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The Long Read: In 1972, a British scientist sounded the alarm that sugar – and not fat – was the greatest danger to our health. But his findings were ridiculed and his reputation ruined. How did the world’s top nutrition scientists get it so wrong for so long?

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We know that too much sugar is bad for our waistlines and our heart health, but now there's mounting evidence that high levels of sugar consumption can also have a negative effect on brain health -- from cognitive function to psychological wellbeing.

While sugar is nothing to be too concerned about in small quantities, most of us are simply eating too much of it. The sweet stuff -- which also goes by names like glucose, fructose, honey and corn syrup -- is found in 74 percent of packaged foods in our supermarkets. And while the Word Health Organization recommends that only 5 percent of daily caloric intake come from sugar, the typical American diet is comprised of 13 percent calories from sugar.

It's easy to see how we can get hooked on sugar. However, we should be aware of the risks that a high-sugar diet poses for brain function and mental well-being.

Here's what you need to know about how overconsumption of sugar could wreak havoc on your brain.

It creates a vicious cycle of intense cravings.

When a person consumes sugar, just like any food, it activates the tongue's taste receptors. Then, signals are sent to the brain, lighting up reward pathways and causing a surge of feel-good hormones, like dopamine, to be released. Sugar "hijacks the brain's reward pathway," neuroscientist Jordan Gaines Lewis explained. And while stimulating the brain's reward system with a piece of chocolate now and then is pleasurable and probably harmless, when the reward system is activated too much and too frequently, we start to run into problems.

"Over-activating this reward system kickstarts a series of unfortunate events -- loss of control, craving, and increased tolerance to sugar," neuroscientist Nicole Avena explained in a TED-Ed video.

In fact, research has shown that the brains of obese children actually light up differently when they taste sugar, reflecting an elevated "food reward" response. This suggests that their brain circuitry may predispose these children to a lifetime of intense sugar cravings.

It impairs memory and learning skills.

A 2012 study on rats, conducted by researchers at UCLA, found that a diet high in fructose (that's just another word for sugar) hinders learning and memory by literally slowing down the brain. The researchers found that rats who over-consumed fructose had damaged synaptic activity in the brain, meaning that communication among brain cells was impaired.

Heavy sugar intake caused the rats to develop a resistance to insulin -- a hormone that controls blood sugar levels and also regulates the function of brain cells. Insulin strengthens the synaptic connections between brain cells, helping them to communicate better and thereby form stronger memories. So when insulin levels in the brain are lowered as the result of excess sugar consumption, cognition can be impaired.

"Insulin is important in the body for controlling blood sugar, but it may play a different role in the brain," Dr. Fernando Gomez-Pinilla, the study's lead author, said in a statement. "Our study shows that a high-fructose diet harms the brain as well as the body. This is something new."

It may cause or contribute to depression and anxiety.

If you've ever experienced a sugar crash, then you know that sudden peaks and drops in blood sugar levels can cause you to experience symptoms like irritability, mood swings, brain fog and fatigue. That's because eating a sugar-laden donut or drinking a soda causes blood sugar levels to spike upon consumption and then plummet. When your blood sugar inevitably dips back down (hence the "crash"), you may find yourself feeling anxious, moody or depressed.

Sugar-rich and carb-laden foods can also mess with the neurotransmitters that help keep our moods stable. Consuming sugar stimulates the release of the mood-boosting neurotransmitter serotonin. Constantly over-activating these serotonin pathways can deplete our limited supplies of the neurotransmitter, which can contribute to symptoms of depression, according to Dr. Datis Kharrazian, functional medicine expert and author of Why Isn't My Brain Working?.

Chronically high blood sugar levels have also been linked to inflammation in the brain. And as some research has suggested, neuroinflammation may be one possible cause of depression.

Teenagers may be particularly vulnerable to the effects of sugar on mood. A recent study on adolescent mice, conducted by researchers at Emory University School of Medicine, found a diet high in sugar to contribute to depression and anxiety-like behavior.

Research has also found that people who eat a standard American diet that's high in processed foods -- which typically contain high amounts of saturated fat, sugar and salt -- are at an increased risk for developing depression, compared to those who eat a whole foods diet that's lower in sugar.

It's a risk factor for age-related cognitive decline and dementia.

