While we don't yet have any conclusive evidence about soy's estrogenic effects in men, there's plenty of other research that might make you reconsider your soy intake:

  1. Weightlifters and bodybuilders may want to avoid soy, or particularly, soy-protein isolates. A study found that soy protein might actually increase protein breakdown in skeletal muscle (1). The researchers fed both casein and soy protein isolate to pigs for 15 weeks and concluded the following:

    "These data suggest that the inferior quality of dietary soybean protein induces hormonally-mediated upregulation of muscle protein breakdown for recruitment of circulatory amino acids in a post-absorptive state."
  2. A 35-year long Hawaiian study of 8,900 Japanese men and 500 of their wives linked consumption of soy (specifically, tofu) to brain aging, shrinking, and the development of Alzheimer's (2).
  3. Unfermented soy (tofu, soymilk, ice cream, yogurt, soy protein isolate) contains large amounts of phytic acid, which blocks absorption of minerals like calcium, magnesium, iron, and zinc.
  4. Soy is known as a goitrogen, which means that it depresses thyroid function as well as the absorption of iodine (which is crucial to thyroid function) (3).
  5. Most non-organic soy protein isolates are made through a process called "hexane extraction." Hexane, a highly combustible compound, is also neurotoxic.
  6. Studies have shown that soy-fed infants have estrogen levels 17,000 times higher than those fed human or cow's milk (4). This is alarming, because this enormous hormone imbalance can have long-term negative effects on behavior, brain chemistry, and physical development, and the health ministers of the United Kingdom and New Zealand have already advised parents not to use soy formula.

Related:  Does Soy Protein Feminize You?

Related:  The Worst Protein for Fit People

References

  1. Löhrke B1, Saggau E, Schadereit R, Beyer M, Bellmann O, Kuhla S, Hagemeister H. "Activation of skeletal muscle protein breakdown following consumption of soyabean protein in pigs," Br J Nutr. 2001 Apr;85(4):447-57.
  2. Wartman, Kristin, "Not Soy Fast," Civil Eats, December 9, 2010.
  3. Heather Patisaul, "The Pros and Cons of Phytoestrogens," Front Neuroendocrinology, 2010, Oct; 31(4): 400-419.
  4. Prof Kenneth DR Setchell, PhD, Linda Zimmer-Nechemias, MS, Jinnan Cai, MS, James E Heubi, "Exposure of infants to phyto-oestrogens from soy-based infant formula," The Lancet, July 5th, 1997.