OK so many men are under the impression that soy protein is bad and riddled with dangerous side effects to the point it is going to turn them into a girly man. Others will tell you soy is the greatest thing since sliced bread (if you're a grain fearing paleo person then let's go with sliced almonds).

Where are people getting this from?

So this is not pulled out of thin air, it originates from the fact that soy beans have isoflavones which have a mild estrogenic effect. Oh my nooooo, put that edamae down! Wait take a breath as we must know how much of an effect something has before freaking out.

What does the evidence say about the dangers and benefits?

In a well designed 12 week double blind, randomized controlled trial researchers took 20 subjects that met the inclusion criteria and put them into four groups. Each group participated in a resistance training program and took 50g of protein from either soy concentrate, soy isolate, soy isolate and whey blend, or whey only. The researchers measured body composition, testosterone, estradiol and sex hormone binding globulin (SHBG) before the intervention and week 12. As noted in the table below estradiol decreased in all groups, even the soy groups! As a result of the decreased estrogen levels and the insignificant effect on testosterone the testosterone/estradiol level increased .¹

In addition to the study discussed above a recent meta analysis and subsequent review found that neither soy nor isoflavones have a significant effect on total or free testosterone levels. ^2,3

What about Soy Lecithin in my Foods or Supplements?

Soy lecithin has no soy (even if it did the amount of lecithin used is minute and as you now know there is nothing to fear), rather it is just lecithin derived from soy. Lecithin is added to protein powder supplements as it is a good emulsifier.

What's the bottom line of all of this?

At the end of the day soy is a rich source of protein that can be utilized by bodybuilders, average joe's, other athletes, etc... just don't overpay for it as it should cost less than whey, casein, and egg.

For those of you who don't like reading, here's a video I made on the topic.

(//www.youtube.com/watch?feature=player_embedded&v=uM5pqRzqbcc)

References

1. Kerksick CM, et al. Impact of differing protein sources and a creatine containing nutritional formula after 12 weeks of resistance training. Nutrition 2007 Sep;23(9):647-56. [Pubmed]
2. Hamilton-Reeves JM, et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010 Aug;94(3):997-1007. doi: 10.1016/j.fertnstert.2009.04.038. Epub 2009 Jun 12. [Pubmed]
3. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence.Fertil Steril.2010 May 1;93(7):2095-104. Epub 2010 Apr 8. doi: 10.1016/j.fertnstert.2010.03.002. [Pubmed]