A growing body of research suggests that a sugar-heavy diet could increase risk for developing Alzheimer's disease. A 2013 study found that insulin resistance and blood glucose levels -- which are hallmarks of diabetes -- are linked with a greater risk for developing neurodegenerative disorders like Alzheimer's. The research "offers more evidence that the brain is a target organ for damage by high blood sugar," endocrinologist Dr. Medha Munshi told the New York Times.

Some researchers, in fact, have even referred to Alzheimer's as "Type 3 Diabetes" -- which suggests that diet may have some role in an individual's risk for developing the disease.

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The Mental Health Effects Of Eating Too Much Sugar

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We know that too much sugar is bad for our waistlines and our heart health, but now there's mounting evidence that high levels of sugar consumption can also have a negative effect on brain health -- from cognitive function to psychological wellbeing.

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A sugar-laden diet may raise your risk of dying of heart disease even if you aren't overweight. So says a major study published in JAMA Internal Medicine.

Added sugars make up at least 10% of the calories the average American eats in a day. But about one in 10 people get a whopping one-quarter or more of their calories from added sugar.

Over the course of the 15-year study on added sugar and heart disease, participants who took in 25% or more of their daily calories as sugar were more than twice as likely to die from heart disease as those whose diets included less than 10% added sugar. Overall, the odds of dying from heart disease rose in tandem with the percentage of sugar in the diet-and that was true regardless of a person's age, sex, physical activity level, and body-mass index (a measure of weight).

Sugar-sweetened beverages such as sodas, energy drinks, and sports drinks are by far the biggest sources of added sugar in the average American's diet. They account for more than one-third of the added sugar we consume as a nation. Other important sources include cookies, cakes, pastries, and similar treats; fruit drinks; ice cream, frozen yogurt and the like; candy; and ready-to-eat cereals.

Nutritionists frown on added sugar for two reasons. One is its well-known links to weight gain and cavities. The other is that sugar delivers "empty calories" - calories unaccompanied by fiber, vitamins, minerals, and other nutrients. Too much added sugar can crowd healthier foods from a person's diet.

Could it be possible that sugar isn't the true bad guy boosting heart disease risk, but that it's the lack of heart-healthy foods like fruits and veggies? Apparently not. In this study, the researchers measured the participants' Healthy Eating Index. This shows how well their diets match up to federal dietary guidelines. Regardless of their Healthy Eating Index scores, people who ate more sugar still had higher cardiovascular mortality.

Exactly how excess sugar might harm the heart isn't clear. Earlier research has shown that drinking sugar-sweetened beverages can raise blood pressure. A high-sugar diet may also stimulate the liver to dump more harmful fats into the bloodstream. Both factors are known to boost heart disease risk.

Federal guidelines offer specific limits for the amount of salt and fat we eat. But there's no similar upper limit for added sugar. According to the American Heart Association's recommendation, women should consume less than 100 calories of added sugar per day (about 6 teaspoons) and men should consume less than 150 per day (about 9 teaspoons).

To put that in perspective, a 12-ounce can of regular soda contains about 9 teaspoons of sugar, so quaffing even one a day would put all women and most men over the daily limit.

If you're going to have something sweet, have a fruit-based dessert. That way, at least you're getting something good out of it. Of course, plain fruit with no added sugar is ideal. If you're trying to curb a soda habit, mix a little fruit juice with seltzer water as a replacement.

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Eating too much added sugar increases the risk of dying with heart disease - Harvard Health Blog

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A sugar-laden diet may raise your risk of dying of heart disease even if you aren't overweight. So says a major study published in JAMA Internal Medicine . Added sugars make up at least 10% of the calories the average American eats in a day.

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Credit: Billion Photos/Shutterstock.com

Each week, MyHealthNewsDaily asks the experts to answer questions about your health. This week, we asked nutritionists and diabetes specialists: Why is excess sugar bad for you? Here's what they said.

Dr. Zachary Bloomgarden, professor at the Mount Sinai School of Medicine in New York City:

Sugar is bad for you a because it has calories, and because if you have diabetes or a diabetes-related condition - lets say high blood fat levels - then having sugar will increase your blood sugar and your triglycerides, which is a risk factor for heart disease. (Triglycerides are a type of fat in the blood.)

If you have someone who has diabetes, their problem is that either they aren't producing insulin, or are resistant to it. Without insulin, eating sugar will increase blood sugar. But, essentially, diabetes is not just about blood sugar. It's about, blood sugar and triglycerides, and lipid levels.

