News Author: Laurie Barclay, MD

Jan. 4, 2006 — A low-fat eating pattern emphasizing fruits, vegetables, and grains does not result in weight gain in postmenopausal women, according to an interim report from the Women's Health Initiative (WHI) Dietary Modification Trial reported in the January 4 issue of JAMA.

"Obesity in the United States has increased dramatically during the past several decades," write Barbara V. Howard, PhD, from MedStar Research Institute in Washington, DC, and colleagues. "There is debate about optimum calorie balance for prevention of weight gain, and proponents of some low-carbohydrate diet regimens have suggested that the increasing obesity may be attributed, in part, to low-fat, high-carbohydrate diets."

In this long-term, low-fat diet trial for which the primary end points were breast and colorectal cancer, 48,835 postmenopausal US women of diverse backgrounds and ethnicities were enrolled between 1993 and 1998 and randomized to a dietary intervention (40%) or control group (60%).

The dietary intervention included group and individual sessions to promote decreased fat intake and increased consumption of vegetables, fruits, and grains, but it did not include weight loss or caloric restriction goals. The control group received diet-related educational materials. The primary endpoint was change in body weight from baseline to follow-up. This analysis includes data from a mean follow-up of 7.5 years, through August 31, 2004.

Compared with the control group, women in the intervention group lost weight in the first year (mean, 2.2 kg; P < .001) and maintained lower weight (difference, 1.9 kg; P < .001 at 1 year and 0.4 kg; P = .01 at 7.5 years). Even when stratified by age, ethnicity, or body mass index (BMI), women in the intervention group showed no tendency toward weight gain. In either group, weight loss was greatest among women who decreased their percentage of energy from fat. A similar but lesser trend was seen with increases in vegetable and fruit servings. There was a nonsignificant trend toward weight loss with increasing intake of fiber.

"A low-fat eating pattern does not result in weight gain in postmenopausal women," the authors write. "This large randomized trial demonstrates that dietary recommendations for reducing fat and replacing it with vegetables, fruits, and grains do not increase body weight, which implies that guidelines that restrict fat intake and advocate increases in complex carbohydrates have not been a contributing factor to the weight gain that has been occurring in the United States throughout the past several decades."

Study limitations include low representation of nonwhites; inclusion only of postmenopausal women aged 50 to 79 years, although the researchers purport that the data are likely generalizable to younger persons and to both sexes; exclusion of women reporting less than 32% fat intake at baseline; biased nature of self-reported food intake; inability to separate carbohydrate intake into the percentage of sugar or simple vs complex carbohydrate; inability to evaluate the potential effects of changes in caloric intake; and possible overestimate of the baseline percentage of energy from fat.

The National Heart, Lung, and Blood Institute funded this study. Dr. Howard has disclosed various relevant financial relationships with Merck, the Egg Nutrition Council, General Mills, Pfizer, and Schering-Plough.

In an accompanying editorial, Michael L. Dansinger, MD, MS, and Ernst J. Schaefer, MD, from Tufts University Human Nutrition Research Center in Boston, Massachusetts, call this a "landmark study... [because the] trial provide[d] a unique opportunity to examine long-term effects of an ad libitum low-fat dietary pattern on body weight and the relationship between weight changes and specific changes in dietary components." However, they continue to describe that the findings on long-term weight change are "somewhat underwhelming." Again, total weight loss at 1 year for the intervention group was only 2.2 kg with a 1.9-kg difference between the intervention and control groups at 1 year and only a 0.4-kg difference after 7.5 years of follow-up.

The editorialists suggest that the dietary intervention should have included the goal of weight reduction. They also stress that weight loss for an individual will not occur without physical activity and belief (by physician and patient) in lifestyle change, regardless of the dietary intervention applied. Citing their own study, published in the January 5, 2005, issue of JAMA, they explain that better adherence leads to better results.

"Much more work needs to be done on the obesity front, including a concerted collective effort focused on developing reliable methods of facilitating high long-term adherence levels to substantial lifestyle efforts — specifically calorie-reduced eating patterns and much more exercise," Drs. Dansinger and Schaefer write. "That is something on which health advocates and popular diet proponents can agree."