Sugar & A Healthy Diet: A Sweet or Not So Sweet Story

There’s no question: Americans are sweet on sugar, consuming 94 grams (376 calories worth) per day on average, according to U.S. Department of Agriculture estimates. What is under debate is whether our sugar habits are harming our health.

Statistics show a clear correlation between increased sugar consumption and an exponential rise in conditions such as diabetes, obesity, heart disease, fatty liver disease, dementia, and even depression and mood issues. But correlation doesn’t prove that sugar is causing these diseases, and some experts contend that it’s simply the extra calories we’re consuming that are the culprit, not sugar itself.

“The problem is total calories,” says Julie Miller Jones, L.N., C.N.S., professor emeritus of nutrition in St. Catherine University’s Department of Family, Consumer and Nutritional Sciences. Citing a systematic review published in 2016 in the British Medical Bulletin, Jones claims that sugar has “no unique diabetogenic effect at physiological levels.”

Not so, argue other experts, such as Gary Taubes, author of The Case Against Sugar (Anchor, 2017). In it he writes: “…sugars like sucrose and high-fructose corn syrup are fundamental causes of diabetes and obesity.…It’s not because we eat too much of these sugars…but because they have unique physiological, metabolic and endocrinological (i.e., hormonal) effects in the human body that directly trigger these disorders.”

So, who’s right? In short, it’s complicated. But here, we tap researchers and mine the scientific data to help you decide if sugar can be part of a healthy diet—and if so, how much.

A Brief History

From the late 1800s to the 1960s and early ’70s, sugar was a prime suspect for the rise of diabetes, heart disease and other maladies associated with a Western lifestyle. By the late ’70s, however, research out of Harvard University and elsewhere had implicated fat as the primary culprit. Manufacturers responded with an increase in highly processed, high-sugar foods, causing per person sugar consumption to grow from 87 daily grams in 1970 to a peak of 111 grams in 1999.

But not everyone was convinced of sugar’s innocence, and as studies continued to examine its effect on our bodies, the finger began to point back at sugar—so much so that the current version of the Dietary Guidelines for Americans, updated every five years, is the first to advocate limiting how much sugar we eat. Its recommendation: Added sugars (those not occurring naturally in a particular food) should make up no more than 10 percent of the calories we consume (see “In Real Terms” for what that looks like in daily life). The American Heart Association pushes for even tighter limits: closer to 5 percent of daily calories (100 calories for women; 150 for men).

What Is Sugar?

Part of the rub in determining how sugar affects us is that there’s no simple definition of sugar. We use it as a catchall term to describe a variety of chemical compounds, primarily with the ending “ose”: glucose, fructose, sucrose, lactose (see “The Many Forms of Sugar” for a rundown of these terms and more). The sugar in most foods is a combination of two or more of these forms. A medium-sized pear, for example, contains 4.9 grams of glucose, 11.1 grams of fructose and 1.4 grams of sucrose (a combo of glucose and fructose).

Our bodies metabolize these various forms of sugar in different ways. For example, glucose and fructose are chemically identical, but they’re shaped differently enough that our bodies metabolize them differently, says Kimber Stanhope, Ph.D., R.D., of University of California Davis, a renowned researcher in the field of sugar science. In short, Stanhope says, the liver uses glucose as it needs it; otherwise it’s distributed to other cells throughout the body that can use it for energy. With fructose, however, the liver takes it in whether it needs it or not. When it gets overloaded with fructose, it turns the fructose into fat, leading to increased triglyceride and cholesterol levels in the blood and increased levels of fat in the liver—all of which are metabolic risk factors associated with insulin resistance, diabetes and heart disease, she explains.

When Sugar Goes Rogue

When we encounter sugar in its natural form—say, in fruit—the fiber (and potentially other nutrients) in the food does a lot to slow down and offset the liver’s response to fructose, Stanhope says.

In practical terms, this means that for most healthy people—those without digestive issues or the metabolic risk factors mentioned above—eating fruit, at least in moderation, is much less likely to increase disease risk factors than sugar that has been separated from its food source and refined, namely table sugar (also called sucrose) and high-fructose corn syrup.

In addition, it’s hard to eat enough fruit to match the level of processed sugar people consume in beverages and other sweets, Stanhope says. “When we asked people to consume 25 percent of their energy in fruit in one day, most couldn’t do it. The ones who did said it hurt to do so,” she explains. “But no one had any problem drinking 25 percent of their calorie requirement as a sugar-sweetened beverage.” 

