Despite many favorable findings about soy protein, critics have pointed out several possible negatives. For example, isoflavones, considered the active ingredients of soy, are similar in structure to estrogen, which has led to concerns that eating soy protein may induce side effects related to increased estrogen intake, particularly in men.
On the other hand, naturally occurring isoflavones are only 1 percent as potent as estrogen, so you would have to take in relatively large doses to experience any true estrogen-based side effects. In fact, studies show that isoflavones exert true estrogen effects only in conditions of high estrogen output, such as in young women. The ubiquitousness of substances in the environment exerting estrogenlike effects, however, has led some researchers to link the ever-increasing estrogen pollution to a worldwide decline in male fertility. Other reputable scientists dispute that notion, but the question remains: Will isoflavones exert detrimental effects on male hormones and reproductive ability?
It has been suggested that isoflavones may be beneficial to men in preventing cardiovascular disease and some types of cancer, such as prostate cancer. If you look at the suggested mechanisms for such effects, though, you’ll find that it’s the estrogenlike activity of isoflavones that cause those beneficial effects.
A recent study examined the effects of a commercially available isoflavone supplement on the hormonal and reproductive functions in normal, healthy men.1 Fifteen nonvegetarian men, ages 18 to 35, with normal hormone and reproductive function, used a supplement containing 40 milligrams of isoflavones for two months. They also donated blood and semen samples monthly for two months before and four months after using the isoflavone supplement.
The results showed that while blood levels of isoflavones rose significantly in the men, the increases had no effect on any hormone levels, including estrogen and testosterone, nor did they affect testicular volume or semen quality. The findings are in contrast to those of previous studies done with animals, where isoflavone supplements did adversely affect testosterone levels; however, the doses used in the animal studies were on average 10 times higher than those consumed by any human population.
Isoflavones have a more definite effect in young women, who usually have the highest levels of estrogen to begin with. Will athletes who use anabolic steroids or pro-hormones that aromatize into estrogens’such as androstenedione’also show estrogen-related side effects if they use isoflavone-rich soy protein? No one has yet examined that question, but I’d conjecture that they likely would.
Even if we assume that only large doses of isoflavones or soy protein negatively affect men’s testosterone levels, there are other potential problems from both health and bodybuilding perspectives. In another recent study pigs were fed either casein, a high-quality milk protein, or soy protein for 15 weeks.2 Both protein sources were supplied in identical amounts and calorie content.
The results showed significantly increased muscle protein breakdown in the pigs that got the soybean protein, apparently because of the comparatively unbalanced level of amino acids in the soy compared to the milk protein. The soy protein used in this study lacked sufficient amounts of the essential amino acid methionine. That led to a biochemical cascade that resulted in an upgraded activity of various catabolic processes in muscle, including a more than twofold rise in cortisol levels. Would commercial versions of soy protein, which have added methionine, have the same effect? That question remains to be answered.
Finally, another new study involving 108 men and 105 older women confirmed many of the protective effects of soy protein against cardiovascular disease onset but also brought out some previously unknown possible adverse effects.3 The subjects, who ranged in age from 50 to 75, took either casein or a soy protein isolate containing 118 milligrams of isoflavones for three months. Those using the soy protein showed significantly lowered blood pressure, LDL and triglyceride levels’all good effects.
On the other hand, men in the study showed a 15 percent increase in lipoprotein (a), a substance that’s linked to an increased risk of cardiovascular disease. Lipoprotein (a) is lowered by estrogens and androgens (including anabolic steroids) but increased by growth hormone use. The men in the study also showed decreased vascular function, another adverse cardiovascular effect. As in the first study discussed above, the men getting the soy protein didn’t show any adverse effects on hormone levels. IM
1 Mitchell, J.H., et al. (2001). Effect of a phytoestrogen food supplement on reproductive health in normal males. Clinical Science. 100:613-618.
2 Lohrke, B., et al. (2001). Activation of skeletal muscle protein breakdown following consumption of soyabean protein in pigs. Brit J Nutr. 85:447-457.
3 Teede, E.J., et al. (2001). Dietary soy has both beneficial and potentially adverse cardiovascular effects: a placebo-controlled study in men and postmenopausal women. J Clin Endocrinol Metab. 86:3053-3060.