February 2006

A dietary modification trial carried out as part of the Women's Health Initiative and published in the February 8, 2006 issue of the Journal of the American Medical Association found that there was not an overall decrease in breast cancer incidence among postmenopausal women who participated in a diet modification intervention to reduce their total fat intake and increase their consumption of vegetables and fruits when compared to women who did not change their diet.

How was the study done?
This is the first dietary study to test whether adopting a low fat diet in the middle to later decades of life reduces the risk of breast cancer, colorectal cancer and heart disease. In NBCCF's analysis of the study's breast cancer findings, 48,835 postmenopausal women (50 to 79 years old) with no prior diagnosis of breast cancer were randomly enrolled to either the dietary modification intervention group (40%; n=19,541) or the comparison group (60%; n=29,294)¹. Participants were on average 62.3 years old, 18.6% minority race/ethnicity, and had an average body mass index of 29.1.

Baseline variables² and breast cancer risk factors such as body weight, age, prior hormone treatment use, family history, education, ethnicity, and the Gail 5-year risk estimate³ were comparable between the two groups. None of the women were taking Tamoxifen or Raloxifene at the beginning of the study and use remained low, and balanced, during the trial's course (1.5% and 2.9%, respectively).

The trial was designed to promote dietary changes with the goals of reducing total fat intake to 20% of total energy (compared to 32% or more total fat intake in the comparison group), and increasing consumption of fruits and vegetables to at least 5 servings per day, and grains to at least 6 servings per day. Trained and certified nutritionists were also available to provide assistance to women in the intervention group. In contrast, women in the comparison group received U.S. dietary guidelines⁴ and other nutritional information but were not asked to make any dietary changes. The investigators did not collect information on physical activity or dietary supplement intake.

What were the results?
After an average follow up of 8.1 years, overall breast cancer incidence was found to be 9% lower for the women in the intervention group compared with women in the comparison group, but these findings were not statistically significant, meaning that they could have been due to chance. Reducing fat intake resulted in modest benefits in terms of weight loss and maintaining a greater weight change compared to women who didn't modify their diet.

In a subgroup analysis, investigators found that the only group that appeared to show evidence of a reduction in breast cancer incidence were women who started out with very high fat intake (more than 76 grams per day), were able to lower it by almost half, and maintained it at low levels over time. Another subgroup analysis indicated that significantly fewer women within the low fat diet intervention group developed estrogen receptor positive, progesterone receptor negative breast cancer than women in the comparison group, suggesting that fat content in the diet has an impact on the type of breast cancer a woman develops.

What are the limitations of the study?
Relatively few women in the intervention group were able to stick to this low fat diet despite nutritional counseling (31.4% of women at year 1, and only 14.4% at year 6), which ultimately might have reduced the study's statistical power necessary to demonstrate the effect of the intervention. There were also only modest differences between the two groups in daily fruit and vegetable intake as well as a small difference in grain intake. The investigators did not gather information on total energy intake from other sources as part of this study. Another limitation is that data on the participants' dietary intake was self-reported, which makes it difficult to assess the reliability and completeness of the data.

Most importantly, the few apparently positive findings in the study were obtained by analyzing subgroups of women. This practice is not considered to offer reliable results by the scientific community because when a large number of associations are studied, as was the case in this trial, some will turn out to be positive just by chance.

What does this study mean for women?
The potential link between fat content in a woman's diet and breast cancer has been studied for decades, and results have been mixed. This is the largest trial yet to explore the link between low fat diet and breast cancer. It found that, overall, a low fat diet does not reduce a woman's chance of getting breast cancer. The study also underscores the difficulty of changing dietary habits despite the availability of counseling and support. While there appears to be a weak trend towards a protective effect from radically lowering the fat content in the diet, the magnitude of this effect is minimal in terms of its potential for reducing risk. Longer follow up of study participants is necessary to better understand the effects of the intervention on breast cancer risk over time.

Breast cancer is a complex disease and we don't know what causes it. Our understanding of the factors that put women at risk for the disease is very limited, and women have little control over the factors we know about such as genetic make up, age at menarche, and age at menopause. The only dietary factor for which there is evidence for increased risk for breast cancer is alcohol intake.⁵

In 2005, results from a well designed study conducted by the Women's Intervention Nutrition Study (WINS) and presented at the American Society of Clinical Oncology conference found evidence that a low fat diet offered protection from breast cancer recurrence to women with a prior diagnosis of breast cancer (see NBCCF News Item The Effect of Low-Fat Diet on Recurrence in Postmenopausal Women with Early Breast Cancer.

The National Breast Cancer Coalition is disturbed by the misleading way in which a press release by National Institutes of Health and the Bulletin of the National Cancer Institute reported on this study. Both pieces stated that the results of the study on diet and breast cancer may be positive, downplaying the fact that the results were not statistically significant and could have been due to chance. Misrepresenting study results to present a positive spin is unacceptable from anyone let alone this country's leading research institutions.

About NBCCF
NBCCF is a grassroots organization dedicated to ending breast cancer through the power of action and advocacy. NBCCF's main goals are to increase federal funding for breast cancer research and collaborate with the scientific community to implement new models of research, improve access to high-quality health care and breast cancer clinical trials for all women, and expand the influence of breast cancer advocates in all aspects of the breast cancer decision making process. NBCCF actively empowers consumers through education and training to be key participants in the effort to improve the quality of health care.

Source Prentice RL, Caan B, Chlebowski RT, Patterson R, 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(6):629-642.

Footnotes
¹The randomization rate of 40% for the intervention group and 60% for the comparison group was chosen in order to minimize study cost at a specified level of power.

²A set of observations or data used for comparison.

³Participants were considered to be at a moderate risk for breast cancer based on a mean (standard deviation) Gail 5-year risk estimate of 1.7% (0.9%).

⁴The 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.

⁵Source: American Cancer Society

Click here to view past NBCCF Breaking News items.