performance benefits is ~20 to 60 g per hour, though higher intakes may help if players can tolerate this. If higher amounts of carbohydrate are taken, it seems wise to use a drink containing a mixture of different carbohydrate sources (glucose, fructose, sucrose, maltodextrins) as the independent transporters of the different carbohydrates allow faster absorption in the small intestine.

Sodium should be included in fluids consumed during exercise lasting longer than 1 to 2 hours or by individuals during any event that stimulates high salt losses. You can recognise 'salty sweaters' by the salt rings on their clothes at the end of a hard session on a hot day. It is possible to collect sweat samples and analyse the salt content in the laboratory, but such a level of precision is not normally necessary. There is some evidence that players who lose a lot of salt may be more prone to muscle cramps. Adding a little extra salt to food and drinks and using the higher sodium version of sports drinks may reduce the risk of cramping for these players, but probably does not benefit other players. Many players may be tempted to restrict salt intake if they have a family history of hypertension, but this is not helpful for players who lose a lot of salt in their sweat.

Drinks are probably best taken cool or even cold. This generally improves palatability and also helps players feel more refreshed. Cold drinks can also provide a cooling effect: if a large volume is taken, this may be sufficient to cause a small reduction in body core temperature, helping players cope better in hot weather. Keeping drinks cool can be a logistical challenge, but the benefits probably repay the effort of organising a coolbox or similar facility to ensure that drinks remain cold throughout the game.

Caffeine is present in many commonly available drinks (tea, coffee, cola etc) and sports foods (e.g. gels, some sports drinks) and can enhance endurance during prolonged exercise. This benefit can be obtained with the relatively small doses of caffeine that are commonly consumed by people of various cultures (e.g. about 2 to 3 mg/kg bodyweight, which is the amount found in 1 to 2 cups of brewed coffee or 750 to 1500 ml of a cola beverage). There is often a concern that the diuretic action of caffeine will increase the risk of dehydration, so players are sometimes told to avoid all caffeine-containing drinks. This is probably not wise and may do more harm than good. At the doses that are effective in improving performance, the diuretic action of caffeine is small, especially in those used to consuming caffeinated drinks. The withdrawal symptoms that may follow acute withdrawal are probably more harmful.

DRINKING BEFORE TRAINING OR GAMES

Many players are in a dehydrated state when they turn up for training and even for games. There are probably many reasons for this, but failure to allow time for breakfast before morning training is probably the most common reason. Players should eat breakfast: carbohydrate will replenish the liver glycogen stores and help maintain blood glucose. Fluids will ensure good hydration. Players who report for training already dehydrated (based on measures of urine osmolality) tend to drink more during training and match play than those who arrive well-hydrated. The time and effort needed to take extra drinks will distract from the focus on training and could be avoided by ensuring an adequate fluid intake before arriving at the training ground or stadium. In very hot weather, training and games may be scheduled for early or late in the day to avoid excessive heat: this has some advantages, but players may need to make extra efforts to make sure they are well-hydrated at the start of the activity.

REHYDRATION AFTER EXERCISE

Recovery after exercise is part of the preparation for the next exercise session, and replacement of sweat losses is an essential part of this process. Both water and salts lost in sweat must be replaced. Players should aim to drink about 1.2 to 1.5 L of fluid for each kg of weight lost in training or matches. Drinks should contain sodium (the main salt lost in sweat) if no food is eaten at this time, but most meals will contain adequate amounts of salt. Sports drinks that contain electrolytes can be helpful, but many foods can also supply the salt that is needed. A little extra salt may be added to meals when sweat losses are high, but salt tablets are seldom or never necessary and should be used with caution.

CONCLUSION

It is hard to measure individual performance in football, so it is hard to produce convincing evidence that any nutrition or hydration intervention will be beneficial in a competitive situation. Nevertheless, there is sufficient evidence to warrant attention to hydration, especially when the weather is hot and even more so when the humidity is also high. Players will benefit from a personalised hydration plan but will need professional help to develop an effective strategy.

Professor of Sport and Exercise Nutrition

Susan Shirreffs B.Sc., Ph.D.

Reader in Human Physiology

Loughborough University

1. Maughan RJ. Alcohol and football. In: Maughan RJ, ed. Nutrition and football. London: Routledge 2007. p. 127-140.

2. Shirreffs SM, Maughan RJ. Water and salt balance in young male football players in training during the holy month of Ramadan. J Sports Sci 2008; 26:S47-54.

3. Kurdak SS, Shirreffs SM, Maughan RJ, Ozgünen KT, Zeren C, Korkmaz S et al. Hydration and sweating responses to hot-weather football competition. Scand J Med Sci Sport 2010; 20:133-139.

4. Grantham J, Cheung SS, Connes P, Febbraio MA, Gaoua N, González-Alonso J et al. Current knowledge on playing football in hot environments. Scand J Med Sci Sports 2010; 20:161-167.

Image via Nathan Forget

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Aspetar Sports Medicine Journal - Hydration strategies for football

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Written by Ron Maughan and Susan Shirreffs, UK KEY MESSAGE Maintaining adequate hydration is important for performance. Fluid intake before, during (where appropriate) and after exercise is important, especially in hot climates. When sweat losses are high, foods and drinks consumed must contain sufficient water and salt to replace these losses.

