In fact, women actually lost a little weight when they consistently ate few fats and lots of fruits, vegetables, and grains. The finding is yet another gem mined from the huge Women's Health Initiative study.
Barbara Howard, PhD, of MedStar Research Institute in Washington, and colleagues signed up nearly 50,000 postmenopausal women aged 50-79 for the WHI Dietary Modification Trial. The majority of the women were overweight or obese and were eating diets that got 39% of calories from fat.
Forty percent of the women got intensive training and support in eating a low-fat diet with five or more servings of fruits and vegetables and six or more servings of grains. This training included 18 group counseling sessions in the first year and four sessions per year for the rest of the study. The other 60% of the women were handed a copy of U.S. dietary guidelines.
None of the women -- in either group -- was told to eat less or exercise more. In fact, they weren't specifically told to lose weight.
Even so, in the first year the women trained to eat a healthy diet lost about 5 pounds. After seven and a half years, they still weighed about a pound less than the women in the group who did not receive special training.
"Restricting fat intake does not lead to weight gain," Howard and colleagues conclude. "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."
The findings appear in the Jan. 4 issue of The Journal of the American Medical Association.
The study may well refute the claims of some popular diet gurus that a low-fat diet makes people fat. Indeed, women who ate the least fat lost the most weight.
And there was also evidence that carbs aren't all bad. Women who ate the most vegetables tended to lose more weight than those who ate the least.
But the study results are "somewhat underwhelming," write diet researchers Michael L. Dansinger, MD, and Ernst J. Schaefer, MD, of Tufts University in Boston, in an editorial accompanying the Howard team's report.
Dansinger and Schaefer note that the majority of the women in the study were obese or overweight.
"Weight loss was not a treatment goal [in the study], but perhaps it should have been," they suggest.
The editorialists note that while the study succeeded in getting women to cut their fat intake from 39% to 30% of total calories, it still fell far short of its goal of cutting fat intake to 20% of total calories.
"Is it time to admit defeat? Is U.S. society doomed to be one in which few individuals maintain normal body weight and one-third of adults are obese?" they ask. "Many believe humankind does not have the self-control to counterbalance the forces that create a predictable wave of obesity in technologically advancing societies."
As an answer, Dansinger and Schaefer point to their own recent study in which they compared several different diet plans. None worked much better than another. But they all worked -- for the relatively few people who stuck with the program.
Eating less and exercising more -- what doctors call "lifestyle change" -- really does help people lose weight. That, the editorialists say, means that all hope is not lost.
"Most able-bodied persons who can find a way to overcome the monumental logistical and psychological barriers that prevent the full application of lifestyle change can reverse obesity within months," Dansinger and Schaefer write. "The medical profession and society in general have under-dosed this potent cure by a long shot."