Influence of soy protein intake on blood isoflavone levels, thyroid and sex hormone concentrations in women
There is a current discussion about the hormonal impact of soy isoflavones in food particularly on circulating of thyroid and sex hormones in women. While official statements argue the hormonal side effects, some weight loss ads recommend an increased energy output induced by changes in hormonal regulation after soy intake. Nowadays, none of these aspects are welcome from a medical point of views.
To describe the possible influence of intake of a commercially available soy-yogurt-honey product (Almased®) used for weight reduction and improvement of metabolic functions, 14 non-obese, euthyroid postmenopausal women (64.2 ± 6.3yrs; 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, started with 25 g and continued with 25 g weekly increase). 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. Health status of the participants was documented in weekly controls and 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 levels. 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 tested 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. A significant increase in ISF blood concentrations was observed after an intake of only 25 g of the soy product.
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 function and sex hormones level in postmenopausal women.