The Benefits of Soy Protein for Athletes: The Evidence Base

Soy protein is often called a 'second-rate protein' for athletes, citing several old studies. The editorial team has gathered acute experiments, randomized training studies and meta-analyses to find out what is really known about its benefit for muscles, strength and health.

How the benefit of a protein supplement is assessed

When it comes to the 'benefit' of a protein supplement, it is important to understand exactly what data lie behind this word. In sports nutrition science, several levels of evidence are distinguished, and they do not always agree with one another. Soy protein is a good example of how the results of short-term experiments can differ from long-term ones.

The first level is acute studies of muscle protein synthesis. In them, participants receive a serving of protein, often with labelled amino acids, and scientists measure how quickly the muscles incorporate new amino acids over several hours. Such work explains mechanisms well but does not show the final result.

The second level is randomized training studies lasting from several weeks to several months, in which muscle mass, strength and body composition are measured. The third is systematic reviews and meta-analyses that combine the results of many such works and give the most balanced assessment.

Below, the editorial team examines all three levels regarding soy protein, and at the end explains which conclusions can be drawn confidently and which so far remain hypotheses.

Acute studies of muscle protein synthesis

The most frequently cited acute study is the work of Tang and colleagues (2009). After strength training, soy isolate stimulated muscle protein synthesis more strongly than casein, but noticeably more weakly than whey hydrolysate (according to the authors' estimate, whey exceeded soy by about 30%). At rest, the differences between soy and whey were smaller.

The study by Wilkinson and colleagues (2007) compared skimmed milk and a soy drink with the same amount of protein and energy. After strength exertion, milk provided greater net accumulation of muscle protein. The authors linked this to the speed of amino acid delivery and to the fact that a larger share of soy's amino acids is directed to the synthesis of liver proteins and to oxidation.

Such results explain why soy is inferior to whey in experiments: less leucine per serving and 'intermediate' absorption kinetics. At the same time, these studies were conducted with moderate protein doses — about 20 g and less. With larger servings, the difference in leucine is partly compensated.

So, from the point of view of mechanisms, soy is an effective but not the strongest stimulus of muscle protein synthesis. The question is whether this difference matters in a real training process, where a person receives protein several times a day over months.

Training studies and meta-analyses

The training study by Hartman and colleagues (2007) showed that in young male beginners who trained with weights for 12 weeks, milk after training gave a greater gain in lean mass than a soy drink. This study is often cited as an argument against soy.

However, other works found no such differences. Kalman and colleagues (2007) compared different protein sources, including soy isolate and concentrate and whey, in men on a strength training program and found no significant difference in changes in body composition and sex hormones.

The most weighty argument is the meta-analysis by Messina and colleagues (2018), which combined randomized studies of direct comparison of soy and animal protein against the background of strength training. The authors found no difference between the groups either in muscle mass gain or in strength.

Interesting data were obtained by Hevia-Larraín and colleagues (2021): vegans who supplemented their diet with soy protein and omnivorous participants who took whey were provided with the same amount of protein — about 1.6 g/kg per day. After 12 weeks of strength training, the gain in muscle mass and strength did not differ between the groups.

The general meta-analysis by Morton and colleagues (2018) also indicates that the main factor is a sufficient daily amount of protein, while the protein source plays a secondary role when this amount is provided.

Other possible effects

Besides muscles, soy protein is credited with an effect on the cardiovascular system. Replacing part of animal protein with soy in many studies moderately reduced LDL cholesterol. The review by Messina (2016) assesses this effect as real but small, and the FDA later proposed to revise the official claim about soy's benefit for the heart.

For athletes this effect may be useful against the background of a diet with a lot of red meat and saturated fats. However, one should not count on soy protein as a means of correcting the lipid profile: for this there are more effective dietary and medical approaches.

As for the antioxidant action of isoflavones, faster recovery after exertion or improvement of endurance, there are few data, they are contradictory and often obtained in small samples. The editorial team does not consider these effects proven.

Another practical advantage of soy is availability and high protein quality among plant sources. For people who do not consume dairy products, this makes it one of the most convenient tools for reaching the recommended 1.4–2.0 g of protein per kilogram of body weight per day (Jäger et al., 2017).

Limitations of the evidence base

Despite the significant number of studies, the evidence base has limitations. Most training works are small, last 8–12 weeks and were conducted mainly with young men. There are fewer data on women, older people and experienced athletes.

In addition, the studies used different protein doses, different forms of soy and different control groups, so a direct comparison of results is complicated. In meta-analyses this heterogeneity is partly smoothed out but does not disappear completely.

Practical conclusion for an athlete: if soy protein helps you regularly meet your daily protein norm, it works. But if you are choosing between soy and whey solely for the sake of 'maximum anabolism' in a single serving, it is sensible to take a somewhat larger portion of soy or to combine it with other proteins.

Editorial conclusions

In acute studies, soy protein stimulates muscle protein synthesis more weakly than whey, which is attributed to the lower leucine content and the features of absorption.

In long-term training programs, with a sufficient daily amount of protein, the gain in muscle mass and strength on soy in most studies and in the meta-analysis by Messina et al. (2018) does not differ from animal proteins.

The effect on LDL cholesterol is real but small; claims about accelerated recovery or improved endurance so far have no reliable confirmation.

We also recommend our materials 'How to Take Soy Protein: Dosage, Timing, Duration', 'Myths About Soy Protein' and 'The Benefits of Pea Protein for Athletes: The Evidence Base'.

References

1. Tang JE, Moore DR, Kujbida GW, Tarnopolsky MA, Phillips SM. 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;107(3):987–992.
2. Wilkinson SB, Tarnopolsky MA, Macdonald MJ, et al. Consumption of fluid skim milk promotes greater muscle protein accretion after resistance exercise than does consumption of an isonitrogenous and isoenergetic soy-protein beverage. Am J Clin Nutr. 2007;85(4):1031–1040.
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(2):373–381.
4. Kalman D, Feldman S, Martinez M, Krieger DR, Tallon MJ. Effect of protein source and resistance training on body composition and sex hormones. J Int Soc Sports Nutr. 2007;4:4.
5. Messina M, Lynch H, Dickinson JM, Reed KE. 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. Int J Sport Nutr Exerc Metab. 2018;28(6):674–685.
6. Hevia-Larraín 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;51(6):1317–1330.
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. 2018;52(6):376–384.
8. Messina M. Soy and health update: evaluation of the clinical and epidemiologic literature. Nutrients. 2016;8(12):754.