This study also had several limitations, which may have weakened the results:
Not many women met the 20% fat intake goal—Low fat wasn't consistently low in fat:
This "low-fat diet" regimen may not have truly been low fat. Since most women did not meet the fat-reduction goal, this study may have proven only that 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, 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. But, because so few women met the requirements of the study, it also means that we don't really know how a diet very low in fat affects breast cancer risk.
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 underreport the amount of fat actually eaten.
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.
Length of follow-up time:
While 48,000 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.
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 healthy weight to begin with.
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.
The type of fat wasn't specified:
There are three basic types of fats: saturated, monounsaturated, and polyunsaturated. 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.
More research is needed to find out if reducing saturated fat will have a significant influence on breast cancer risk.
The bottom line is that, while this study did not produce significant results, it does suggest that more research with longer follow-up time is needed to try and tease out the relationship between a low-fat diet and breast cancer risk in specific groups of women. So don't fill your plate with cookies and ice cream just yet. Reducing fat and increasing fruits and vegetables and whole grains in your diet will ensure your body is getting enough nutrients and contribute to your overall health. Also, a low-fat diet will probably help you lose weight, if you are trying to do that as well.
Stay tuned to Breastcancer.org for the latest information on diet and breast cancer.
Reviewed study: "Large Study Finds No Direct Link for Postmenopausal Women, But Reducing Fat Intake Offers Health Benefits" by Ross L. Prentice and others, Journal of the American Medical Association, February 8, 2006
Is this for me? If you're interested in changing your diet to reduce your risk of breast cancer, you might want to read this article.
Background and importance of the study: Overweight women have an increased risk of getting breast cancer after menopause. And being overweight can increase the risk of breast cancer coming back in women who have had the disease. This may be because fat cells make extra estrogen and other hormones, which might stimulate breast cell growth.
Researchers have long thought that a diet low in fat could reduce breast cancer risk. A low-fat diet usually reduces total calories consumed, which tends to result in weight loss. Preliminary studies comparing women in countries who ate higher-fat diets with women in countries who ate lower-fat diets seemed to support this idea. Other studies didn't find an association between low-fat diets and a reduction in the risk of breast cancer.
In the large study reviewed here, researchers compared the risk of developing breast cancer in women who were randomly assigned to one of two groups: continue to eat their regular diet or change to a low-fat diet.
Study design: Conducted by researchers at 40 clinical centers across the United States, this study is known as the Women's Health Initiative Randomized Controlled Dietary Modification Trial (a big name for a big study!). More than 48,800 postmenopausal women, aged 50 to 79 with no history of breast cancer, were studied from 1993 to 2005. This is one of the largest studies of a low-fat diet and breast cancer risk.
The women were randomly assigned to either a low-fat diet group (19,541 women) or a regular diet group (29,294 women).
The goal of the low-fat diet group was to reduce total fat intake to 20% of total calories. This means that, if you eat 2,000 calories per day, only 400 calories could come from fat. Two other goals were to eat more fruits and vegetables (at least five servings per day) and more grains such as whole-wheat bread or brown rice (at least six serving per day). The type of fat in the diet was not specified—meaning that women were not told to specifically reduce saturated fat (which is considered "bad" fat because it raises unhealthy cholesterol) or increase the amount of "good" fat (which tends to lower cholesterol). The women were simply told to reduce their fat intake. The women in the low-fat diet group participated in a series of small-group nutritional counseling sessions. A specially trained certified nutritionist led the sessions. Each woman in the low-fat diet group was given her own fat intake goal, based on her height.
Women in the regular diet group received a copy of the U.S. Department of Health and Human Services "Dietary Guidelines for Americans" and other health-related publications but were not asked to change their diets.
None of the women in either group were asked to change their use of dietary supplements or other health-related behaviors, such as exercise or smoking.
Four-day food records were collected from all the women before they were randomly assigned to either group. At the beginning of the study and at one year, the women filled out a questionnaire on their nutrient consumption (how much fat, vegetables, fruits, grains, folate, and alcohol they consumed per day). A rolling sample of about one-third of the women filled out this survey again each year. The women's height, weight, waist measurement, and blood pressure were measured every year. Also, about 5% of the women filled out a four-day record after one year of the study and a 24-hour food record after three and six years.
The women had a mammogram at the beginning of the study and every two years after that.
The women in each group were similar in terms of:
The researchers compared how many women were diagnosed with breast cancer in the low-fat diet group with the number diagnosed in the regular diet group. They also looked at the levels of specific hormones and proteins in the women's blood to see if there were differences between the groups.
Results: After about eight years of follow-up, 655 women (0.42%) in the low-fat diet group and 1,072 women (0.45%) in the regular diet group were diagnosed with breast cancer. This difference was very small and not significant, which means that it could have happened by chance and might not be due to the low-fat diet regimen.
But for women who ate a high-fat diet before the study started, switching and sticking to a low-fat diet did seem to make a difference. Of the women who got more than 37% of their calories from fat to begin with, 291 of the women who continued their relatively high-fat diet developed breast cancer, whereas 151 of women who changed to the low-fat diet got breast cancer. This difference wasn't significant either, but it was very close to being significant. This means that the low-fat diet might have helped reduce the risk of breast cancer.
Dietary fat intake was significantly lower in the low-fat diet group. But only a small percentage of women met the goal of reducing their fat intake to 20% of their calories. Only 31% met the goal after one year, and only 14% continued to meet the goal after six years.
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.
The women in the low-fat diet group had slightly lower levels of estradiol (a type of estrogen) in their blood after one year. This difference was not significant. The estradiol levels were about the same after one year for the women in the regular-diet group.
Conclusions: The researchers concluded that a low-fat diet did not appear to significantly reduce the risk of breast cancer in postmenopausal women. But the researchers also noted that the low-fat diet seemed to be more beneficial for women who ate more fat to begin with. Also, the benefits of a low-fat diet may take longer than eight years to become significant, so a longer study may offer more definitive conclusions.
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