Low-Fat Diet Not a Cure-All
Results from large, long Women’s Health Initiative Dietary Modification Trial shows no effect on heart disease, breast cancer, colorectal cancer, or weight.
The low-fat, high-starch diet that was the focus of dietary advice during the 1990s-as reflected by the USDA food guide pyramid-is dying out. A growing body of evidence has been pointing to its inadequacy for weight loss or prevention of heart disease and several cancers. The final nail in the coffin comes from an eight-year trial that included almost 49,000 women. Although the media have made much of the "disappointing" results from the Women’s Health Initiative (WHI) Dietary Modification Trial, it would be a serious mistake to use these new findings as reason to load up on sausage, butter, and deep-fried fast food.
The trial and its findings
The Women’s Health Initiative Dietary Modification Trial was started back in 1993, at a time when dietary fat was seen as a dietary evil and the low-fat diet was thought to be a straightforward route to preventing heart disease, some cancers, and the epidemic of obesity that was beginning to sweep the country. With funding from the National Heart, Lung, and Blood Institute, researchers recruited almost 50,000 women between the ages of 50 and 79 years. Of these, 19,541 were randomly assigned to follow a low-fat diet. Their goal was to lower their fat intake from almost 38% of calories to 20%. They were helped in this effort by a series of individual and group counseling sessions.Another 29,294 women were randomly assigned to continue their usual diets, and were given just generic diet-related educational materials.
After eight years, the researchers looked at how many (and what percentage) of women in each group had developed breast cancer or colorectal cancer. They tallied up heart attacks, strokes, and other forms of heart disease. They also looked at things like weight gain or loss, cholesterol levels, and other measures of health.
The results, published in the Journal of the American Medical Association, showed no benefits for a low-fat diet. Women assigned to this eating strategy did not appear to gain protection against breast cancer,(1) colorectal cancer,(2) or cardiovascular disease.(3) And after eight years, their weights were generally the same as those of women following their usual diets.(4)
The researchers saw a trend toward a lower risk of breast cancer among women in the low-fat group. This trend was not statistically significant, meaning it could have been due to chance. It could also have been due to the very small weight loss during the early years of the study among women in the low-fat group, who received intensive dietary counseling. There is strong evidence from many studies that being overweight increases the risk of breast cancer after menopause, and that staying slim after menopause is an effective way to reduce risk of breast cancer, along with many other diseases.
Limitations of the study
Some nutrition experts say that the WHI Dietary Modification Trial doesn’t really lay to rest the low-fat hypothesis because the women in the study only modestly lowered their fat, from 38% to 29%. Had they reached the trial’s target of 20%, benefits from the low-fat approach may have become more apparent, these nutritionists suggest.
It is possible that the participants in the low-fat group may have actually overstated how much they reduced their fat intake. This has happened in other studies, as shown by comparisons between self-reported changes and biochemical measures of dietary change. Significant reductions in fat intake are usually reflected in a decrease in HDL (good) cholesterol and an increase in triglycerides. Yet in the WHI trial, there were no differences in blood levels of HDL cholesterol or triglycerides between the low-fat and usual diet groups. This casts doubt on the degree of fat reduction achieved in this study.
Two other limitations of the trial are the study population and duration. The trial included women who were aged 50 to 79 years at the beginning of the trial. By this time in life, it may be too late for changes in diet to reduce risks of cancer and other chronic conditions. In addition, it takes years for the effects of dietary change to be seen, and so it is possible that eight years wasn’t enough time to see the true impact of a low-fat diet.
The debate will likely continue as to why the WHI observed little benefit for a low-fat diet. Was it because reducing the intake of dietary fat truly has little benefit? Was it because the women in the trial didn’t lower fat intake enough? Or had the study focused on a younger population, or lasted longer, would it have revealed a benefit?
In any case, the dietary intervention didn’t work, even though the WHI trial was, by far, the most expensive study of diet ever conducted (costing many hundreds of millions of dollars) and even though the women in the low-fat group received intensive dietary counseling from some of the best nutritionists and dietitians in the country.
