Effect of Soy in Men With Type 2 Diabetes Mellitus and Subclinical Hypogonadism: A Randomized Controlled Study

Context: Isoflavones found in soy products have a chemical structure similar to estrogen, leading to concerns of an adverse estrogenic effect in men, particularly in those with type 2 diabetes mellitus (T2DM) who have low testosterone levels due to hypogonadism. Objective: The primary outcome was change in total testosterone levels. The secondary outcomes were the changes in glycemia and cardiovascular risk markers. Design: This was a randomized double-blind parallel study. Setting: This study occurred in a secondary care setting in United Kingdom. Participants: Two hundred men with T2DM and a total testosterone level ⩽12 nmol/L were included. Intervention: Fifteen grams of soy protein with 66 mg of isoflavones (SPI) or 15 g soy protein alone without isoflavones (SP) daily as snack bars for 3 months were administered. Results: There was no change in either total testosterone or in absolute free testosterone levels with either SPI or SP. There was an increase in thyrotropin (TSH) and reduction in free thyroxine (fT4; P < 0.01) after SPI supplementation. Glycemic control improved with a significant reduction in hemoglobin A1c (−4.19 [7.29] mmol/mol, P < 0.01) and homeostasis model of assessment - insulin resistance after SPI. Cardiovascular risk improved with a reduction in triglycerides, C-reactive protein, and diastolic blood pressure (DBP; P < 0.05) with SPI vs SP supplementation. There was a 6% improvement in 10-year coronary heart disease risk after 3 months of SPI supplementation. Endothelial function improved with both SPI and SP supplementation (P < 0.01), with an increased reactive hyperemia index that was greater for the SPI group (P < 0.05). Conclusions: Testosterone levels were unchanged and there was a substantial improvement in glycaemia and cardiovascular risk markers with SPI compared with SP alone over 3 months. There was also a substantial increase in TSH and a reduction in fT4.