'Good Carbs' Yield Newfound Benefits
Today's debates over the pros and cons of low-carb diets may someday seem quaintly oversimplified. A growing body of research suggests that the kinds of carbohydrates a person eats—not just the quantity—have wide-ranging health effects. Much has been written about how "bad carbs"—the white bread and potatoes that cause blood sugar levels to quickly spike and then crash—make people more prone to obesity and diabetes. Now, researchers are linking such foods with health problems as dissimilar as acne and eye diseases.
Bad carbs are said to rate high on the "glycemic index," which assigns each food a numerical value based on how quickly it increases a person's blood glucose. High-index carbs include sugars and easily digested starches; healthier carbs are lower on the index and include those found in whole grains, brown rice, fruits, and vegetables. These produce slower rises in blood sugar and more tempered release of insulin, a hormone involved in moving glucose out of the bloodstream and into the cells, where it is used as fuel or stored as fat.
Originally developed as a tool to keep diabetics' blood sugar on an even keel, a diet consisting of low-glycemic foods also seems to lower a healthy person's risk of developing diabetes and heart disease—probably because it limits the damage that elevated blood sugar inflicts on cells. Some experts think a low-glycemic diet also helps people lose weight, because smoothing out fluctuations in blood sugar may curb appetite. According to a recent analysis by the Cochrane Collaboration, an international medical research group, dieters choosing foods that have low index values tend to shed more pounds than those using other weight-loss plans. Some experts, however, say it's unclear whether a diet based on the index is really more effective than any other.
The newly identified benefits, however, have little to do with that debate. One new study found that elderly people who had followed a low glycemic index diet during the previous year had a reduced rate of age-related macular degeneration, an eye disease that can lead to blindness. Another found that sticking to a diet low in "bad carbs" helped young men fight off acne better than a low-fat diet that had just as many total calories. Both new studies appeared this month in the American Journal of Clinical Nutrition.
Where's the link? Many researchers now believe that elevated sugar levels, even those technically within the normal range, can cause cellular damage that leads to disease. Jennie Brand-Miller, an expert on the glycemic index and a professor of nutrition at the University of Sydney, Australia, points out that it's the extremely high levels of sugar in the blood that cause many of the complications of diabetes—from nerve damage to kidney failure to, yes, blindness. "The question," she says, "is whether people without diabetes...are doing damage" if they eat foods that cause their blood sugar to spike.
"If blood sugar is surging and crashing after every meal and snack, that produces wide oscillations in powerful hormones, including insulin and adrenalin," says David Ludwig, an endocrinologist and director of an obesity research program at Children's Hospital Boston. "This affects our biology in ways that could affect diabetes, heart disease, possibly even cancer."
Despite the possible health benefits of a low-glycemic diet, people shouldn't become obsessed with the values that the index assigns to specific foods, says dietitian Joan Salge Blake. Use common sense as a guide, she recommends. Blake, a spokesperson for the American Dietetic Association, points out that potato chips score lower on the glycemic index than air-popped popcorn, yet they contain unhealthy amounts of saturated fat that aren't in unbuttered popcorn.
Most nutritionists now recommend a diet rich in fruits, vegetables, and whole grains—a diet that turns out to be low on the glycemic index. All whole-grain foods are not created equal, however. "Food advertisers and marketers have become masterful at misrepresenting products," says Ludwig. To find a loaf of bread, for instance, that is both whole grain and low on the glycemic index, look for the word "whole" in the first ingredient, and make sure that sugar isn't one of the first ingredients listed. The less processed the grain is, the better, says Ludwig. "If you really want to go for the nutritional prize, look for a bread in which you can actually see the kernel."