Soy intake and other lifestyle determinants of serum estrogen levels among postmenopausal Chinese women in Singapore.

Endogenous estrogen levels are strongly associated with breast cancer risk, but its determinants are not well understood. We conducted a cross-sectional study of 144 healthy postmenopausal women, participants in a population-based prospective investigation of diet and cancer risk among Chinese in Singapore. The relationships between plasma levels of estrone (E(1)), estradiol (E(2)), and androstenedione and dietary intake of soy and other food groups were investigated. Data on diet and other lifestyle factors were obtained from a structured questionnaire with a validated dietary component that was administered in-person to all participants. Few dietary factors emerged as determinants of plasma estrogen levels in this population. An exception was soy, which was significantly associated with plasma E(1) levels. Specifically, E(1) levels were 15% lower among individuals in the highest quartile of soy protein intake compared with those in the lower three quartiles of intake (P = 0.047). E(1) levels did not differ between individuals in the lower three quartiles of soy protein intake. Similar patterns of differences in E(1) levels emerged in analyses by intake of isoflavones and total soy products. These findings on soy were independent of the four nondietary factors (see below) that significantly influenced estrogen levels. Both E(1) and E(2) levels increased with high body mass index [BMI (weight/height(2))]; the respective levels were 41% (two-sided P = 0.02) and 17% higher (P = 0.34) among women in the highest BMI category (BMI >/= 24) compared with those in the lowest category (BMI < 20). After adjustment for BMI and age, women with a late age at menarche (age 17 years or older) showed significantly lower E(1) (30% lower; P = 0.02) and E(2) levels (24% lower; P = 0.02) compared with women with earlier age at menarche (before age 17 years) and who were nulliparous or had a late age at first live birth (after age 31 years). Current smokers showed significantly higher E(2) levels (28%) than nonsmokers (P = 0.04). These findings are discussed in relation to the recent doubling of breast cancer incidence among Chinese women in Singapore.