Men do not produce less testosterone as a result of eating soya proteins. American nutritionists came to this conclusion after doing a meta-analysis of other studies in which men had been given proteins derived from soya or soya-isoflavones.
The researchers confined their study to trials in which subjects had been given proteins or isoflavones, and their hormone levels were then compared with those of other men. Isoflavones are diphenols attached to soya protein, and they can interact with the beta receptor for estradiol. One gram of soya protein represents about 3.5 milligrams of isoflavones – not including any sugar chains that might be involved.
The highest intake in the trials that the researchers used was 450 milligrams of isoflavones per day. In the trials in which the men were given proteins, the highest intake was 70 grams per day. The average length of the trails was 74 days. The figure below shows the effect on the total testosterone level. The diamond at the bottom of the figure represents the effect of all studies together. The researchers were unable to find sufficient data on the effect on bioavailable testosterone.
The effect that soya and isoflavones have may well depend on the – probably genetically determined – bacteria population in your intestines. The researchers describe a study in which all forty test subjects were given 110 mg of isoflavones daily. The concentration of isoflavones in their blood varied by a factor of 1500. And the amount of the active metabolite equol varied by a factor greater than 1000. “This finding is not surprising because only approximately 25% to 35% of Westerners possess the intestinal bacteria capable of producing equol”, the researchers write.
The researchers also looked at the effect on SHBG. This effect was even smaller than the effect on testosterone.
If the findings are correct, then three-quarters of the western male population are not absorbing enough isoflavones to experience any hormonal effects from soya proteins. If westerners are experiencing effects, they still may not be negative. There are some animal studies in which the experimental animals actually started to make more androgens as a result of long-term exposure to isoflavones. [Reprod Biol. 2006 Nov;6(3):275-90.] It’s possible that soya has the same effect in human males: soya increases the conversion of estradiol into inactive metabolites in men. [J Nutr. 2007 Oct;137(10):2258-63.] That might be why soya appears to raise the concentration of androgens in trials on men with a low testosterone level. [Fertil Steril. 2007 Dec;88(6):1632-6.] This effect has also been recorded in normal males. [Eur J Clin Nutr. 2006 Dec;60(12):1423-9.]
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