Change was already in the air
The dietary fat reduction arm of the WHI (it also has a hormone replacement therapy component and a calcium and vitamin D component) was controversial from the beginning.(5-8) Members of the HSPH Department of Nutrition argued that the hypothesis that a reduction in total fat intake would have major health benefits was not supported by existing data. It also noted that maintaining a contrast in diets between two groups over many years was difficult, and for this reason the study might not provide a clear answer even if the hypothesized benefits were true.
Such a study had failed in the past. The Multiple Risk Factors Intervention Trial (MRFIT, often called Mister Fit) aimed to decrease risk of coronary heart disease by a program aimed at controlling key risk factors for the disease. Some of the participants received intensive counseling to stop smoking, control high blood pressure, and reduce their intake of saturated fat. At the end of the trial, there was no significant difference in rates of coronary heart disease.(9) Even so, the investigators argued that their hypotheses were still correct because the members of the control group had, on their own, began to stop smoking and eat less saturated fat, making differences in smoking rates and diet between the two randomized groups very small.(10) The results of the WHI add further evidence that clear answers to questions about the long term effects of diet on risks of cancers and other major diseases may not be obtainable by large randomized intervention trials, no matter how much money is spent conducting them.
Type trumps percentage
The findings from the Women’s Health Initiative Dietary Modification Trial came as a surprise to many Americans who have been hearing for years that reducing fat is important for long-term health. Yet long-term follow-up studies such as the Nurses Health Study have consistently found little relation between the percentage of calories from fat and risks of breast cancer, colon cancer, or coronary heart disease. Such studies are one reason why major reviews of diet and health during the last five years, including those conducted by the U.S. Institute of Medicine and the U.S. Dietary Guidelines Committee, have moved away from advocating low fat intake to an emphasis on the type of fat.
Many lines of evidence indicate that the type of fat is very important to long-term health. Replacing saturated and trans with natural vegetable oils can greatly reduce the risk of heart disease and diabetes. In the Nurses’ Health Study II we have seen that women who consume high amounts of red meat and high-fat dairy foods during their early adult years are at increased risk of developing breast cancer.
Making good dietary choices does really matter, but it is the type of fat, not the amount, that is most important. And keep in mind that too many calories from both fat and carbohydrate will lead to weight gain, which will increase risks of breast cancer, colon cancer, and heart disease.
1. Prentice RL, Caan B, Chlebowski RT, et al. Low-fat dietary pattern and risk of invasive breast cancer: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006; 295:629-42.
2. Beresford SA, Johnson KC, Ritenbaugh C, et al. Low-fat dietary pattern and risk of colorectal cancer: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA.2006; 295:643-54.
3. Howard BV, Van Horn L, Hsia J, et al. Low-fat dietary pattern and risk of cardiovascular disease: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006; 295:655-66.
4. Howard BV, Manson JE, Stefanick ML, et al. Low-fat dietary pattern and weight change over 7 years: the Women’s Health Initiative Dietary Modification Trial. JAMA. 2006; 295:39-49.
5. Michels KB, Willett WC. The women’s health initiative: will it resolve the issues? Recent Results in Cancer Research. 1996; 140:295-305.
6. Prentice RL, Sheppard L. Dietary fat and cancer: consistency of the epidemiologic data, and disease prevention that may follow from a practical reduction in fat consumption. Cancer Causes and Control. 1990; 1:81-97; discussion 99-109.
7. Prentice RL, Sheppard L. Dietary fat and cancer: rejoinder and discussion of research strategies. Cancer Causes and Control. 1991; 2:53-8.
8. Willett WC, Stampfer MJ. Dietary fat and cancer: another view? Cancer Causes and Control. 1990; 1:103-109.
9. Multiple risk factor intervention trial. Risk factor changes and mortality results. Multiple Risk Factor Intervention Trial Research Group. JAMA. 1982; 248:1465-77.
10. Willett W. Nutritional epidemiology. New York: Oxford University Press, 1998.
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