"One study is never enough to change a recommendation, but this study is interesting in that it shows that a low-fat diet is no better than a low-carbohydrate diet in preventing type 2 diabetes," said Thomas Halton, lead author of a study in the current issue of the American Journal of Clinical Nutrition. "The one diet that did seem to show a protective effect was a vegetable-based, low-carb diet which consisted of higher amounts of vegetable fat and vegetable protein, and lower amounts of carbohydrate."

The findings, Halton added, were a bit surprising in that most doctors and nutritionists recommend a low-fat diet to prevent type 2 diabetes. "This study showed that a low-fat diet didn't really prevent type 2 diabetes in our cohort when compared to a low-carb diet. I was also surprised that total carbohydrate consumption was associated with type 2 diabetes, and that the relative risk for the glycemic load was so high."

Halton is a recent graduate of the Harvard School of Public Health and has founded his own nutrition consulting company, Fitness Plus, in Boston.

Type 2 diabetes, which is associated with overweight and obesity, is a pressing health problem around the world. In the United States, two-thirds of adults weigh more than they should.

And, according to background information in the study, some 45 percent of women and 30 percent of men in the United States are trying to lose weight at any one time.

While low-fat, high-carb diets are often recommended, the long-term effects of such a regimen are not known.

People who reduce their carb intake generally take in more total and saturated fat and less whole grains, cereal fiber, fruit and vegetables, which can heighten the risk of type 2 diabetes.

For this study, Halton and his colleagues examined the association between low-carb diets and the risk of diabetes among 85,059 women participating in the Nurse's Health Study. The data included 20 years of follow-up.

Women were ranked according to what they ate. "We calculated a low-carbohydrate diet score based on the women's percent consumption of fat, protein and carbohydrate," Halton explained. "A higher score reflected a higher intake of fat and protein and a lower intake of carbohydrate. Therefore, the higher a woman's score, the more closely she followed a low carb-diet, and the lower her score, the more closely she followed a low-fat diet."

Women with a higher score did not have a heightened risk of diabetes. In fact, they seemed to have a small decreased risk when they derived their fat and protein from vegetable rather than animal sources.

Such a low-carb diet is similar to a healthy Atkins diet, meaning one which does not include large amounts of animal fat and animal protein, Halton said.

"When focusing on vegetable sources of fat and protein, this version of Atkins is similar to a low-glycemic Mediterranean diet," he said.

How easy will it be for people to follow such a diet?

"It's probably a very good thing to do . . . [but] people don't understand how to eat well. People don't know what simple and complex carbohydrates are and what it takes to have a good, balanced diet. People go to extremes," said Dr. Stuart Weiss, a clinical assistant professor of medicine at New York University School of Medicine in New York City. "In general, carbs should be limited just like saturated fat needs to be limited. . . If you eat too much of anything, you're bound to get into trouble."