Childhood soy intake and breast cancer risk in Asian-American women.

B163

Background: Historically breast cancer incidence rates have been 4-7 times higher in the United States than in China or Japan. When Asian women migrate to the United States, their breast cancer risk rises over several generations and approaches that for U.S. Whites, which suggests that modifiable factors are responsible for the international variation in rates. In a case-control study of breast cancer in Asian-American women, designed to take advantage of their diversity in lifestyle and risk, we have demonstrated a 6-fold gradient in risk by migration history, comparable to the international differences in breast cancer incidence. In this analysis, we explore the contribution of childhood, adolescent and adult soy intake to breast cancer risk. Methods: A population-based case-control study of breast cancer was conducted among women of Chinese, Japanese and Filipino ancestry, aged 20-55 years, and living in San Francisco-Oakland (CA), Los Angeles (CA) and Oahu (HI) between 1983 and 1987. We successfully interviewed 597 cases (70% of those eligible) and 966 controls (75% of those eligible) during 1985-89. In addition, we attempted to contact the subject's mother if the mother were alive and living in the U.S. Mothers of 99 cases (43%) and 156 controls (40%) were successfully interviewed about their daughter's early exposures. Diet during childhood, adolescence and adulthood were assessed with a food frequency questionnaire; and cultural and religious practices during the same periods were assessed to characterize acculturation to Western lifestyles. Results: Childhood soy intake was strongly and significantly associated with reduced breast cancer risk. Comparing highest to lowest tertiles, the adjusted relative risks (RRs) for childhood, adolescent, and adult soy intake were, respectively, 0.42 [95% confidence interval (CI) 0.20-0.90, p-trend=0.02]; 0.77 (95% CI 0.57-1.04, p-trend=0.08; and 0.71 (95% CI 0.53-0.95, p-trend=0.01). The inverse association with childhood soy intake was noted in all three races, all three study sites, and women with and without a family history of breast cancer. Analyses of the relative contributions of soy intake during different time periods suggested that childhood soy intake was the most critical. The protective effect of childhood soy was similar in Asian women born in Asia and in the West. Furthermore, while adjustment for measures of acculturation attenuated the associations with adolescent and adult soy intake, it did not affect the inverse relationship with childhood soy intake. Conclusions: Soy intake during childhood, adolescence, and adult life were each associated with a decreased risk of breast cancer, with the strongest and most consistent effect seen for childhood intake. Soy itself seemed to be protective and not simply a marker of Asian lifestyles. Soy intake may be one more hormonally related, early life exposure that influences the incidence of breast cancer in adult life.