The benefits of soy protein for athletes: the evidence base

Soy protein is often called a "second-rate protein" for athletes, with reference to a few old studies. The editorial team gathered acute experiments, randomized training trials and meta-analyses to find out what is actually known about its benefits for muscle, strength and health.

How the benefit of a protein supplement is assessed

When we speak of the "benefit" of a protein supplement, it is important to understand exactly what data lie behind the word. In sports nutrition 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 portion of protein, often with labeled 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 trials lasting from a few weeks to a few 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 studies and give the most balanced estimate.

Below the editorial team considers all three levels for soy protein, and at the end explains which conclusions can be drawn with confidence and which remain hypotheses for now.

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 (by the authors' estimate, whey exceeded soy by about 30%). At rest the differences between soy and whey were smaller.

A study by Wilkinson and colleagues (2007) compared skim milk and a soy beverage with the same amount of protein and energy. After strength exercise, milk provided greater net accumulation of muscle protein. The authors linked this to the rate of amino acid delivery and to the fact that a larger share of soy amino acids is directed toward the synthesis of liver proteins and oxidation.

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

So, from the standpoint of mechanisms, soy is an effective but not the strongest stimulus for 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

A training study by Hartman and colleagues (2007) showed that in young untrained men who trained with weights for 12 weeks, milk after training produced a greater gain in lean mass than a soy beverage. This study is often cited as an argument against soy.

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

The weightiest argument is the meta-analysis by Messina and colleagues (2018), which combined randomized trials directly comparing soy and animal protein against a background of strength training. The authors found no difference between the groups in either muscle mass gain or 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 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, and the protein source plays a secondary role once this amount is ensured.

Other possible effects

In addition to muscle, soy protein is credited with an effect on the cardiovascular system. Replacing part of animal protein with soy in many studies moderately lowered LDL cholesterol. A review by Messina (2016) rates this effect as real but small, and the FDA later proposed to reconsider its official statement about the heart benefits of soy.

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

As for the antioxidant action of isoflavones, faster recovery after exercise or improved endurance, the data are scarce, 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 a considerable number of studies, the evidence base has limitations. Most training studies 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 entirely.

The practical conclusion for the athlete: if soy protein helps you regularly reach your daily protein target, it works. But if you are choosing between soy and whey solely for the sake of "maximum anabolism" in a single meal, it makes sense 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 its lower leucine content and 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 of accelerated recovery or improved endurance do not yet have 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.