Does Sugar Cause Obesity?

From the lecture series: The Skeptic's Guide to Health, Medicine, and the Media

By Roy Benaroch, M.D., Emory University

In the 1980s and 1990s, low-fat diets were all the rage. Now the media’s tune has changed, though, and sugar is the new culprit in the obesity epidemic. But does sugar cause obesity? As with fat, the answer is more complicated than it appears to be on the surface.

Word Obesity written in and with sugar with capital letters
On the most basic level, sugar adds empty calories to your diet, leading to an energy excess.

How Sugar Became the New Fat in the Fight against Obesity

On the most basic level, sugar adds empty calories to your diet, leading to an energy excess. But the harmful impact of sugar extends beyond extra calories.

Ingested carbohydrates drive up insulin levels, which forces glucose—that’s the sugar in your blood—to enter cells. Your blood sugar then drops, and that can cause hunger and lead to additional eating.

Endocrinologist Robert Lustig, in his influential bestseller Fat Chance and the movie Fed Up, uses almost-apocalyptic language, calling sugar evil, toxic, and poisonous. And it’s certainly true that increased sugar consumption is tied with more obesity, and more complications of obesity, like Type 2 diabetes.

In the media, what’s happened to sugar lately is kind of a rehash of what happened to fat 30 years ago. From The Guardian comes this headline, “Sugar, Not Fat, Exposed as Deadly Villain in Obesity Epidemic.”

That’s some strong language—“deadly villain”—and it’s a kind of silly oversimplification of what we know about calories, metabolism, and weight gain.

This is a transcript from the video series The Skeptic’s Guide to Health, Medicine, and the Media. Watch it now, on The Great Courses Plus.

Or, from The New York Times, a more specific headline about one kind of sugar that’s become a big media focus, “Fructose May Increase Cravings for High-Calorie Foods.” Let’s look a little closer at where that New York Times headline comes from.

The article is about a study published in the Proceedings of the National Academy of Sciences—that’s a good, reputable journal—that found that people who consume fructose were more likely than people who consumed glucose to crave high-calorie foods.

True, that’s what the study showed. But we need some context here to understand what that means.

Learn more about the media focus on weight loss

Is There “Good” Sugar and “Bad” Sugar?

The main dietary sugars—what we find in both natural foods, fruits and vegetables, but also in processed foods—are table sugar, also called sucrose, and high fructose corn syrup.

There’s also honey, but we don’t eat a whole lot of that. And by the way, honey is essentially a combination of sucrose and fructose.

Sucrose, that’s ordinary table sugar, is broken down by the body into the simpler smaller sugars glucose and fructose to be absorbed. And high fructose corn syrup also contains a mix of glucose and fructose.

Structural chemical formula and model of sucrose. White and brown sugar cubes in bowls
Sucrose is broken down by the body into the simpler smaller sugars glucose and fructose.

In fact, the proportions are similar to the proportions of glucose and fructose found in table sugar.

The study that The New York Times article reviewed involved 24 healthy adult volunteers given a sweet drink made with either pure fructose or pure glucose, and did find that those consuming fructose had more high-calorie food cravings afterwards.

But in the real world, we almost never consume pure glucose nor pure fructose. Ordinary foods, whether containing added table sugar, high fructose corn syrup, or the natural sugars found in foods, contain a blend of glucose and fructose-containing sugars.

The New York Times story didn’t explain any of this. The study is interesting as a demonstration of physiology, but in practice, it’s not useful. Whichever kinds of food we eat, we’re consuming both fructose and glucose.

In fact, the article might be misleading in two important ways. The headline, “Fructose May Increase Cravings for High-Calorie Foods” might lead one to try to avoid fruit, which is rich in fructose.

But that would be a mistake. Though one kind of sugar in fruit is fructose, there’s still far less total sugar in fruit than there is in soda or other processed foods.

The net calories from fruit are relatively low, and fruit is a healthful food for other reasons, too. The article does mention, down near the bottom, in an interview with one of the investigators, that people shouldn’t avoid fruit. But the headline is all about the downside of the fruit sugar, fructose.

Articles like this also mislead by implying that there’s something especially bad about high fructose corn syrup. Though it’s called high fructose, corn syrup is actually a mix of sugars, and the most commonly used version contains about the same amount of fructose as good old ordinary table sugar.

The bottom line from obesity and heart-health research now is that all simple sugars, especially those added to processed foods, are problematic. They all provide too many calories, and they can all contribute to that insulin surge that can lead to the development of diabetes.

Ironically, by the way, fructose, the main sugar in fruit, causes less insulin release than ingested glucose. That’s probably good.

But it also stimulates more fat production in the liver, and seems less effective than glucose in controlling other hormones that regulate hunger and satiety.

It is complicated, that’s for sure, but remember that almost all foods actually contain a blend of sugars, and too much of any of these will not help you reach or maintain a healthy weight.

Learn more about food, diet, and health

What Actually Causes People to Gain Weight?

Both medicine and the media portray obesity in two different ways: either it’s the individual’s fault for eating too much or it’s a hormonal disorder related to metabolism, genetic influences, and things that change our so-called microbiome, the bacteria that live in our guts to help us digest food.

In the second theory, weight is not about individual choices and decisions, but about these and other environmental influences over which we have little control.

For now, we don’t have a definite answer as to which theory is more accurate. The data is in many ways contradictory.

On the one hand, approaches based on only reducing calorie intake have been tried for decades now, in all sorts of variations, and in general they don’t seem to work. But there’s no denying the simplicity of the energy balance equation, and how net calories consumed from all sources is still an important predictor of weight change.

Cropped portrait of overweight woman choosing healthy food refusing chocolate bar
Diet approaches based on only reducing calorie intake don’t seem to work.

On the other hand, there are measurable, objective, and unhealthful metabolic consequences to the over consumption of sugar, even though, so far, low-carbohydrate diets have not been reliably shown to result in better weight loss than other calorie-restricted diets.

That’s not only been shown in studies of individuals following different meal plans, but even on a population level. In Australia, sugar consumption has dropped precipitously, even as rates of obesity have continued to increase.

Maybe the real enemy isn’t sugar or fat, and it’s not the overweight people depicted as eating too much in news stories. Strategies that demonize particular foods or particular people aren’t helping, and media stories that hype breakthrough medications and the newest way to boost metabolism aren’t helping either.

Learn More: The Media’s Role in Improving Health

What might help would be more media attention to a few neglected topics. Instead of spilling ink and clicks on preliminary studies on mice, there should be more attention on the need for high-quality, long-term studies in human beings.

There should be less attention to the latest superfood or diet fad, and more attention to simple ways of eating more healthfully: smaller portions; family meals; eating slower; drinking water; and making home-cooked food with more fruits, vegetables, and whole grains.

Staying active, too, is crucial. In fact, good studies have shown that even if increased exercise doesn’t lead to weight loss, it improves health in many other ways, including reducing complications like diabetes and heart disease.

Do diet and exercise changes work? Yes, but they may not be easy.

Common Questions About Sugar and Obesity

Q: Can eating sugar result in obesity?

Yes. Eating sugar can lead to obesity as sugar turns to fat in the body and makes you crave more sugar, leading to overeating.

Q: Is sugar worse for a diet than fat?

Sugar is what makes people gain weight. Conversely, quality fat from nuts and vegetables is healthy and necessary for survival.

Q: Where does sugar make you fat?

Too much sugar primarily leads to fat in the belly and liver.

Q: Does sugar act as a fat or a carbohydrate?

Carbohydrates are made up of different types of sugars but result in the creation of fat.

This article was updated on 8/29/2019

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