A large nationwide study of nearly 49,000 postmenopausal women aged 50 to 79 has determined that a low-fat diet is not as protective against disease as some previously thought. Study participants did not experience lower rates of breast or colorectal cancer, heart disease, or stroke.
Some people have been shocked at this report. The popular belief held by many people (but few researchers) is that a low-fat diet is the best way to prevent cancer and heart problems. Over the last eight to nine years, however, very little research has shown any benefit of a low-fat diet for either of these problems. In fact, the vast majority of research confirms this same finding.
For example, in the Nurses? Health Study by Harvard, total fat intake was not associated with cancer or heart risk. And five years ago the Institute of Medicine and National Instituts of Health both recommended an increase in the upper limit of total fat intake from 30 percent to 35 percent of calories.
The consistent dietary recommendation for nearly a decade is this: It?s not the amount of fat that is so important for preventing heart disease or stroke, but the kind of fat eaten.
The theory of the study was that eating a low-fat diet would decrease the risk of breast cancer. The study diet focused on reducing total fat, and unlike diets used to reduce heart disease risk, did not differentiate between ?good? fats found in fish, nuts, and vegetable oils, and ?bad? fats like saturated fat and trans-fat found in processed foods, meats, and some dairy products. Also, weight loss was not a study goal.
Fat - How Much, What Kind
By the end of the first year of the study, the low-fat diet group reduced average total fat intakes to 24 percent of calories from fat, but did not meet the study?s goal of 20 percent. At year six, the low-fat diet group was consuming 29 percent of calories from fat. The comparison group averaged 35 percent of calories from fat at year one and 37 percent at year six. Women in both groups started at 35 to 38 percent of calories from fat. The low fat diet group also increased their consumption of vegetables, fruits, and grains.
The U.S. Dietary Guidelines for Americans recommend that adults keep total fat intake between 20 and 35 percent of calories, and saturated fats less than 10 percent of calories, with most fats coming from sources of polyunsaturated fats and monounsaturated fats, such as fish, nuts, and vegetable oils. For people with heart disease or at high risk for heart disease, targets for saturated fats may be further lowered.
At the end of eight years, there were no significant differences in the rates of colorectal cancer, heart disease, or stroke between the group who followed a low-fat dietary plan and the comparison group who followed their normal dietary patterns. Although the women in the study who reduced their total fat intake had a nine percent lower risk of breast cancer than did women who made no dietary changes, the difference was not large enough to be statistically significant — meaning it could have been due to chance.
?The results of this study do not change established recommendations on disease prevention. Women should continue to get regular mammograms and screenings for colorectal cancer, and work with their doctors to reduce their risks for heart disease including following a diet low in saturated fat, trans-fat and cholesterol,? said Elizabeth G. Nabel, MD, director of the National Heart, Lung, and Blood Institute.
The study also found that following a high-carbohydrate, low-fat eating pattern does not increase body weight, triglycerides, or indicators of increased risk of diabetes such as blood glucose or insulin levels in women. Neither does eating a low-fat diet alone decrease weight, according to study results.
Small Benefits
?While the participants? overall change in LDL ?bad? cholesterol was small, we saw trends towards greater reductions in cholesterol and heart disease risk in women eating less saturated and trans-fat,? said Jacques Rossouw, MD, WHI project officer.
Even though the reduction in breast cancer was considered ?nonsignificant? over the eight-year study, follow-up over several more years may indeed show that eating more fruits and vegetables and less fat could have a positive impact in reducing the risk of breast cancer.
?Because the health implications of a low-fat dietary pattern may take years to be fully realized, longer, planned, nonintervention follow-up may yield a more definitive comparison,? the authors wrote in the February 8 issue of the Journal of the American Medical Association.
Leslie G. Ford, MD, with the National Cancer Institute agrees, stating, ?Study data indicate that women who started with the highest fat intake and who had greater changes in fat intake, show stronger evidence for reduction in their risk of breast cancer. Longer follow-up may be needed to show the effects of diet on cancer risk over time.?
Bottom Line
This was the largest study of its kind to determine the benefits of a low-fat diet. While the results proved statistically insignificant, small improvements were seen in just a few years. Undoubtedly, this group of women will continue to be observed.
While this study didn?t specifically limit saturated and trans-fat foods, other studies have shown them to be harmful to health. On the other hand, fruits, vegetables, and whole grains provide many important nutrients. Healthy fats, such as nuts, are also important for good health. Simply restricting fats does not appear to be enough. Focus on limiting "bad" fats, and incorporate other healthful lifestyle such as regular exercise and stress management.
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