A controlled 2-month dietary fat reduction and soy food supplementation study in postmenopausal women

Nutrition Research Newsletter, June, 2005

The role of diet in the etiology of breast cancer remains controversial in observational epidemiologic studies. Pooled analyses of cohort studies conducted in Western countries, which used food-frequency questionnaires, showed no relation between fat intake and breast cancer risk. However, in the Norfolk component of the European Prospective Investigation of Cancer study, which used a 7-day food diary, risk of breast cancer increased significantly with increasing intakes of total and saturated fat. These differences in cohort findings emphasize the importance of measurement error in dietary assessment. The role of soy in the etiology of breast cancer is also inconclusive. There is compelling evidence that estrogen concentrations are a critical determinant of breast cancer risk. Dietary fat reduction and soy supplementation intervention studies have been conducted with the rationale that a reduction in endogenous estrogen concentrations in short term settings will lend support to a role of dietary fat or soy on breast cancer risk. However, the quality of these intervention studies, particularly the dietary fat reduction studies, has been questioned.

To further examine the influence of dietary fat reduction and soy food supplementation on circulating hormone concentrations in postmenopausal women, the researchers conducted a randomized, controlled, dietary, intervention study, investigating the separate effects on hormonal endpoints of a soy food-supplemented Step I diet (30% of energy as fat, 50% of energy as carbohydrate, 20% of energy as protein and designed to provide 50 mg isoflavones/d; SFD) and a very-low-fat, high-carbohydrate diet (designed to provide 12% of energy as fat, 68% of energy as carbohydrate, 20% of energy as protein, and no soy food; VLFD), compared with a control Step I diet (30% of energy as fat, 50% of energy as carbohydrate, 20% of energy as protein, and no soy food; CD) in free-living postmenopausal women. Fifty-seven postmenopausal women participated in this randomized dietary intervention study.

During the 8 weeks of study, participants received all foods to be consumed from the research kitchen at the General Clinical Research Center (GCRC). Each daily menu included foods for breakfast, lunch, dinner, and a morning and an evening snack. Written instructions regarding food safety and reheating methods were provided. Participants were given a daily log to check off all of the foods on the menu that they consumed and to record any extra foods eaten that were not on the menu. They were also asked to return to the kitchen all uneaten food or leftovers in their original containers, which were weighed and deducted from their daily nutrient intake.

There were no significant differences in total estradiol and sex hormone binding globulin at the completion of the intervention, between women in the SFD and VLFD groups and those in the CD group. Serum insulin decreased significantly in the SFD group, and leptin decreased significantly in the SFD and VLFD groups; however, these changes did not differ significantly from the changes in the CD group. The results of this 8-week intervention study indicate that a VLFD and an SFD produced very small reductions in serum estrogen concentrations in postmenopausal women; these changes did not differ significantly from those observed in women after a CD. These results were unexpected because the fat intake in the VLFD was very low, akin to fat intakes in Japan during the 1950s. The amount of soy isoflavones added (50 mg isoflavones or 15 g soy protein in the form of tofu) in the SFD was not dissimilar to soy intakes in Japan. The fat intake in the CD and SFD groups did not differ significantly from the fat intake in a Step I diet, which has <30% of energy as fat and < 10% of energy as saturated fat.

In summary, the present study does not provide evidence that short-term ingestion of soy food or a VLFD, resembling intakes in a traditional Asian diet, significantly reduces estrogen concentrations in postmenopausal women. In all 3 diet groups, insulin and leptin concentrations decreased. Results on the effects of a low-fat intervention diet in the Women's Health Initiative will be extremely informative in elucidating the longer-term effects of dietary fat reduction in postmenopausal women.

A. Wu, F. Stanczyk, C. Martinez, et al. A controlled 2-month dietary' fat reduction and soy food supplementation study in postmenopausal women. Am J Clin Nutr; 81 (5): 1133-1141 (May, 2005). [Correspondence: to A. Wu, University of Southern California/Norris Comprehensive Cancer Center, 1441 Eastlake Avenue, MC 9175, Los Angeles, CA 90089-9175. E-mail: annawu@nsc.edu]