In 2006, results from the world's largest low fat diet project were published (see reference at end). This was a US government-funded study of 48,835 postmenopausal women in a multicenter prospective, randomized clinical trial known as the Women's Health Initiative Randomized Controlled Dietary Modification Trial. The study was conducted from 1993 to 2005 at 40 centers around the country. The volunteers were randomly assigned to either a low-fat diet group (19,541 women) or a regular diet group (29,294 women). After about eight years of follow-up, this large and costly study did not find any significant differences in breast cancer incidence between postmenopausal women who were asked to eat a low-fat diet and those who continued to eat their regular diet. On the other hand, the results did suggest that changing to a low-fat diet may reduce the risk of breast cancer for women who had diets very high in fat to begin with.
According to most experts, the following may be reasons why this study showed no significant benefit:
1) Not many women met the 20% fat intake goal. This "low-fat diet" regimen may not have truly been low in fat. Since most women did not meet the fat-reduction goal, this study may have proven only that the approach to the intervention did not work. It did not prove that a truly low-fat diet doesn't help protect you from breast cancer. Getting only 20% of your calories from fat is very difficult to do. This means that, if you eat 2,000 calories per day, only 400 calories could come from fat. So it's not surprising that less than a third of the women met this goal after the first year, and only 14% continued to meet the goal after six years. Because so few women met the requirements of the study, it means that we don't really know how a diet low in fat affects breast cancer risk.
2) Diets were self-reported and infrequent, which may make them unreliable. This study relied on the women's written reports of what they ate, which may not be an accurate reflection of true intake. These reports were done occasionally. No daily food log or journal was done and checked. Most of us aren't proud to admit in an interview or questionnaire that we've broken the rules and haven't stuck to the "prescribed diet." So there may be a tendency to under-report the amount of fat actually eaten.
3) Other changes besides the low-fat diet. The study did not separate out the effects of reducing the amount of fat eaten vs. the effects of increasing fruit and vegetable servings. Women in the low-fat diet group ate almost two more servings per day of fruits and vegetables than women in the regular diet group and about one more serving of grains.
4) Length of follow-up time. While 48,835 women is a lot of people, eight years isn't a lot of follow-up time. Eating a low-fat diet for 15 or 20 years may offer more significant benefits and show a closer relationship between dietary fat and breast cancer risk.
5) Baseline body mass index. In this study, 74% of the women were classified as overweight by body mass index at the beginning of the study. So we don't really know if a low-fat diet would offer benefits to women who are at a normal weight to begin with.
6) All the women were postmenopausal. It may be that dietary fat plays a more important role in the diets of younger, premenopausal women. It makes sense that your diet in the first 50 years of your life might affect your cancer risk in the second half of your life. This study doesn't address that question.
7) The type of fat wasn't specified. There are three basic types of fats: saturated, mono-unsaturated, and poly-unsaturated. In this study, women were asked only to reduce fat. They weren't asked to consider the various types of fat or told that reducing saturated fats may offer more health benefits than reducing unsaturated fats. Saturated fats are only found in foods that come from animals and are the types of fats that raise your blood cholesterol level. Trans fats (also called trans-saturated fats) are man-made fats. (Vegetable oils are modified to form margarine and vegetable shortening, both of which are trans fats.) Trans fats also are added to prepackaged foods. Saturated fats raise your blood's "bad" cholesterol (low-density lipoprotein or LDL) level and lower your "good" cholesterol (high-density lipoprotein or HDL) level. Mono-unsaturated and poly-unsaturated fats are found in plant foods like vegetables, nuts, and grains, as well as oils made from these nuts and grains (canola, corn, soybean). Omega-3 and omega-6 fatty acids are poly-unsaturated. Besides vegetables, nuts, and grains, omega-3 and omega-6 fatty acids are also found in coldwater fish such as tuna, salmon, and mackerel. Some studies have shown that eating foods that have mono- or poly-unsaturated fats can help reduce your levels of "bad cholesterol." Mono- and polyunsaturated fats may also keep your triglyceride levels low. Triglycerides are a form of fat in your bloodstream.
8) None of the women in either group were asked to change their health-related behaviors, such as exercise, drinking or smoking. These known risk factors for breast cancer were left uncontrolled and may obscure any benefit from eating less fat.
9) This study is about a low fat diet; it is not about going from an overweight/obese state to a normal weight. Eating less fat while staying persistently fat may not help anyone! Since this study cost nearly half a billion dollars, it is unlikely that another lifestyle intervention clinical trial at this large scale would be done again anytime soon, particularly in the current US economy.
Reference: Prentice RL, et al. Low-fat dietary pattern and risk of invasive breast cancer: the Women's Health Initiative Randomized Controlled Dietary Modification Trial. Journal of the American Medical Association, volume 295, pages 629-42, 2006.
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