Keep in mind that Soy contains high levels of plant estrogens that have differing effects on men and women. For every study you find showing that soy's perfectly fine for you, you can find one where it says men might not want to consume it. Consider that the US is the largest consumer of soy in the world and that it's used as a bulk filler in the vast majority of processed products in our supermarkets it's not hard to assume funding for pro-soy research is much easier to come by. It is known that estrogens consumed in the diet can have massively detrimental effects on males of any species including humans but estrogens are a very diverse group of steroids that can have different effects based on their form/source. Here's a study looking specifically at soy in male hormone levels:
Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males
Authors: Habito RC, Montalto J, Leslie E, Ball MJ
Published in: Br J Nutr, 84(4): 557-63 2000
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, androstanediol glucuronide, oestradiol, sex hormone-binding globulin (SHBG), and the free androgen index (total testosterone concentration/SHBG concentration 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 providing an equivalent amount of macronutrients, with only the source of protein differing between the two diets. Each diet lasted for 4 weeks, with a 2-week interval between interventions. Fasting blood samples were taken between 07.00 and 09.30 hours. Urinary excretion of genistein and daidzein was significantly higher after the tofu diet (P < 0.001). Blood concentrations of sex hormones did not differ after the two diets, but the mean testosterone
estradiol 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 between the difference in SHBG and testosterone
estradiol and weight change. Adjusting for weight change revealed SHBG to be 8.8% higher on the tofu diet (mean difference 3 (95% CI 0.7, 5.2) nmol/l; P = 0.01) and testosterone
estradiol to be significantly lower, P = 0.049). Thus, replacement of meat protein with soyabean protein, as tofu, may have a minor effect on biologically-active sex hormones, which could influence prostate cancer risk. However, other factors or mechanisms may also be responsible for the different incidence rates in men on different diets.
