Depending on whom you ask, soy products are either a great boon to mankind or the worst crap on Earth. Those who advocate the use of soy in its various forms, including soy protein, tofu and soy flour, point to many studies that show protective effects. A study of Seventh Day Adventist men, for example, associated drinking soy milk with a 70 percent reduction in prostate cancer.
In vitro, or isolated-cell, studies show that the primary active ingredients in soy, isoflavones, can block the growth of prostate cancer cells. Soy also appears to offer protective effects against estrogen-related cancers in women, such as breast and uterine cancers. Incidence of these types of cancer is lower in Asian countries, where soy intake is higher.
Those who eschew soy in any form point out that soy isoflavones are phytoestrogens, or plant estrogens. Although the naturally occurring estrogenlike compounds have only about 1/10,000 the potency of direct estrogen, they can still interact with hormones in the body. In fact, that's the main controversy about soy: It may not only exert estrogenlike effects in men but also interfere with androgen, or testosterone, activity.
Scientists like soy's interference with testosterone because test is linked to prostate cancer. While little or no evidence indicts it as a direct cause of prostate cancer, it does speed the growth of that disease. Many forms of therapy used to treat prostate cancer block the effects of testosterone and such other androgens as dihydrotestosterone (DHT). Blocking androgens before the prostate cancer has spread can lead to a shrinkage or even disappearance of the cancer. Prostate cancer is unaffected by any other type of androgen.
From a bodybuilding perspective it's difficult to justify the use of soy protein. Although considered a complete protein, soy is naturally low in the essential amino acid methionine. Companies that make soy products circumvent that problem by adding methionine, which makes soy protein compare favorably to milk and egg proteins. Like whey protein, it's also rich in the beneficial branched-chain amino acids.
Those pesky soy isoflavones, though, may still interfere. Studies examining the effects of soy protein on testosterone are equivocal, with some indicating an interference and others showing no effect.
In the latest study to examine the issue, 35 men, aged 20 to 40, got either a milk protein isolate or one of two types of soy protein, one having a high isoflavone content and the other having a low isoflavone content.1 After 57 days those getting the soy proteins showed lower levels of DHT than those in the milk group.
DHT is testosterone's evil twin. It has little or no anabolic effect in muscle but promotes prostate enlargement and cancer, acne and male-pattern baldness. Soy lowered DHT without influencing 5-alpha reductase, the enzyme that converts testosterone into DHT. Only the low-isoflavone soy lowered total testosterone, and that occurred at the 29th day of the study. Free, or active, testosterone was also lowered but not to a significant degree. The low-isoflavone soy also increased DHEA-S, the circulating form of the adrenal androgen DHEA in the body. That may have affected the increased estrogen level seen in the low-isoflavone group at day 5, since DHEA is usually converted into estrogen in younger men.
Collectively, these results don't paint a pretty picture of the effects on young men who use soy. The mysterious aspect of the study, which the authors didn't explain, was why only the low-isoflavone soy produced those hormonal effects. Since the isoflavones are considered the active components of soy that affect hormonal activity, the higher-isoflavone soy should have produced the greatest effects, but that wasn't the case. There may be something else in soy that affects hormones. IM
1 Dillingham, B.L.,et al. (2005). Soy protein isolates of varying isoflavone content exert minor effects on serum reproductive hormones in healthy young men. J Nutr. 135:584-91.