ScienceDaily (May 17, 2007) — Overweight individuals who secrete insulin at a higher level may experience greater weight loss by selecting a low-glycemic load diet, compared to a low-fat diet, according to a study in the May 16 issue of JAMA. The researchers also found a low-glycemic load diet to have beneficial effects on HDL cholesterol and triglyceride concentrations.

"With prevalence approaching one-third of the population, obesity is among the most important medical problems in the United States and identification of effective dietary treatment has become a major public health priority. Three popular diets--low fat, low carbohydrate, and low glycemic load--have recently received much attention.

However, clinical trials have produced inconsistent findings, with some suggesting that one diet is superior for weight loss and others indicating no difference between diets," the authors write. They add that one explanation for the inconsistent findings could relate to the inherent physiological differences among study participants. "One physiological mechanism that might relate weight loss to dietary composition is individual differences in insulin secretion."

Cara B. Ebbeling, Ph.D., of Children's Hospital Boston, and colleagues conducted a study to determine whether insulin secretion affects weight loss and body fat loss among obese individuals with different diets. This randomized controlled trial, conducted from September 2004 to December 2006, included 73 obese young adults (age 18-35 years) and consisted of a 6-month intensive intervention period and a 12-month follow-up period. Serum insulin concentration at 30 minutes after a 75-g dose of oral glucose was determined at baseline as a measure of insulin secretion. Outcomes were assessed at 6, 12, and 18 months. Participants consumed either a low--glycemic load (40 percent carbohydrate and 35 percent fat) or low-fat (55 percent carbohydrate and 20 percent fat) diet.

The researchers found that change in body weight and body fat percentage did not differ between the diet groups overall. However, for those with insulin concentration at 30 minutes above the median (midpoint), the low--glycemic load diet produced a greater decrease in weight (12.8 lbs. vs. 2.6 lbs.) and body fat percentage (--2.6 percent vs. --0.9 percent) than the low-fat diet at 18 months. There were no significant differences in these end points between diet groups for those with insulin concentration at 30 minutes below the median level. Among all the participants in the study, high-density lipoprotein cholesterol (the "good" cholesterol) and triglyceride concentrations improved more on the low--glycemic load diet, whereas low-density lipoprotein cholesterol (the "bad" cholesterol) concentration improved more on the low-fat diet.

"The main finding of our study is that a simple measure of insulin secretion predicted weight and body fat loss on low--glycemic load and low-fat diets," the authors write. "For obese individuals with high insulin concentration at 30 minutes during an oral glucose tolerance test, a low--glycemic load diet may promote more weight and body fat loss than a low-fat diet. Regardless of insulin secretion, a low--glycemic load diet has beneficial effects on concentrations of HDL cholesterol and triglycerides but not on LDL cholesterol. Additional research is needed to examine these effects in other populations and to explore the mechanistic basis for the observed diet-phenotype interaction."

JAMA. 2007;297:2092-2102.

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