Isoflavones are a subgroup of phytoestrogens and have a chemical structure similar to oestrogen, leading to concerns regarding possible adverse effects of isoflavones in men. Testosterone levels in men with type 2 diabetes mellitus (T2DM) are lower.
Materials and methods: A randomised double-blind, parallel study was undertaken with 200 men with T2DM, of median age of 50 years (25th/75th centiles; 50, 55) years with a total testosterone level ≤12 nmol/l and normal gonadotrophins. They were randomised and administered either 30 g soy protein with 66 mg isoflavones (SPI)/day, or 30 g soy protein alone without isoflavones (<300 parts/billion) (SP) for 12 weeks.
Results: There was no significant change in serum total testosterone with either SPI or SP supplementation. There was no significant changes in absolute free testosterone levels with either SPI or SP. DHEAS reduced significantly with SPI and increased with SP suggesting SPI reduces adrenal androgens rather than gonadal androgens. Regarding glycaemic control, there was a significant reduction in HbA1c, fasting glucose, fasting insulin, and HOMA-IR after SPI that were significantly greater than the changes observed in SP. Triglycerides and hsCRP reduced significantly with SPI that was not seen with SP supplementation. Diastolic blood pressure reduced significantly after both SPI and SP. There were no changes in systolic blood pressure in either group. There was a significant improvement in endothelial function with an increased reactive hyperemia index (RHI) with SPI whereas there was a significant reduction with SP administration.
Conclusions: It is reassuring that soy protein with and without 66 mg isoflavone/day do not have any effects on total and free testosterone levels in men with T2DM, and who had compromised gonadal function with low testosterone levels, after 12 weeks. There was a significant improvement in both glycaemic control and cardiovascular risk markers including lipids, diastolic blood pressure, and hsCRP with the soy protein/isoflavone combination compared to soy protein alone.