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Soy Vs. Whey: Which Protein Is Better?

Soy protein gets a bad rap among strength athletes. If you’re a male athlete focused on gaining muscle mass, you probably know all the talking points by now. Soy will will raise your estrogen levels and decrease testosterone. Its biological value is too low. Soy’s amino profile isn’t really complete. It contains “anti-nutrients” like lectin and protease inhibitors that interfere with digestion. It will lower your energy levels. And, of course, real athletes don’t eat tofu.

The Good News About Soy

Some of the above charges about soy are true, some only partially true, and others false. Given its bad reputation, it might surprise you to learn that soy protein actually has a number of distinct nutritional and wellness benefits in its favor. Soy, for instance, has been linked in clinical testing to improvements in thyroid function in test animals. In fact, soy protein may be superior to other proteins in its effect on increasing thyroid hormone output, which plays a key role in successful weight loss. Thyroid hormone has a catabolic effect on dietary fats and carbohydrates, while promoting protein synthesis. When faced with caloric restriction, your body will actually decrease thyroid hormone production in response, as a way of preserving body fat. Soy protein, then, may help dieters cut body fat over a longer sustainable interval.

There is also evidence that soy protein intake may have beneficial antioxidant effects that would reduce post-workout muscle soreness and inflammation. A study published in the Journal of Sports Nutrition and Exercise Metabolism determined that the phytochemical antioxidants in soy were superior in decreasing the plasma serum peroxide values associated with delayed onset muscle soreness (DOMS), as compared to whey consumption over a 4-week period.

Soy has also been associated in some research with positive effects on blood sugar and may help maintain healthy blood cholesterol and triglyceride levels when already in the normal range. Finally, soy is high in the aminos arginine (associated with muscle vasodilation and nutrient delivery) and glutamine (a building block of recovery).

Soy Protein and Muscle Building

So it turns out that a case can be made for soy, right? Okay, we know what you’re thinking. But what about muscle growth? How does soy help me there? Well, here the picture isn’t quite as rosy. Soy simply does not have the nitrogen-retention, muscle-building capacity of whey protein or meat sources of protein, and this is partly due to methionine levels in soy.

Soy is actually unique among plant-based proteins in that it contains all nine of the essential amino acids needed to support muscle repair and recovery. However, conventional soybeans contain lower levels of methionine (one of those nine EAAs). Methionine is a sulfur-containing amino found primarily in meat, fish, and dairy products that is important for protein synthesis and growth, proper immune system function, and the body's production of glutathione (GSH). Manufacturers of soy-based products have actually acknowledged this problem by, for instance, adding methionine to soy-derived animal feed. Soy protein isolate, an almost a pure form of soy protein that contains at least 90 percent protein with most of the carbs and fat removed, also often has methionine added to it. Raw soybeans, tofu or typical soy protein concentrates, however, are still going to be deficient in methionine.

And then, too, there’s the matter of Biological Value (or BV). Soy protein is a slow-absorbing protein source with a BV roughly approximating that of casein. It has a significantly lower BV than that of whey protein, whole eggs, egg whites, beef or chicken. Its BV is certainly superior to that of purely plant-based sources like wheat or pea protein (which have incomplete amino profiles), but it’s not in the same class as whey protein for sustaining muscle growth and repair.

Indeed, a study published in the Journal of Applied Physiology compared the impact of soy and whey protein on changes in muscle protein synthesis at rest as well as after resistance exercise. In the study, young college-aged men completed three experimental trials in which they ingested similar amounts of either whey, casein or soy protein. Muscle protein synthesis was measured both at rest and after a single bout of resistance exercise. At rest, rates of muscle protein synthesis after whey protein ingestion were 18% greater than when soy protein was ingested.  When the same response was measured after completing a single bout of heavy resistance exercise, the results favored whey protein even more, leading to 31% higher values of muscle protein synthesis.  


Finally, it should be noted that soy protein contains lectins and protease inhibitors that can inhibit the absorption of important nutrients. The enzymes trypsin and chymotrypsin are important for the digestion and absorption of proteins in the gastrointestinal tract. The lectins and protease inhibitors in soy interfere with this process, allowing critical proteins to pass unabsorbed. Clearly, not an outcome you're looking for when trying to maximize protein intake.

Soy Protein and Hormonal Response

Ah yes, the elephant in the room. What about those phytoestrogens in soy? Aren’t they inhibiting testosterone levels and undermining muscle growth? It turns out, not nearly as much as had been previously assumed. While there have always been studies that suggest that the phytoestrogens in soy may effect hormone levels in men, more recent studies suggest its impact on free testosterone levels may have been over-estimated.

In a study published in the Journal of the International Society of Sports Nutrition, 20 test subjects supplemented for twelve weeks with 50 grams per day of one of four different protein sources (soy concentrate, soy isolate, soy isolate and whey blend, and whey blend only) in combination with a resistance-training program. Body composition, testosterone, estradiol and sex hormone binding globulin (SHBG) were measured at baseline and week 12. No significant differences were observed between the four groups for total and free testosterone or SHBG. These results led researchers to conclude that 12 weeks of supplementation with soy protein did not decrease serum testosterone or inhibit lean body mass changes in subjects engaged in a resistance exercise program.

What accounts for the discrepancy between this finding and previous beliefs about soy protein intake and hormonal response? Researchers theorize that the mixed estrogen agonist and antagonist properties in estrogenic compounds in soy may simply be "tissue specific” and have no apparent negative effects on the reproductive system. This is a provisional theory at best and is subject to much more research at this time.


In the final analysis, is there a place for soy protein in your dietary regimen? The answer is probably yes, although in modest amounts compared to your intake of whey and animal sources of protein. Soy protein does provide some antioxidant, fat loss, and cholesterol-moderation benefits and isn’t nearly as negatively impactful on male hormone levels as was previously assumed. It is also, it should be said, much less stressful on our global environment than animal and dairy production (for those of you who like to think green). For muscle growth and physique enhancement, however, whey is the way to go.

Scientific References

Forsythe, W. A. (1995). Soy protein, thyroid regulation and cholesterol metabolism. The Journal of Nutrition, 125(3), 619S.

Rossi, A., Disilvestro, R. A., & Blostein-Fujii, A. (1998, March). Effects of soy consumption on exercise-induced acute muscle damage and oxidative stress in young adult males. In FASEB Journal (Vol. 12, No. 5, pp. A653-A653). 9650 Rockville Pike, Bethesda, MD 20814-3998 USA: Federation of American Societies for Experimental Biology.

Tang, J. E., D. R. Moore, G. W. Kujbida, M. A. Tarnopolsky and S. M. Phillips (2009). "Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men." J Appl Physiol 107(3): 987-992.

Kalman, D., Feldman, S., Martinez, M., Krieger, D. R., & Tallon, M. J. (2007). Effect of protein source and resistance training on body composition and sex hormones. Journal of the International Society of Sports Nutrition, 4(1), 4.


The articles featured herein are for informational purposes only and should not be construed as medical advice. Specific medical advice should only be obtained from a licensed health care professional. No liability is assumed by ProSource for any information herein.