It’s become popular to think of foods as either good or bad, something to eat or something to avoid. Carbohydrates, which had their moment as a good food back when fat was the bad guy, are now being blamed in part for the epidemic of obesity, diabetes and heart disease. And a slew of diet books proposes that you will feel better and be healthier if you never eat bread, pasta or sugar again.
But are carbs really so bad?
Science makes the answer pretty clear: no. While bread, pasta and sugar are hard-to-resist sources of calories without much in the way of nutrition, other carbohydrate-heavy foods — whole grains, legumes and fruit — are nutrient-rich. Carbohydrates can play a healthful role in your diet or they can be your undoing, depending on which, and how many, you eat.
The biggest beef against carbs is that it’s easy to eat too much of them, which is a problem because it can lead to weight gain and because they can crowd out more-nutritious foods. There’s also speculation that the way our bodies digest sugar and certain processed grains such as those found in white bread and white rice makes us hungry again soon after eating.
“Carbs aren’t the enemy,” says Julie Jones, a professor emeritus of food and nutrition at St. Catherine University in St. Paul, Minn. who is also a scientific advisor for the Grain Foods Foundation, a baking and milling industry funded group which promotes grain-based food as part of a healthy diet. “Overconsumption, of anything, is the enemy.”
Even so, the good-or-bad notion gets traction. “It’s easier for a lot of people to cut off whole categories of food than to eat moderately,” says Marion Nestle, a professor in New York University’s department of nutrition, food studies and public health. And a lot of people report that they feel better and lose weight when they cut out sugar and refined carbohydrates, she says. Yet there’s no reason, she adds, that bread, pasta and plain old sugar should be completely off-limits, as some popular diets recommend. In moderation, they’ll do you no harm.
To help navigate the world of carbs the foods, it’s helpful to spend a little time with carbs the molecules.
Carbohydrates run the gamut from very simple molecules that your body breaks down easily to very complex molecules that your body breaks down more slowly, or not at all. Since carbohydrates that you eat are mainly converted to glucose, the sugar that every cell of your body can use for energy, the faster the carbohydrate is digested, the quicker it’s turned into blood sugar.
There are questions about possible negative health effects of some carbs, such as fructose, which is found in sugar and high-fructose corn syrup, and galactose, which is found in milk. But the question of how carbs affect health is mostly focused on how quickly and efficiently the body can break the molecule down and deliver glucose to the bloodstream.
But you don’t eat carbohydrates, you eat food, so it’s useful to categorize foods by the type of carbohydrates that predominate.
Simple-carb foods are those that your body breaks down quickly and easily, such as sweeteners (sugar, honey, maple syrup) and refined grains (white flour, pasta, white rice). These are the carbs that tend to get the bad rap because they cause spikes in blood sugar. Complex-carb foods, which include whole grains and legumes, have large, complex molecules that are more difficult to digest and consequently don’t cause the same rapid increase in blood sugar.
The simple/complex classification isn’t perfect. Many fruits and vegetables contain both types of carbohydrates: Some get broken down quickly, others more slowly. And it’s not always true that whole foods are digested slowly while refined foods are digested quickly. Potatoes, for example, have lots of carbohydrates in the form of starch, which is broken down quickly.
Let’s look at some of the simple carbohydrates, starting with sugar.
In the complex world of food, it’s refreshing to find an idea on which there is universal agreement: Everyone thinks it’s important to limit sugar consumption. There is, however, a range of opinion on just how bad sugar is.
Some doctors and scientists believe that the problem with sugar is that it’s empty calories — tasty empty calories that go down very easily, particularly in sweetened drinks. Others believe that the ease with which our bodies turn sugar in soda into sugar in our bloodstream messes with our metabolism in a way that disposes us to overeat.
Because the carbohydrates in refined grains — bread, white rice, pasta — come packaged with some fiber, some protein and even a few other nutrients, their calories aren’t quite as empty, and the speed with which they’re digested varies. (Refined flour is also fortified with folate, essential to reducing the risk of fetal neural tube defects.)
White bread, for example, lets loose a flood of glucose, so your blood sugar spikes, but pasta, particularly if it’s not overcooked, doesn’t have that effect. Although the ingredients of the two foods are almost identical, pasta has a difficult molecular structure that your body can’t break down as quickly.
There is a measure for how much a particular food increases your blood sugar: the glycemic index, or GI. When carbohydrates in a food get converted quickly, that causes a spike in insulin, which your pancreas releases to prompt cells to absorb the glucose. The hormones that your body releases in response can make you feel hungry. The higher the GI, the higher the blood sugar level. If you eat high-GI foods often, the repeated stressing of your insulin-producing machinery may have other effects, such as increasing your risk for diabetes.
There is disagreement about the importance of the glycemic index. While some scientists believe it’s an essential measure of diet quality and while many diets have been designed around it, Nestle isn’t sold. “I’m not a great believer in its importance,” she says, and points out that the GI measures foods eaten alone, and what you eat with your carbs affects subsequent blood sugar levels.
“People don’t usually eat those things without anything else,” Nestle points out. “They put butter on their bread. They put cheese on their pasta.” Both have fat, she explains, and fat slows down the glucose-delivery mechanism which is why the glycemic index of bread with butter is lower than that of bread alone.
Glycemic index response is also affected by how the food was cooked (not only the method but how long it was cooked), how thoroughly you chew and other factors, says Susan Roberts, director of the energy metabolism laboratory at Tufts University in Boston. A person can have a different response to the same food from one day to another.
A study published in December in the Journal of the American Medical Association compared high- and low-GI diets and found that people lost the same amount of weight on both. The researchers found little difference in cholesterol, triglyceride levels and insulin resistance (a condition in which the body doesn’t use insulin efficiently), and concluded that “using glycemic index to select specific foods may not improve cardiovascular risk factors.”
Roberts, while acknowledging the many factors that affect the GI of food, does pay attention to it. “If you look at the epidemiological [population-based] studies, in every study I’ve seen, the higher the GI, the greater the risk of Type 2 diabetes, cardiovascular disease and breast cancer.”
Why is it that when researchers look at the diet of the population as a whole, they find that a low-GI diet has benefits, but when they bring people into the lab and feed them low- and high-GI diets, they don’t find those advantages?
It could be because low-GI foods tend to be healthful for reasons other than their GI values — they’re nutrient- or fiber-rich — while high-GI foods tend to be unhealthful. In general, whole grains, legumes and vegetables have lower — i.e., better — GI scores than refined-grain breads, baked goods and sugary drinks. So “high-GI” may be a marker for an unhealthful diet. Experimental diets, though, don’t include lots of junk; instead, they use the most healthful of the high-GI foods, because the point is to change as little as possible about the diet to get at the effect of only the glycemic index.
If you eat a lot of junk food, your diet is definitely high-GI. As Julie Jones says, “We don’t need any kind of index to tell us we shouldn’t eat Doodles, Ding-Dongs and doughnuts.”
Roberts says that it could be the higher nutrient levels of low-GI foods, and not the glycemic response, that’s responsible for the lowered disease risk. Luc Tappy of Switzerland’s University of Lausanne, who chairs the committee revising carbohydrate recommendations for France, says we don’t have conclusive evidence of the glycemic index’s importance. He calls it an “open question.”
When questions are open, it’s often hard to know what to eat. But everyone agrees that limiting sugar is important, and Jones points to the U.S. government’s Dietary Guidelines for Americans as a simple rule for other carb-heavy foods: Make half your grains whole.
Clarification: Julie Jones is also a scientific advisor. This story has been updated.
Tamar Haspel writes about food and science. Follow her on Twitter: @TamarHaspel.