Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males

A randomised crossover dietary intervention study was performed to evaluate the effects of replacing meat protein in the diet with a soyabean product, tofu, on blood concentrations of testosterone, dihydrotestosterone, andros tanediol glucuronide, oestradiol, sex hormone-binding globulin (SHBG), and the free androgen index (total testosterone concentration/SHBG concentratio n x 100; FAI). Forty-two healthy adult males aged 35-62 years were studied. Diets were isoenergetic, with either 150 g lean meat or 290 g tofu daily p roviding an equivalent amount of macronutrients, with only the source of pr otein differing between the two diets. Each diet lasted for 4 weeks, with a 2-week interval between interventions. Fasting blood samples were taken be tween 07.00 and 09.30 hours. Urinary excretion of genistein and daidzein wa s significantly higher after the tofu diet (P<0.001). Blood concentrations of sex hormones did not differ after the two diets, but the mean testostero ne:oestradiol value was 10 % higher (P=0.06) after the meat diet. SHBG was 3 % higher (P=0.07), whereas the FAI was 7 % lower (P=0.06), after the tofu diet compared with the meat diet. There was a significant correlation betw een the difference in SHBG and testosterone:oestradiol and weight change. A djusting for weight change revealed SHBG to be 8.8 % higher on the tofu die t (mean difference 3 (95 % CI 0.7, 5.2) nmol/l; P=0.01) and testosterone:oe stradiol to be significantly lower, P=0.049). Thus, replacement of meat pro tein with soyabean protein, as tofu, may have a minor effect on biologicall y-active sex hormones, which could influence prostate cancer risk. However, other factors or mechanisms may also be responsible for the different inci dence rates in men on different diets.