Influence of soy protein intake on blood isoflavone levels, thyroid and sex hormone concentrations in women

Abstract

T is a current discussion about the hormonal impact of soy isoflavones in food on circulating of thyroid and sex hormones in women. While official statements argue the hormonal side effects, weight loss ads recommend an increased energy output induced by changes in hormonal regulation after soy intake. To describe the possible influence of intake of a commercially available soy-yogurt-honey product (Almased®) used for weight reduction and metabolic functions improvement, 14 non-obese, euthyroid postmenopausal women (64.2±6.3 years; 24.4±3.2 kg/m2) were examined over a period of 8 weeks consuming a standardized amount of the product (25 -125 g/d). The amount of soy-protein-isolate of the product was 44.2 % and the content of biologically available isoflavonoids was 1.45 mg/g powder. Blood sample were drawn to analyze blood ISF levels (genistein, daidzein, glycitein, o-DMA, equol) by a LCMS method as well as basal TSH, fT3 and fT4. Estradiol, progesterone, testosterone and DHEA were also examined. Despite the significant increases in blood ISF concentrations after all intake periods, the baseline levels of all thyroid and sex hormones was stayed within the normal range during the intervention and even after a daily intake of 125 g. ISF, e.g., genistein rose from 3.8±1.9 ng/ml to 519.2±382.7 ng/ml. Results showed that the daily intake of 25-125 gram of the tested product significantly increased the blood ISF levels. It can be assumed that at this amount of ISF intake, the supplement used had no specific influence particularly negative side effects on thyroid and sex hormones in postmenopausal women.