Soy protein – your body doesn’t always use it

Soy protein has had a bad reputation for years. Bodybuilders worry not only that it’s often an incomplete protein but also that it may have potent hormonal effects similar to estrogen, and that consumption may lower testosterone levels and cause estrogenic effects. Though the evidence has shown that ingested soy protein doesn’t in fact produce these effects, the debate still rages, with some experts recommending that male bodybuilders avoid ingesting soy protein altogether. A new study might just make the whole argument moot. The real reason you might not bother to consume soy protein is your body simply might not use it.

The study examined the retention of consumed wheat-derived soy protein, both before ingesting other forms of protein and afterward, to measure the effects of a high-protein diet on soy protein retention and utilization. The researchers found that when protein from other sources was eaten prior to ingesting soy protein, soy protein utilization and nitrogen retention was reduced dramatically.


Although you shouldn’t avoid soy protein on the basis of possible hormonal issues, you should limit its consumption if you’re already eating a lot of protein from other sources. In the presence of these other types of protein, your body shies away from using soy protein in favor of the other kinds, so it becomes of limited benefit for muscle building.


Juillet B, Fouillet H, Bos C, et al. Increasing habitual protein intake results in reduced postprandial efficiency of peripheral, anabolic wheat protein nitrogen use in humans. Am J Clin Nutr. 2008;87:666-78.

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