One of the hottest diet trends focuses on the Glycemic Index, which ranks carbohydrates according to their ability to affect blood glucose. The premise is that a diet of carbs with a low Glycemic Index will help people lose weight and reduce their risks for heart disease and diabetes.

But a study by a researcher at the University of South Carolina's Arnold School of Public Health has found that the Glycemic Index may not help people determine the foods that they should eat -- or avoid -- to improve their health.

The use of glycemic index (GI) to rank carbohydrates to their ability to affect blood glucose has become more and more mainstream, despite being complicated for the consumer to understand.

The general ‘take-home’ message for consumers has been “low GI good, high GI bad.”

But a recent published reported: “The present results [of the study] call into question the utility of GI to reflect glycemic response to food adequately, when used in the context of usual diet.”

The researchers from the University of South Carolina evaluated GI in relation to blood sugar levels for 813 volunteers using a food frequency questionnaire.

After five years of follow-up the results showed that the GI of the diet was not related to any of the measures of blood glucose.
“There are valid reasons to question the Glycemic Index scientifically,” said lead-author Elizabeth Mayer-Davis.

"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.

“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,” she said.

Mayer-Davis explained that in scientific literature, a food’s GI 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,” said Mayer-Davis. “In general, the GI does not seem to be useful in understanding how diet impacts health, and use of the GI may not be an effective way to identify foods for optimal health,” she said.

Dr Glenn Gaesser, co-chair of the US Grain Food Foundation's clinical advisory board suggested last summer that the growth in popularity was being pushed by the industry.

“With the GI craze we have a case of the tail wagging the dog- everyone is following along for fear of losing market share.

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.