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A carefully controlled animal study provides clear evidence that a low-glycemic-index (low-GI) diet -- one whose carbohydrates are low in sugar or release sugar slowly -- can lead to weight loss, reduced body fat, and reduction in risk factors for diabetes and cardiovascular disease.

''The study findings should give impetus to large-scale trials of low-GI diets in humans,'' says senior author David Ludwig, MD, PhD, director of the Optimal Weight for Life (OWL) obesity program at Children's Hospital Boston. His group's findings appear in the August 28th issue of The Lancet.

Many studies, including small studies in humans, have suggested that low-GI diets are beneficial, but due to study design, the observed benefits could have come from other aspects of the subjects' diets, such as fiber or overall caloric intake. For this reason, no major health agency or professional association references glycemic index in their dietary guidelines, Ludwig says.

In the current study, rats were fed tightly controlled diets with identical nutrients, except for the type of starch. Both diets were 69 percent carbohydrates, but 11 rats were randomly assigned to a high-GI starch and 10 to a low-GI starch. Food portions were controlled to maintain the same average body weight in the two groups.

In a further experiment, rats were randomly assigned to one of the two diets, and, at week 7, were crossed over to the alternate diet for another 3 weeks. Rats that switched from a low to high GI diet showed greater increases in blood glucose and insulin than rats that were switched from high to low GI. Finally, 24 mice were randomly assigned to the low- or hi-GI diet. At week 9, the high-GI group had 93 percent more body fat than mice on the low-GI diet.

''What the study shows is that glycemic index is an independent factor that can have dramatic effects on the major chronic diseases plaguing developed nations -- obesity, diabetes, and heart disease,'' says Ludwig. ''This is the first study with hard endpoints that can definitively identify glycemic index as the active dietary factor.''

''The Atkins diet tries to get rid of all carbohydrates, which we think is excessively restrictive,'' says Ludwig. ''You don't have to go to this extreme if you pay attention to the glycemic index and choose low-GI carbs.''

Children's Hospital Boston is now recruiting adult subjects for a large-scale, 18-month human study of the low-GI diet. Subjects will receive comprehensive dietary and behavioral counseling in individual and group sessions that will enable them to put low-GI diets into effect. To enroll, subjects must be overweight, 18 to 35 years old, and motivated to attend weekly sessions for four months and return for follow-up through 18 months. People interested in enrolling should contact Erica Garcia-Lago at 617-355-2500.