In a clinical trial of over 48,000 post-menopausal women, a low-fat diet that includes increased consumption of fruits, vegetables and whole grains is not associated with weight gain over an average of 7.5 years, according to a study in the January 4 issue of JAMA.

The prevalence of obesity in the United States has increased dramatically during the past several decades, according to background information in the article. A number of popular diet books have suggested that increasing obesity may be attributed to the diets recommended for chronic disease prevention by various national health organizations, specifically, diets that are lower in total and saturated fat and high in carbohydrates from vegetables, fruits, and whole grains or fiber-rich foods. Proponents of the popular alternative diets have claimed that the higher proportion of carbohydrates in the standard diets may promote weight gain.

Barbara V. Howard, Ph.D., of the MedStar Research Institute, Washington, D.C., and colleagues examined long-term data on the relationships between weight changes and specific changes in dietary components and macronutrient composition.

The data were from the Women’s Health Initiative (WHI) Dietary Modification Trial, which was designed to examine the long-term benefits and risks of a dietary pattern low in fat, with increased vegetable, fruit, and grain intake, on breast and colorectal cancers and cardiovascular disease in postmenopausal women. Between 1993 and 1998, 48,835 postmenopausal women were randomly assigned to either a low-fat dietary intervention or self-selected dietary control group. The data included body measurements and nutrient data through August 31, 2004, with an average follow-up of 7.5 years. Forty percent (19,541) of the participants were randomized to the intervention and 60 percent (29,294) to a control group. The intervention included group and individual sessions to promote a decrease in fat intake and increases in vegetable, fruit, and grain consumption and did not include weight loss or caloric restriction goals. The control group received diet-related education materials.

“Results show that after losing 2.2 kg [4.8 lbs.] in the first year, women in the intervention group maintained a modest weight loss, compared with the control group, during an average 7.5 years of follow-up and showed no increase from their baseline weight at any point during the study. Weights in the intervention group were lower than those of the control group, who followed their usual eating pattern during the follow-up period, suggesting that a low-fat dietary pattern may help attenuate the tendency for weight gain commonly observed in postmenopausal women,” the authors write.

No tendency toward weight gain was observed in intervention group women overall or when stratified by age, ethnicity, or body mass index. Weight loss was greatest among women in either group who decreased their percentage of energy from fat. A similar but lesser trend was observed with increases in vegetable and fruit servings, and a nonsignificant trend toward weight loss occurred with increasing intake of fiber.

“In summary, the results of this long-term trial of diverse postmenopausal women demonstrate that long-term recommendations to achieve a diet lower in total and saturated fat with increased consumption of fruits, vegetables, and whole grains, and without focus on weight loss, do not cause weight gain. Long-term effects of this dietary pattern on other health outcomes will be available after confirmation of end points and data analyses are completed, and long-term weight-loss studies designed to compare hypocaloric diets of varying macronutrient intake will be needed to establish the relative merits of different weight-loss regimens,” the researchers conclude.

(JAMA.2005; 295:39-49. Available pre-embargo to the media at http://www.jamamedia.org)