Low-Fat Dietary Pattern and Risk of Invasive Breast Cancer: The Women???s Health Initiative Randomized Controlled Dietary Modification Trial

Despite a longstanding belief that a low-fat diet can reduce the risk of breast cancer, no controlled intervention trials have been reported. This randomized, controlled, primary prevention trial, based on the Women’s Health Initiative cohort, enrolled 48,835 postmenopausal women 50 to 79 years of age at 40 U.S. clinical centers in the years 1993–2005. Forty percent of participants received a diet designed to lower the intake of total fat to 20% of energy, provide at least 5 servings daily of fruit and vegetables, and include at least 6 daily servings of grains. There was no explicit weight loss goal. The remaining 60% of women continued on their usual diets. The intervention group took part in an intensive behavioral modification program administered by a specially trained nutritionist. Participants received an individualized goal of total fat grams based on height and were trained to monitor their dietary intake. The comparison women were given informational materials, including dietary guidelines. At baseline, dietary and other variables, including body weight, were nearly identical in the intervention and control groups. Women in the diet group had a small decrease in energy consumption. The difference between groups in change from baseline in the percentage of energy from fat ranged from 10.7% after 12 months to 8.1% at year 6. The intervention group ate more fruits and vegetables, but there was less difference in grain consumption. The annualized incidence rate of invasive breast cancer during an average follow up of 8.1 years was 0.42% in the intervention group and 0.45% in the comparison group. The hazard ratio was 0.91% with a 95% confidence interval of 0.83 to 1.01. Rates of total cancer (excluding non-melanoma skin cancer), breast cancer mortality, and total mortality all were slightly lower for the intervention group, but the differences were not significant at the 0.05 level. Women who actively participated in the dietary intervention program and those consuming a high-fat diet at baseline had lower hazard ratios. The breast cancer risk also was lower in women with a relatively high baseline percentage of energy from fat. Tumor grade and size, lymph node status, and stage of breast cancers all were similar in the intervention and comparison groups. A lower hazard ratio was noted for progesterone receptor-negative tumors, but there was no apparent association with estrogen receptor status. A low-fat diet failed to lessen the risk of invasive breast cancer in this large trial of postmenopausal women during a follow up averaging approximately 8 years. Nevertheless, the trends observed, although not statistically significant, warrant longer-term studies. Any beneficial effects of reduced fat intake might take years to fully emerge.