Even refined starches (white bread, rice, pasta) don’t seem to be as detrimental, at least in terms of metabolic disease. Stanhope points to a study published in 2015 in The Journal of Clinical Endocrinology & Metabolism that compared fructose-beverage consumption with the same number of calories eaten as refined, white, starchy foods like bread and rice. Metabolic-disease markers rose when subjects consumed the fructose drinks compared with when they consumed the starch diet. “Compared to refined pasta, white bread and white rice, sugar-sweetened beverages are so much worse,” Stanhope says. “There’s no doubt about that.”

So how bad are processed sugars? In another study, published in 2015 in the American Journal of Clinical Nutrition, Stanhope found a very strong dose response effect of sugar, meaning that the more high-fructose corn syrup in the beverage provided to the subjects, the bigger the jump in their disease risk factors. To her astonishment, even the group that received just 10 percent of their daily energy requirement as a beverage sweetened with high-fructose corn syrup had increases in metabolic disease risk factors compared with baseline levels in just two weeks. “This was in young, healthy subjects drinking the equivalent of one and a half cans of soda per day for 14 days,” Stanhope says. “I was floored—very, very surprised that risk factors would increase that quickly.”

Fortunately, lowering sugar consumption may reverse metabolic-disease markers just as quickly. A 2016 study in the journal Obesity reported that obese adolescents with metabolic syndrome who reduced sugar from 28 percent of their calories to 10 percent, substituting those calories with starch, saw improvements in blood pressure; liver fat; triglyceride, insulin and LDL-cholesterol levels; and glucose tolerance—in just nine days, whether or not they lost weight.

What We Don’t Know

These studies are just a few of many that suggest processed sugar is a—if not the—major factor in Western-lifestyle diseases like diabetes and heart disease. In fact, Stanhope contends that the totality of the evidence—meaning all the various studies that exist—indicates that sugar, at least in the form of sugar-sweetened beverages, promotes obesity, diabetes and cardiovascular disease. The effect of sugar in solid food, like cookies or ice cream, is much harder to measure, she says, because it’s difficult to isolate its effects from other components of the food, such as fat.

Stanhope concedes that many experts still do not agree that sugar has negative health effects beyond being a potential source of too many calories that can lead to weight gain. This may be partly because the “causal evidence” from human dietary-intervention studies relies on increases in risk factors such as insulin resistance and blood triglyceride and cholesterol levels rather than direct evidence that sugar consumption causes heart disease—largely because it’s unethical to conduct an intervention study with the intent to see if the provided diet will cause heart disease. However, Stanhope says, it’s notoriously hard and expensive to measure insulin resistance, the major risk factor for diabetes, which means many studies rely on a less expensive—and much less reliable—measurement process, leading to inconclusive results (like the ones Miller Jones noted)—hence the difficulty of pinning the rise in diabetes on sugar consumption.

Consequently, we can’t say for sure if modest sugar consumption can be part of a healthy diet. But Stanhope points to a population study in JAMA Internal Medicine indicating that people who consumed an average of 11 percent of their daily calories as added sugar—just a smidge higher than the government’s “10 percent of calories” recommendation—had an 8 percent greater risk of dying of cardiovascular disease over a 15-year period than people who got about 7 percent of their daily calories from added sugar.

“How low do we really need to go to ensure we are not increasing our risk of disease?” Stanhope asks. “I would like to know that answer, but I don’t know where the money’s going to come from for us to be able to find out.”

The Takeaway

The science may be incomplete, but enough evidence exists to confirm the wisdom of restricting calories from added sugar to 10 percent or fewer of total calories, as the 2015–2020 Dietary Guidelines for Americans recommends.

Doing so, however, may be a challenge—partly because sugar shows strong signs of being addictive, says Nicole Avena, Ph.D., a research neuroscientist in the fields of nutrition and addiction. “Whether the studies are done on lab rats or people, they all point back to the fact that if we let them have lots and lots of sugar, they end up showing signs of addiction and showing changes in the brain that are similar to what you’d see with an addiction,” she says. “The physical withdrawal patterns aren’t the same as you would see with, say, an opiate, but that’s true for nicotine as well, and there’s no question that it’s addictive.”