All proteins are not created equal! Whether it comes from beef, milk, soy, lentils, peanuts, and so on, each protein source has a different proportion of amino acids. Every week, I read a new study that shows the unique effects of individual amino acids on human or animal physiology. Some proteins, such as those found in grains, aren’t complete— meaning they are deficient in an essential amino acid. Notably, corn protein is low in lysine and tryptophan, thus it is not a “complete protein.”
The U.S. Food and Drug Administration (FDA) and World Health Organization recognize the Protein Digestibility Corrected Amino Acid Score (PDCAAS) as the preferred method for evaluating protein quality, based on both the amino acid requirements of humans and their ability to digest it.1 A PDCAAS value of 1 is the highest, and 0 the lowest.2
PDCAAS of Various Proteins
1.00 casein (milk protein)
1.00 egg white
1.00 soy protein
1.00 whey protein
0.75 black beans
0.70 other legumes
0.59 cereals and derivatives
0.42 whole wheat
Based on the PDCAAS, whey protein, soy protein and egg white are the complete proteins with the best digestibility. Because of this, many in the supplement industry have focused on these sources of protein when developing new products. However, there is still a significant difference in the individual amino acid makeups of each of these various proteins. In particular, there is a significant contrast between soy and whey. Whey has nearly two times the L-threonine and ~25 percent more branched-chain amino acids (BCAAs, including leucine) than soy.3 Alternatively, soy has more than two times the L-arginine content of whey protein.
Scientists have recognized this difference between protein types, and have done many comparison studies on the effects that various proteins have on body composition and recovery from weight training. A great deal of interest has been on the comparison of whey protein to soy protein. Other than economic reasons, comparing the two types of protein is of particular interest because whey is from an animal source and soy is, of course, a plant. Many believe the difference between these two proteins is related to the isoflavones found in soy, but research would suggest differently. In fact, in studies where isoflavone-free soy isolate was used, a significant difference was still noted between whey and soy in their effects on muscle protein synthesis.3
A study reported in 2009 from McMaster University by Tang et al. showed that at rest and after resistance exercise, consumption of whey protein produced ~20-30 percent greater increases in muscle protein synthesis over soy supplementation.5 Multiple studies suggest that the BCAA L-leucine has stimulatory effects on muscle protein synthesis, and it just so happens that whey protein has nearly 30 percent more leucine than soy. Could there be a cause and effect? Perhaps a study that uses an L-leucine-depleted form of whey or leucine-supplemented soy could help explain this.
Some researchers have tried to combine whey, soy and casein to get the best of the three proteins. They suggest that by combining fast, slow and intermediately absorbed proteins, blood levels of amino acids could be better sustained. This doesn’t necessarily translate to better muscle growth or recovery from exercise— as protein synthesis machinery needs a little rest now and then to have a maximal response to food/protein. In ICU patients, constant infusions of nutrients do not work as well at preventing muscle loss as does providing nutrients every two to three hours in a cyclical fashion. Just as bodybuilders have learned to have multiple, small meals every two or three hours, the body responds well to this feeding schedule even in critical condition.
A study published in Nutrition & Metabolism in June 2012 by the same lab out of McMaster University demonstrated further comparisons of soy and whey, and their effects on muscle protein synthesis in the elderly.4 Researchers noted that the elderly do not respond as well to 20 grams of protein as their younger counterparts do. They showed that the higher, 40-gram dose of soy was necessary to get a statistically significant effect on muscle protein synthesis only after exercise, with no effect when given at rest.
While the elderly subjects did not respond to supplementation of soy protein at rest, the whey protein groups given 20 and 40 grams experienced a significant rise in muscle protein synthesis. Furthermore, whey protein demonstrated a dose-dependent effect on muscle protein synthesis with exercise. The authors note that whey protein is made of 12 percent leucine and soy is made of 8 percent; again, this is a 30 percent difference. Researchers also found that more leucine was being oxidized and thus not being used to generate muscle protein in the soy-treated subjects. Multiple studies have confirmed the ability of L-leucine to improve muscle protein synthesis rates in the elderly and to activate the mTOR-signaling pathway.6 Maybe adding L-leucine to soy supplements would improve the quality?
In my opinion, the data really seems to point toward a significant benefit to consuming whey protein supplements over soy. Not only could you get a better muscle-building effect at lower doses, but you also avoid concerns over the phytoestrogenic effects of soy products. I would think that future studies manipulating the levels of leucine in these proteins may answer some questions of the biological differences between soy and whey. The more we learn about how amino acids like L-leucine signal increases in muscle growth, the better we will understand the supplements we already know to work!
Dr. Victor Prisk is a board certified orthopaedic surgeon and IFBB professional bodybuilder in Pittsburgh, PA. Dr. Prisk is an active member of the GNC Medical Advisory Board and creator of the “G.A.I.N. Plan.” He is an NCAA All-American gymnast, champion swing dancer and NPC Welterweight National Champion.
1. Schaafsma G. The protein digestibility-corrected amino acid score. J Nutr 2000 Jul;130(7):1865S-7S.
2. Hoffman JR, Falvo MJ. Protein— Which Is Best? Journal of Sports Science and Medicine 2004;3, 118-130. ISSN symposium, June 2005. Las Vegas, NV.
3. Baer DJ, et al. Whey protein but not soy protein supplementation alters bodyweight and composition in free-living overweight and obese adults. J Nutr 2011;Aug;141(8):1489-94.
4. Yang Y, et al. Myofibrillar protein synthesis following ingestion of soy protein isolate at rest and after resistance exercise in elderly men. Nutrition & Metabolism 2012;June; 9:57.
5. Tang JE, et al. 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 2009;Sep;107(3):987-92.
6. Katsanos CS, et al. A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly. Am J Physiol Endocrinol Metab 2006;291:E381-87.