Sugar is very calorie-dense. So you can easily consume a lot of sugar in soft drinks, and in all kinds of food that contain added sugar. It's not that the sugar calories are more fattening than any other calories. It's just calories are calories, and sugar packs a lot.

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Rachel Johnson, professor of nutrition at the University of Vermont in Burlington:

The bottom line is that sugar does one of two things. It either displaces more nutritious foods in your diet, which means you're screening out nutritious-dense foods, or it adds calories to your diet. So if you're adding calories on top of an already nutritious diet that puts you at risk for weight gain.

There's been a lot of research in recent years looking at the impact of added sugars - not the sugar naturally occurring in fruits and dairy products. What we know is that added sugars put you at a higher risk for a poor lipid (fats) profile: Higher triglycerides, lower HDL (the good cholesterol) and higher LDL levels (the bad cholesterol) in the blood. A poor lipid profile increases your risk for coronary heart disease.

There's also been some research that has shown that added sugars increase your risk of high blood pressure. And added sugars seem to promote inflammation, and more and more we find that inflammation is not good for the heart.

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Marisa Moore, spokesperson for the Academy of Nutrition and Dietetics

There are several problems with excess sugar. All types of sugars do promote tooth decay. The more often you're eating sugary foods, then the more often you give that bacteria in your mouth time to go to work on the sugar.

When we look at the types of foods that are high in added sugar, they often are not high in nutrition. So eating a lot of sugar is a general indication that maybe you aren't eating as healthy of a diet as you could.

The other problem is that it can be a contributor to obesity and weight gain, that is because you are replacing foods that would be nutrient-dense with empty calories.

Weight gain has been associated with high risk of heart disease, diabetes and certain types of cancer. Sugar, no matter, where it's coming from, does tend to increase our triglycerides, or fat levels in the blood, and higher triglycerides increase your risk for heart disease. Everything goes back to heart disease; it is the number one killer of Americans.

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Stephanie Dunbar, director of nutrition and medical affairs at the American Diabetes Association

There's a couple issues with sugar. The biggest thing is that there's no nutritional value. Foods that tend to have a lot of sugar added, they don't provide the satiety that you get from other more healthful foods, so people tend to consume more calories when they eat foods with more sugar in it.

The other worry is sugar-sweetened beverages. If you eat a cookie or a piece of cake, you have hormones to signal that you feel like you've had something to eat. That doesn't happen with a soda.

It doesn't mean that people can't have a treat occasionally, but most Americans are getting too many calories through sugary foods and sugar-sweetened beverages. That means you're not getting fiber, you're not getting vitamins, minerals.

What we recommend for diabetics, is if someone's going to have a small piece of cake, or a small piece of pie, then you would cut back on the other carbohydrates in the meal. If you normally have half a cup of rice, if the treat is going to be one cookie, then you have to have very little rice. The portion sizes for the most sweets are very small, but the sweets aren't. The cookies they sell in the stores, some have 400 calories. A cookie is equal to a large salad in terms of carbohydrates.

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Mary Ellen DiPaola, dietitian at the University of California San Francisco Medical Center

Glucose is the body's major fuel and is broken down from carbohydrates, a combination of sugar molecules, in the foods we eat. Simple sugars such as sucrose (table sugar) and juice (fructose) are composed of only one or two sugar molecules and are converted to blood glucose faster than more complex carbohydrates like whole grains and vegetables.

Carbohydrates come from all food groups except proteins and fats. Fruits, vegetables, starchy foods, milk and yogurt are all sources of carbohydrates. Sweet foods and beverages such as baked goods, frozen desserts, candy, juice, soft drinks and other sweet liquids also contain carbohydrates in the form of simple sugar. Table sugar, honey, molasses, syrup, jelly/jam, and any other sweet additions are also carbohydrates termed as "simple."

All these very tasty and fun foods and beverages can displace more nutritious complex carbohydrates, which provide vitamins, minerals, phytochemicals, and fiber - a multitude of power-packed nutrition. Simple sugar is also correlated with tooth caries (cavities), poor energy levels, and can lead to sugar cravings as the body never becomes fully satisfied from healthful foods.

Eating naturally sweet fruits and dairy or dairy substitutes (i.e. soy milk) can also satisfy sweet cravings as well as take the place of simple sugar treats.