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Como é que a cafeína nos mantém acordados? 

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Como é que a cafeína nos mantém acordados? 

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Hanan Qasim

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How does caffeine keep us awake? - Hanan Qasim

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Emma Bryce

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This mobile device can monitor child dehydration - Wef

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This mobile device can monitor child dehydration - Wef

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A prototype mobile device shows promise for monitoring dehydration in children.

Two identical blue cuffs attach to a child's hand and foot, each with two embedded electrodes connected by a cable. Through the electrodes, a weak electrical current goes into the body and the resistance is calculated.

Walter Karlen, professor at ETH Zurich, and his team of researchers have developed the inexpensive mobile device that could be used by laypeople to more effectively treat dehydration.

After pneumonia, diarrhea and the associated dehydration of the body are the second most frequent cause of death among children under five years old-more fatal than malaria, HIV, or tuberculosis. In 2013, 1.3 million people died of dehydration, mostly children in impoverished regions, where poor hygiene and contaminated water make diarrheal infections commonplace.

"Many of these fatalities could be avoided by proper prevention and early treatment," says Karlen, who works as part of ETH Zurich's Mobile Health Systems Laboratory.

Karlen lived in South Africa for two years and carried out research at Stellenbosch University near Cape Town. He is familiar with the conditions that plague rural Africa, where the closest clinic with trained doctors is often hundreds of kilometers away and resources at village health centers are scant. Circumstances are the worst in the summer months, when conditions are ideal for the multiplication of bacteria and viruses, and the heat accelerates dehydration .

"If a doctor suspects a child is suffering from dehydration, they will check the moisture of the child's eyes and the elasticity of their skin with purely visual methods. They also examine the inside of the child's mouth to determine if the mucous membranes are dry," says Karlen. However, this is a subjective procedure that requires a lot of experience.

"We were looking for a solution that could objectively measure dehydration over longer periods of time." The result is AMBICA (Accurate Model for Bio-Composition Analysis)-a system for measuring hydration in young children.

This bioelectrical impedance analysis can be used to draw conclusions about the concentration of water in the body. A red or green LED array on the wrist indicates whether hydration is increasing or decreasing and an alarm sounds when the situation is critical-for example, when an infusion is required.

"Monitoring takes place in real time and eliminates the need for the presence of medical professionals," explains Karlen. This would allow laypeople such as parents or relatives to track the success of rehydration therapy. "This relieves the strain on medical personnel at stations, while increasing the chance of survival for dehydrated children."

AMBICA is tailored to conditions in developing countries and is intuitive to use. The cuffs are made of a light EVA plastic used in orthopedic applications, which could potentially be finished locally.

While the contacts of the electrodes need to be replaced after each use for hygienic reasons, the cable and cuffs are reusable. This is a crucial development, since cables in medical applications often have to be replaced, despite usually being one of the most expensive components in a measuring device. Karlen believes that AMBICA could be mass-produced for less than 100 Swiss francs ($103 USD).

"The small sensors in the wristbands are fairly intelligent," says Karlen. "They could be connected to an internet of things." This would let experts visualize, store, and evaluate the data, which could go to other devices via the sensor.

Big data analyses could then be used to determine exactly when cases of dehydration peak and in which regions-knowledge that would be put towards more effective organization of information and hygiene campaigns. AMBICA could also aid in long-term epidemiological studies.
Who'll make the device?
AMBICA is still just a prototype. The device exhibited at Zurich University of the Arts (ZHdK) was not yet ready for use, as the team is still looking for a sensor of the right size-the cuffs were tested using a larger, external sensor. However, the first field study is set to take place in December. Researchers will test AMBICA over the course of three to four months during the hot summer season in the Western Cape province of South Africa.

But even if the study yields positive results, it will take several years before AMBICA can be used on a wider scale. "It will be very difficult to find a manufacturer for the device, since there isn't a lucrative market for it yet," says Karlen.

This is mainly because dehydration is a rare cause of death for young children in developed western countries. But with heat waves becoming more frequent as a result of climate change, a dehydration warning system could also be a lifesaver for elderly, forgetful users in colder climates.

Nevertheless, Karlen only sees this application coming into play after the system has been established in developing countries, where the demand and benefits are the greatest.

He is currently pinning his hopes on large foundations and NGOs, which could play a key role in the funding of development costs through to the production stage as part of their humanitarian commitments. The benefits of this would be twofold: "Ideally, the devices would be produced locally, which would not only save lives, but also create desperately needed jobs."

World Economic Forum reports may be republished in accordance with our Terms of Use.

This article is published in collaboration with Futurity.

The views expressed in this article are those of the author alone and not the World Economic Forum.

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This mobile device can monitor child dehydration

# data[].attributes.attachment_link.description
A prototype mobile device shows promise for monitoring dehydration in children. Two identical blue cuffs attach to a child's hand and foot, each with two embedded electrodes connected by a cable. Through the electrodes, a weak electrical current goes into the body and the resistance is calculated.