Soy Isoflavone Supplementation Does Not Alter Lymphocyte Proliferation and Cytokine Production In Postmenopausal Women

A growing body of evidence has demonstrated that soy isoflavone consumption may protect against the development of various chronic diseases. This protection could be linked to isoflavone-induced alterations in immune function. However, recent in vitro and animal studies suggest that soy isoflavones may either enhance or suppress immunocompetence, depending upon the isoflavone concentration, target tissue, and a number of other factors. To date, no study has investigated the effect of dietary soy isoflavone supplementation on immune parameters in humans. Therefore, the purpose of this double-blind, placebo-controlled, 4 wk intervention trial was to investigate whether supplementation with soy isoflavones alters indices of immune function in postmenopausal women. Twenty healthy women (50-69 yr), who were not on hormone replacement therapy, were randomly divided into 2 treatment groups. The supplemented group (n=10) consumed soy isoflavone tablets (100 mg/d) for 4 wk, while the control group (n=10) received placebo tablets. Fasting blood samples were drawn at baseline and on d 28 to assess specific immune parameters. In addition, plasma concentrations of genistein and daidzein were quantified at baseline and at the end of the intervention period. Despite high individual variability among subjects, there was a significant increase (p<0.005) in plasma isoflavone concentration in the supplemented group. However, all assessed immune parameters remained unchanged after supplementation and did not differ between the 2 treatment groups. In conclusion, this study suggests that short-term soy isoflavone supplementation at physiologically attainable concentrations does not alter the aforementioned immune parameters in healthy postmenopausal women. Due to the conflicting data concerning the effect of dietary soy isoflavones on immune function, further research in this area is warranted.