NutritionSoy Protein

Sports Nutrition Paper Falls Short In Analysis of Soy Protein and Strength

A new position paper on protein and exercise from the International Society of Sports Nutrition (ISSN) provides useful information for health professionals.1  But it falls short in its conclusions about soy protein.

The paper presents 14 conclusions/recommendations about protein and exercise including the following:

  • It is not necessary to consume protein within a limited timeframe following exercise for maximal stimulation of muscle protein synthesis (MPS)
  • MPS is likely stimulated by both pre- and post-workout ingestion.
  • The post exercise meal should contain 700–3,000 mg of leucine in addition to a balanced array of the essential amino acids since leucine plays a key role in stimulating MPS.
  • For building muscle mass and for maintaining muscle mass through a positive muscle protein balance, overall daily protein intake should be in the range of 1.4–2.0 g protein/kg body weight/day
  • Protein intake should ideally be evenly distributed, every 3–4 hours, across the day
  • While it is possible for physically active individuals to obtain their daily protein requirements through the consumption of whole foods, supplementation is a practical way of ensuring intake of adequate protein quality and quantity, while minimizing caloric intake, particularly for athletes who typically complete high volumes of training.

The above cited conclusions and recommendations from the ISSN are well supported by the literature. Their comments about soy protein are not.

The ISSN concluded that soy is considered a lower quality complete protein. They based this conclusion on a few studies showing soy protein was not as effective as milk or whey protein at stimulating MPS in an acute setting or at increasing muscle mass in longer-term studies.  However, they failed to include any of the many studies showing that among individuals engaged in resistance exercise training, there is little if any difference in strength increases between those consuming animal protein (including whey) and those individuals consuming soy protein.2-5

Although lean tissue accretion is often greater in response to dairy protein, it appears this difference is entirely attributable to the greater leucine content of whey protein in comparison to soy protein.  With very few exceptions, corn protein being one, whey is considerably higher in leucine than other proteins.  However, athletes can compensate for the lower concentration of leucine by simply consuming a bit more protein.

Strangely, the ISSN highlighted a study that found the consumption of rice protein led to similar increases in lean tissue and strength in men enrolled in a resistance exercise program as men consuming whey protein.   However, the ISSN neglected to mention that in this study the participants received 48 g/d of the test protein.6 Rice protein is no higher in leucine than soy protein, but at the intake level used in this study, the leucine threshold would have been met.  The leucine threshold is the amount of leucine required to maximally stimulate MPS. It’s likely that a similar result would have been observed if 48 grams of soy protein had been compared to 48 grams of whey protein.

Finally, the ISSN noted that research has found that soy phytoestrogens inhibit mTOR expression in skeletal muscle. Activation of the mechanistic target of rapamycin (mTOR) complex 1 plays a key role in stimulating MPS.7 However, the paper does not mention that this inhibition occurred in mice. This should have been noted given the limitations of nutrition research in rodents. Importantly, mice metabolize isoflavones differently than humans.

In contrast to the views of the ISSN, the literature shows that soy protein can be used to increase strength in response to resistance exercise. Stay tuned for more on this in an upcoming blog post.


  1. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition Position Stand: protein and exercise. J Internal Soc Sports Nutr. 2017;14:1-25.
  2. Haub MD, Wells AM, Tarnopolsky MA, Campbell WW. Effect of protein source on resistive-training-induced changes in body composition and muscle size in older men. Am J Clin Nutr. 2002;76:511-7.
  3. Hartman JW, Tang JE, Wilkinson SB, et al. Consumption of fat-free fluid milk after resistance exercise promotes greater lean mass accretion than does consumption of soy or carbohydrate in young, novice, male weightlifters. Am J Clin Nutr. 2007;86:373-81.
  4. Candow DG, Burke NC, Smith-Palmer T, Burke DG. Effect of whey and soy protein supplementation combined with resistance training in young adults. International journal of sport nutrition and exercise metabolism. 2006;16:233-44.
  5. Kalman D, Feldman S, Martinez M, Krieger DR, Tallon MJ. Effect of protein source and resistance training on body composition and sex hormones. Journal of the International Society of Sports Nutrition. 2007;4:4.
  6. Joy JM, Lowery RP, Wilson JM, et al. The effects of 8 weeks of whey or rice protein supplementation on body composition and exercise performance. Nutrition journal. 2013;12:86.
  7. Anthony JC, Yoshizawa F, Anthony TG, Vary TC, Jefferson LS, Kimball SR. Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway. J Nutr. 2000;130:2413-9.
Dr. Mark Messina

Author Dr. Mark Messina

PhD in Nutrition, Executive Director, Soy Nutrition Institute. Expert in soyfoods and isoflavones.

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