Soy Intake and Insulin‐like Growth Factor‐I Levels among Women in Hawaii and Japan

Abstract

Insulin-like growth factor-I (IGF-I) has been proposed as the link between diet and breast cancer risk. Due to their estrogen-like structure, soy isoflavones may affect IGF-I levels in a similar way as exogenous estrogens. In a cross-sectional design, we compared IGF-I levels between women with high and low soy intake. The analysis included 611 pre- and postmenopausal women: Japanese in Japan (JJ), and Japanese (JH) and Caucasians (CH) in Hawaii. The subjects had participated in a previous study, were never diagnosed with breast cancer, and provided a screening mammogram and blood sample. The same lab analyzed all serum samples for IGF-I and IGF binding protein (IGFBP)-3 by double-antibody ELISA. Soy intake was assessed by validated food frequency questionnaires. We estimated mean IGF-I and IGFBP-3 levels by tofu intake after adjustment for covariates. The respective mean IGF-I levels were 213, 257, and 255 ng/ml for JJ, JH, and CH. Tofu intake was higher among JJ than among JH and CH (11.0 vs. 9.4 and 4.9 g/1000kcal). Mean IGF-I levels were 11% lower (p = 0.01) and mean IGFBP-3 levels were 5% higher (p = 0.22) among women in the highest tofu intake category as compared to the lowest, but the relation was strongest among JJ women. These findings suggest that a diet rich in soy foods may be related to lower IGF-I levels. This project was funded by NCI grants R03 CA105948 and R01 CA80843 and by AICR grant 02A021.