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Each week, MyHealthNewsDaily asks the experts to answer questions about your health. This week, we asked nutritionists and diabetes specialists: Why is excess sugar bad for you? Here's what they said.

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That it makes us fat is something we take for granted. That it might also be making us sick is harder to accept.

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The rise of obesity is usually blamed on too much eating and not enough exercising, but Robert Lustig, MD, a UCSF pediatric neuroendocrinologist, asks us to look beyond the obvious. Yes, more Americans are overweight today than 30 years ago. Kids are still getting heavier, compared with prior generations of kids. That leads some UCSF researchers to warn that heart disease and other health problems will grow in future decades. But behaviors that some might refer to as gluttony and sloth are merely consequences of the true cause of the epidemic, Lustig says. Food was just as abundant before obesity's ascendance. The problem is the increase in sugar consumption. Sugar both drives fat storage and makes the brain think it is hungry, setting up a "vicious cycle," according to Lustig.

Robert Lustig, MD

More specifically, it is fructose that is harmful, according to Lustig. Fructose is a component of the two most popular sugars. One is table sugar — sucrose. The other is high-fructose corn syrup. High-fructose corn syrup has become ubiquitous in soft drinks and many other processed foods. Lustig presented his case against fructose in a recent UCSF Mini Medical School course on diet and nutrition, part of a series sponsored by the Osher Lifelong Learning Institute. Audience members may have been surprised to hear such unequivocally strong statements from a researcher. Lustig framed the obesity epidemic as a societal issue that pits the food-selling agenda of federal agencies and profit-seeking behavior of major corporations against public health needs. Lustig quit working in the lab a decade ago. Now he spends more time with pediatric patients. He is on the front lines of the world's weight woes, treating kids who already are obese, a condition that sets the stage for health problems that begin long before these children become adults. Lustig still conducts clinical research. He evaluates dietary lifestyle, as well as pharmacologic interventions that might hold the pounds at bay. He tracks down associations between diet, lifestyle and health outcomes in an effort to identify biological mechanisms that will explain them.

Insulin and Leptin

Lustig's own groundbreaking studies more than a decade ago stimulated the development of his controversial ideas about metabolism and biological feedback in weight control. One not-yet-popular idea is that, calorie for calorie, sugar causes more insulin resistance in the liver than other edibles. The pancreas then has to release more insulin to satisfy the liver's needs. High insulin levels, in turn, interfere with the brain's receipt of signals from a hormone called leptin, secreted by fat cells, Lustig believes. In the 1990s, Lustig worked with children diagnosed with hypothalamic obesity, a disorder that can occur after brain tumor surgery. The children were making more insulin than was necessary for normal energy storage in fat cells. Lustig thought the kids were not receiving signals from leptin, which helps send a message that the appetite has been sated. Lustig concluded that the children's brains were fooled into thinking that they were starving. Lustig administered a drug called octreotide, known to block insulin release. Insulin levels fell; the children ate less, lost weight, spontaneously became more active and improved their quality of life. Lustig tried the same treatment with obese adults, and found that a subset responded in the same way as the children with hypothalamic obesity. Eating stimulates secretion of insulin and leptin. The conventional view holds that insulin, like leptin, feeds back in the brain to limit food intake, Lustig explains. But Lustig does not think that chronically elevated insulin levels feed back negatively to curb eating. Instead, chronically elevated insulin blocks leptin's negative feedback signal, Lustig believes. "Most people think insulin does the same thing as leptin," he says. "I think it does just the opposite." Lustig believes that fructose generates greater insulin resistance than other foodstuffs, and that fructose calories, therefore, fail to blunt appetite in the same way as other foods.

A Calorie Is Not Just a Calorie

Lustig also is at odds with mainstream scientific viewpoints when it comes to explaining how fructose is shunted through biochemical pathways and converted into fat and other molecules. Unlike conventional calorie counters, Lustig does not believe all food calories have the same impact on fat storage and energy expenditure, regardless of whether they come from fat, protein or carbohydrate. Fructose, a type of carbohydrate, is not metabolized like other foodstuffs, and not even like glucose, the other major carbohydrate, Lustig says. In addition, Lustig claims that fructose is just as bad as alcohol in causing fat storage in the liver — and in causing fatty liver disease. Lustig advances these controversial ideas primarily by citing already published studies, most of them by other researchers. But he also tries to enlist bench scientists in research collaborations in the hopes that additional studies will prove to others that these ideas are correct.

