The findings, published in the February issue of the British Journal of Nutrition, show that people should exercise caution with the Glycemic Index diet, says Dr. Elizabeth Mayer-Davis, a noted diabetes researcher and the study's lead author.

"There are valid reasons to question the Glycemic Index scientifically," Mayer-Davis says. "This is an area in the field of nutrition that is controversial. It turns out that despite all of the interest in the Glycemic Index, the scientific literature is very mixed."

Some studies show beneficial effects of low Glycemic Index diets on diabetes or other conditions, and other studies show no effect, she says.

The basis for the Glycemic Index is this: When a specific carbohydrate is eaten, its effect on the body is consistent among individuals. Therefore, a specific number can be attached to it. Apples, plums and oranges, for example, have a low Glycemic Index, while french fries, watermelon and dried dates have a high Glycemic Index.

The limitation of the Glycemic Index, Mayer-Davis says, is that the numbers in the index are based on blood-sugar levels recorded two hours after the ingestion of test foods, in a controlled experimental setting and after a person has fasted overnight.

"However, many factors can affect the impact of food on glucose levels in a 'real life' setting, including the length of time that food is cooked, your body's hormones and other foods that are eaten at the same time," she says.

"In scientific literature, the Glycemic Index of foods is based on fasting. This is unrealistic because we eat throughout the day, and a certain food eaten at lunchtime can have a different impact on blood-glucose levels compared to eating that same food for breakfast after fasting overnight."

The USC study, funded by the National Institutes of Health and conducted over five years, followed more than 1,000 people at four clinical sites. Participants included African Americans, Hispanics and Caucasians.

The researchers wanted to determine whether study participants with a relatively low Glycemic Index diet had lower overall blood-glucose levels compared to participants with a relatively high Glycemic Index diet. Using several different measures of blood-glucose levels, the researchers found that the Glycemic Index of the diet was not related to any of the measures of blood glucose.

This means that the Glycemic Index is probably not picking up the specific effects of food on blood glucose, Mayer-Davis says.

"Several recent studies show that dietary fiber is important to heart disease, diabetes and obesity," she says. "Typically, foods high in fiber have a relatively low Glycemic Index."

This means that, in some studies, the Glycemic Index may have been related to good health because of dietary fiber, not because of a unique characteristic of food called the Glycemic Index, Mayer-Davis says.

"In general, the Glycemic Index does not seem to be useful in understanding how diet impacts health, and use of the Glycemic Index may not be an effective way to identify foods for optimal health," she says.

Many of the chronic diseases that have been related to diets with high Glycemic Index, including diabetes and heart disease, are much more strongly related to obesity than to other aspects of diet. The key to losing weight and lowering the risk for diabetes, heart disease and obesity, in simple terms, is this: Consume fewer calories and burn more calories through physical activity.

"A diet that is low in saturated fat and includes whole grains, fiber, fruits and vegetables will support weight management as long as the total calories are reduced," she says. "And, moderate physical activity is key to improving health."

The Glycemic Index only makes life more complicated for those trying to adopt a healthier lifestyle, she says.