a1 School of Biological and Chemical Sciences, Deakin University, Melbourne, Australia
a2 Department of Clinical Biochemistry, Royal Children's Hospital, Melbourne, Australia
a3 School of Health Sciences, Deakin University, Melbourne, Australia
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×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: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 between the difference in SHBG and testosterone:oestradiol 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:oestradiol 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.
(Received March 30 1999)
(Revised February 21 2000)
(Accepted April 24 2000)