Adherents of low-carbohydrate diet plans such as Atkins, South Beach, or the Zone should be somewhat chagrined at the results of one of the largest diet studies yet published. (1) The Women's Health Initiative (WHI) found no evidence that postmenopausal women who decrease the fat content of their diets (and thus increase carbohydrate) are prone to gain weight. Indeed, they showed a tendency to lose modest amounts of weight in a follow-up that spanned up to seven and one half years.
The WHI was designed to assess the effects of a low-fat dietary pattern on the risk of breast cancer, heart disease, and colorectal cancer in postmenopausal women. As part of that study, the women's weight and dietary intake was monitored for over seven years. Over 19,500 women (the intervention group) were instructed on reducing the fat in their diets, while approximately 29,000 continued their usual diets -- with information about how to eat healthfully (the control group). Neither group was instructed to try to lose weight.
Overall, the participants were sixty-two years of age and overweight or obese, with an average BMI of 29. Initially, all the women's diets derived about 39% of their calories from fat. The goal was to have the intervention group reduce this figure to 20%, but they only attained a reduction to 30%. The women in the control group didn t reduce their percent of dietary fat.
In spite of their failure to achieve the desired fat reduction, the women in the intervention group did reduce their average calorie intake -- by significantly more than the control group did (reductions of 361 vs. 241 calories). It is not unusual for people taking part in studies that monitor food intake to alter what they eat, even if not specifically instructed to do so. Thus it makes sense that all these women actually lost some weight over the course of the study.
By the end of the first year of the study, the women in the intervention group had lost significantly more weight than those in the control group (1.9 kilograms more), but this difference was not totally maintained for the whole follow-up period. By 7.5 years, the difference had diminished to 0.4 kgm, which was still statistically significant.
The main take-home message to be derived from this study is that, contrary to what low-carb advocates would have us believe, a diet high in carbohydrate does not doom one to weight gain nor the body to obesity,as we've said in the past. As one might expect, since fat contains more calories than protein or carbohydrates, a reduction in dietary fat will tend to reduce the number of calories consumed (if, of course, one doesn't compensate by eating more calories from carbs or protein).
The results of this study simply reinforce the dictum that it's still the calories that count!
(1) Howard BV, Manson JAE, Stefanick ML, Beresford SA, et al. Low-fat dietary pattern and weight change over 7 years. JAMA 2006; 295(1):39-49.