Supplementing Plant-Based Diets with Soy Protein Increases Muscle Mass and Strength

Considerable work has been devoted to determining the optimal protein for building muscle mass and strength in response to resistance exercise training (RET). Until recently, there was a consensus within the scientific community that whey protein performed better than other proteins in this regard.¹ However, studies published over the past couple of years showed that soy protein performed as well as whey.² Now comes a study utilizing a novel design that confirms the role soy protein can play in building muscle mass and strength.³

The superiority of whey protein over other proteins, including soy, is based on the results of acute studies measuring changes in muscle protein synthesis over a 3–4-hour period, not on long-term studies measuring gains in muscle mass and strength. These acute findings are consistent with the high leucine content of whey. Most proteins are approximately 8% leucine (1 of 3 branched chain amino acids) whereas whey is 10 to 14% leucine.^3,4 This difference is relevant because leucine is generally considered to be the key amino acid for triggering muscle protein synthesis.⁵

In 2020, researchers showed that supplementing diets with more soy protein than whey (26 vs. 19 g/d), so that leucine intake would be similar, would lead to similar gains in muscle mass and strength in untrained men and women who underwent RET 3 times per week for 12 weeks.⁶  However, the results of a meta-analysis published two years earlier that included nine studies, showed that it was unnecessary to provide more soy protein than whey protein to produce the same results.² Four of the studies compared soy with whey, and five compared soy with beef, milk, or dairy protein. The authors concluded that “… soy protein supplementation produces similar gains in strength and LBM [lean body mass] in response to RET as whey protein.”

One limitation to the studies cited above is that, in all cases, participants were following an omnivorous diet. Whether the same results apply to those following a vegan diet, could not be determined from the existing literature. That is, until the recent publication of a Brazilian study designed to answer that question.³

For this study, 19 young men who were habitual vegans (age, 26 ± 5 y) and 19 young men who were omnivores (age, 26 ± 4 y) undertook a 12-week, twice weekly, supervised resistance training program involving the lower body. Vegans were supplemented with soy protein and the omnivores, whey protein. The goal was for each group to consume protein in the amount of 1.6 g/kg body weight (bw), which is the amount recently recommended by a team of internationally recognized experts for increasing strength and lean body mass.⁷ Achieving this goal necessitated that the vegans consume on average 58 g/d (0.79 g/kg bw) of soy protein and the omnivores, 39 g/d (0.52 g/kg bw) of whey protein.

At study completion, there were statistically significant increases in both groups in leg lean mass, whole muscle, and muscle fiber cross-sectional area as well as the maximum leg-press, but there were no differences between groups.³ The authors concluded that “a high-protein (~1.6 g/kg/d), exclusively plant-based diet (plant-based whole foods + soy protein isolate supplementation) is not different than a protein-matched mixed diet (mixed whole foods + whey protein supplementation) in supporting muscle strength and mass accrual.”

One detail about the design of the Brazilian study warrants mention. As noted, because baseline vegan protein intake was lower than the omnivores, more soy protein was added to the diet of vegans than whey protein was added to the diet of omnivores. Therefore, the study did not directly compare the effects of isonitrogenous amounts of whey and soy protein. However, in total, the vegan diet plus soy protein, contained less leucine than the omnivore diet plus whey protein (9 vs. 11 g/d).

The key take home message regarding RET and dietary protein is that the amount of protein consumed is far more important than the type of protein consumed. Secondarily, this new research shows that soy protein, when added to a completely plant-based diet, leads to gains in muscle mass and strength comparable to adding whey protein to an omnivore diet. Finally, it is clear, and has been for some time, that acute studies measuring changes in muscle protein synthesis over a 3–4-hour period are not predictive of the results of long-term studies assessing changes in muscle mass and strength.⁸ This lack of predictability is likely because following a bout of exercise, muscle protein synthesis continues for as long as 24 hours, not just 3 or 4.^8-11

References

1. Devries MC, Phillips SM. Supplemental protein in support of muscle mass and health: advantage whey. J Food Sci. 2015;80 Suppl 1:A8-A15.
2. Messina M, Lynch H, Dickinson JM, et al. No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. International journal of sport nutrition and exercise metabolism. 2018;28:674-85.
3. Hevia-Larrain V, Gualano B, Longobardi I, et al. High-protein plant-based diet versus a protein-matched omnivorous diet to support resistance training adaptations: A comparison between habitual vegans and omnivores. Sports Med. 2021.
4. van Vliet S, Burd NA, van Loon LJ. The skeletal muscle anabolic response to plant- versus animal-based protein consumption. J Nutr. 2015;145:1981-91.
5. Stipanuk MH. Leucine and protein synthesis: mTOR and beyond. Nutr Rev. 2007;65:122-9.
6. Lynch HM, Buman MP, Dickinson JM, et al. No significant differences in muscle growth and strength development when consuming soy and whey protein supplements matched for leucine following a 12 week resistance training program in men and women: A randomized trial. Int J Environ Res Public Health. 2020;17.
7. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2017;52:376-84.
8. Mitchell CJ, Churchward-Venne TA, Parise G, et al. Acute post-exercise myofibrillar protein synthesis is not correlated with resistance training-induced muscle hypertrophy in young men. PloS one. 2014;9:e89431.
9. Burd NA, West DW, Moore DR, et al. Enhanced amino acid sensitivity of myofibrillar protein synthesis persists for up to 24 h after resistance exercise in young men. J Nutr. 2011;141:568-73.
10. MacDougall JD, Gibala MJ, Tarnopolsky MA, et al. The time course for elevated muscle protein synthesis following heavy resistance exercise. Can J Appl Physiol. 1995;20:480-6.
11. 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.