Soy Protein, Weightlifting, and Testosterone

A reader asked me to comment on a study comparing soy protein, whey protein, and a carbohydrate placebo and their effects on hormones (1).

The study was mainly conducted to see if soy increased estrogen levels. Unsurprisingly, it did not.

On all 3 supplements, testosterone levels went up during the workout. But compared to both whey and carbohydrate placebo, testosterone levels were not sustained as high post-workout. And cortisol, which can break down muscle tissue for energy, was higher on the soy and carbohydrate regimens than on whey.

The study was done on 10 young men who took each supplement for 2 weeks, with washout periods between regimens. The supplements were taken 30 minutes before a bout of exercise: 6 sets of squats, 10 repetitions each, 2 minutes rest between sets, weight was 80% of their maximum. The hormones were measured before, during, and at a number of times after the exercise up to one hour.

All 3 supplements were 80 calories worth of energy.

It’s possible that the lower testosterone levels are a result of either the much lower amounts of leucine in soy versus whey protein (about 39% lower) or due to the isoflavones. The authors of the study say, “The majority of evidence from previous research on isoflavones alone has shown no effect on testosterone,” making me think it’s the lower leucine leading to lower testosterone.

As for the cortisol, that soy didn’t perform as well as carbohydrate is odd and might be due to the fairly intense workout, soy and carbohydrate caused higher cortisol levels. When I lift weights, I try not to push my muscles to the point where I’m breaking down muscle for a significant portion of energy, and six intense sets of squats to 10 reps seems like a lot. The carbohydrate, or the extra leucine in the whey (which muscles can use for energy), could have been enough to prevent muscle from being degraded for energy.

It was not clear from the write-up whether the participants were blinded to which regimen they were on, though I assume they were. If not, that could have played a role.

More research is needed – we don’t even know if this would result in a noticeable difference in strength gains, particularly on a different weight-lifting program that isn’t quite so focused on so many reps for the same muscle groups.

But if you are a serious weightlifter and this worries you, adding some leucine to your soy protein powder, or a non-soy protein powder, might be the way to go. Or perhaps just carbohydrate instead, to prevent the cortisol response.

The Vegetarian Resource Group has information on where leucine is sourced in their 2011 article, Leucine, Isoleucine, and Valine May Be Derived from Duck Feathers or Human Hair.

Addendum 12-31-2013:

For testosterone, whey and carbohydrate achieved similar results. Soy was not as high from 5 to 30 minutes post exercise, but was about the same at 60 minutes. Although the differences were statistically significant, they were not all that great – the biggest difference was about 30% between soy and whey.

For cortisol, I had said that soy didn’t perform as well as carbohydrate, but the differences were so small that I shouldn’t have said this. Soy and carbohydrate resulted in statistically significant, higher levels than whey at 5, 15, and 30 minutes post exercise. The biggest difference between soy and whey was about 45%.


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1. Kraemer WJ, Solomon-Hill G, Volk BM, Kupchak BR, Looney DP, Dunn-Lewis C, Comstock BA, Szivak TK, Hooper DR, Flanagan SD, Maresh CM, Volek JS. The effects of soy and whey protein supplementation on acute hormonal reponses to resistance exercise in men. J Am Coll Nutr. 2013;32(1):66-74. | link

4 Responses to “Soy Protein, Weightlifting, and Testosterone”

  1. Fredrik Says:

    Doesn”t the fact that carbohydrates prevented muscle breakdown point to a calorie deficiency rather than a difference in amino acid profile? Or did I misunderstand the results?

    Good to have yet another article that debunks the soy haters estrogen argument.

  2. Jack Norris RD Says:


    I misspoke about the soy being worse than carbohydrate for cortisol — the differences were quite small. I also added an addendum at the end of the article with a bit more information.

    The caloric content of all 3 regimens was 80 kcal (as I mention above). In recognizing the soy and carbohydrate performed about the same for cortisol, it seems to me that the muscles preferring leucine, either for energy or repair, is the most likely the explanation.

    But, I don’t know if this would make a noticeable difference in strength or muscle size.

  3. Ashley Says:

    This is not related to this article, but has been bothering me for days. My husbands aunt (not vegan) is in the currently in the hospital with bronchitis, pneumonia, and a few others that hit all at once. Anyway, the doctor said that she had absolutely no protein in her body because all the carbs she was eating had cancelled it out. It sounds like bs to me (doctors.. nutrition..), but is there any truth to that? Seems like that would be a big problem for us vegans if it were true.

  4. Jack Norris RD Says:


    Taken literally, that makes no sense. But what the doctor might have meant was that your husband’s aunt wasn’t eating enough protein for her body to properly fight her infection, and that she didn’t have enough protein in her diet because she was eating mostly carbohydrate.

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