Dieters may not gain health or weight-loss benefits from following the trendy so-called low-GI diets, a new study suggests.
In recent years, researchers have taken to classifying carbohydrates based on their GI, or glycemic index -- a measure of the effects of a given food on blood sugar levels. High-GI foods, like white bread and potatoes, tend to produce a quick surge in blood sugar, and some studies have suggested that diets heavy in such foods can contribute to weight gain, diabetes and heart disease.
Book titles and Web sites espousing "low-GI" diets have followed suit.
But the science is mixed. Some studies have failed to find links between high-GI foods and elevated blood sugar and diabetes.
The new research suggests on reason is that it's hard to translate lab findings on glycemic index to the much more complicated realm of everyday eating, according to Dr. Elizabeth Mayer-Davis, the lead author of the new study.
One problem, she explained, is that a food's GI is determined under artificial conditions where a person eats the test food after a fast, then has blood sugar tests taken two hours later. But a food has different blood sugar effects when it's not eaten after a fast, she said.
In addition, many factors sway blood sugar levels after a meal, according to Mayer-Davis, a diabetes researcher at the University of South Carolina in Columbia. These include the length of time a carbohydrate is cooked, the foods it is eaten along with, and the workings of an individual's hormones, among other things.
In her team's study, published in the British Journal of Nutrition, there was no association between high-GI eating habits and elevated blood sugar among 813 adults who were followed over 5 years.
The findings, Mayer-Davis said, reinforce the notion that GI is "simply not a good index of how food impacts blood sugar."
The health benefits that some studies have attributed to low-GI foods may actually reflect other qualities of those foods -- like high fiber content, according to the researcher. Fiber-rich foods like whole grains are often lower on the GI scale.
What's more, GI is a complicated way to judge a food's value. Certain vegetables, for instance, have a fairly high GI, but actually contain very few grams of carbohydrate and few calories. On the other hand, a dish of ice cream may have a lower GI than a bowl of brown rice.
Mayer-Davis said that in her view, health-conscious consumers should not bother with seeking out the GI of their favorite foods.
The best move, she advised, is to keep calories in check and eat plenty of fiber-rich whole grains, fruits, vegetables and beans -- and to burn calories through regular exercise.
Many studies, she noted, show that obesity is the "major player" in the risk of diabetes, and weight control is essentially a matter of balancing calorie intake with calorie expenditure through physical activity.