Sugar No Better Than Fat

Each sucrose molecule consists of one molecule of fructose joined to one molecule of glucose. In the gut, these two components are quickly split apart. High-fructose corn syrup is a less expensive mixture of glucose and fructose. There is no point in belaboring the difference, Lustig says. "High-fructose corn syrup and sucrose are exactly the same," Lustig says. "They're equally bad. They're both poison in high doses." Over the past century, Americans have increased their fructose consumption from 15 grams per day to 75 grams per day or more, Lustig explains. The trend accelerated beginning about three decades ago, when cheap, easy-to-transport high-fructose corn syrup became widely available. Much of processed food labeled "reduced fat" instead has sugar added to make it more palatable, Lustig says. But when it comes to harmful health effects, sugar is worse than fat, he claims. Consumption of either results in elevated levels of artery-clogging fats being made by the liver and deposited in the bloodstream. But fructose causes even further damage to the liver and to structural proteins of the body while fomenting excessive caloric consumption, Lustig says.

Four Simple Guidelines

Lustig prescribes four simple guidelines for parents coping with kids who are too heavy:

Get rid of every sugared liquid in the house. Kids should drink only water and milk.

Provide carbohydrates associated with fiber.

Wait 20 minutes before serving second portions.

Have kids buy their "screen time" minute-for-minute with physical activity.

Fructose is abundant in fruit. Fruit is fine, Lustig says, but we should think twice before drinking juice or feeding it to our kids. The fiber in whole fruit contributes to a sense of fullness. Lustig says it is rare to see a child eat more than one orange, but it is common for kids to consume much more sugar and calories as orange juice. Eating fiber also results in less carbohydrate being absorbed in the gut, Lustig notes. In addition, he says, fiber consumption allows the brain to receive a satiety signal sooner than it would otherwise, so we stop eating sooner. Exercise burns only a modest amount of calories, Lustig notes. But it does have other benefits. Exercise improves insulin sensitivity in skeletal muscle, lowering insulin levels in the bloodstream. Exercise reduces stress and, therefore, reduces stress-induced eating, according to Lustig. Lastly, exercise increases metabolic rate. The directive to balance active play with computer, video and TV time is the most difficult one to comply with, Lustig says. But failure to limit sugar intake appears to be the most predictive of poor weight control in children, he adds. "You are not what you eat; you are what you do with what you eat," Lustig concludes. "And what you do with fructose is particularly dangerous."
Related Links:

UCSF Mini Medical School for the Public
UCTV

Childhood Obesity: Adrift in the "Limbic Triangle"
Michelle L. Mietus-Snyder and Robert H. Lustig
Annual Review of Medicine 59:147-162 (February 2008)

Childhood Obesity: Behavioral Aberration or Biochemical Drive? Reinterpreting the First Law of Thermodynamics
Robert H. Lustig
Nature Clinical Practice Endocrinology & Metabolism 2(8):447-458 (2006)

Adolescent Overweight and Future Adult Coronary Heart Disease
Kirsten Bibbins-Domingo, Pamela Coxson, Mark T. Pletcher, James Lightwood and Lee Goldman
New England Journal of Medicine, 357(23):2371-2379 (Dec. 6, 2007)

Overweight Adolescents Projected to Have More Heart Disease in Young Adulthood
UCSF News Release, Dec. 5, 2007

Prevalence of Overweight and Obesity in the United States, 1999-2004
Cynthia L. Ogden, Margaret D. Carroll, Lester R. Curtin, Margaret A. McDowell, Carolyn J. Tabak and Katherine M. Flegal
JAMA, 295(13):1549-1555 (April 5, 2006)

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Robert H. Lustig Annual Review of Medicine

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First Law of Thermodynamics Robert H. Lustig Nature Clinical Practice Endocrinology & Metabolism

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Lee Goldman New England Journal of Medicine

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Sugar Is a Poison, Says UCSF Obesity Expert

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The rise of obesity is usually blamed on too much eating and not enough exercising, but Robert Lustig, MD, a UCSF pediatric neuroendocrinologist, asks us to look beyond the obvious. Yes, more Americans are overweight today than 30 years ago. Kids are still getting heavier, compared with prior generations of kids.