Whether you’re an all-star athlete or just starting your fitness journey, it is important to ensure adequate protein intake is consumed daily to help maintain muscle mass and support muscle growth. However, not all protein sources are created equal. Protein sources can be divided into two main categories: complete and incomplete. Complete protein sources contain sufficient levels of all nine essential amino acids the body needs to consume through food and drink whereas incomplete protein sources do not.1 The quality of protein can be ranked based on the digestible indispensable amino acid score (DIAAS).2 A high DIAAS will be 1.0, with whey protein isolate having a DIAAS of 1.09. Soy protein isolate has a DIAAS of 0.91, making it an effective protein source as well. Incomplete protein sources will having a lower DIAAS, such as rice or pea protein.2
In order for skeletal muscle to grow, muscle protein synthesis must occur. The key amino acid that triggers muscle protein synthesis is leucine,.2 Complete protein sources are animal-derived, such as dairy, eggs, poultry, seafood, or beef, and they will have a high DIAAS because they digest easily and contain all amino acids.2 Soy is one of the few plants recognized as a complete protein. Common soy protein foods are edamame, tofu, tempeh, textured soy protein, and soy-based protein powders and shakes.1
Soy is a complete protein and can be beneficial for athletes and active individuals. Other benefits of soy for active adults include its antioxidant content which can help reduce exercise induced inflammation. Soy protein may also have a beneficial effect on blood pressure, osteoporosis, 3 and cholesterol levels.4 A recent meta-analysis looked at multiple studies and found that there was no difference between the effects of supplementing with soy protein compared to animal protein on muscle mass gains and strength in response to resistance training, making soy protein an effective protein option.5
Soybeans contain compounds called isoflavones, which are phytoestrogens. Phytoestrogens act like estrogen in certain parts of the body, but they are not the same as the hormone estrogen.6 One of the common myths related to soy intake is that soy increases the risk of breast cancer due to the presence of estrogen. The phytoestrogens in soy (soy isoflavones) may act like estrogen, but they also contain anti-estrogenic properties which can actually block natural estrogens and prevent estrogen formation. Many studies have now shown that there is no link between soy intake and the risk of breast cancer.5 Additionally, studies have shown that soy does not affect testosterone or estrogen levels in men either. A recent meta-analysis found that neither soy nor isoflavones affect male reproductive hormones.7
Soy is a great protein source to incorporate into the diet. Soy is a plant-based protein that is a complete protein and has been shown to be just as effective as animal-based proteins, such as whey protein, for building muscle.5 So if you are looking for a powerhouse plant-based protein to introduce more variety into your diet, soy is a solid choice.
- Michelfelder AJ. Soy: a complete source of protein. Am Fam Physician 2009;79(1):43-7
- Phillips SM. The impact of protein quality on the promotion of resistance exercise-induced changes in muscle mass. Nutr Metab (Lond) 2016;13:64 doi: 10.1186/s12986-016-0124-8[published Online First: Epub Date]|.
- Kostrakiewicz-Gieralt K. An overview of soybean derived products for sportsmen. Movement & Sport Sciences – Science & Motricité 2020 doi: 10.1051/sm/2020002[published Online First: Epub Date]|.
- Ramdath DD, Padhi EM, Sarfaraz S, Renwick S, Duncan AM. Beyond the Cholesterol-Lowering Effect of Soy Protein: A Review of the Effects of Dietary Soy and Its Constituents on Risk Factors for Cardiovascular Disease. Nutrients. 2017 Mar 24;9(4):324. doi: 10.3390/nu9040324. PMID: 28338639; PMCID: PMC5409663.
- 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. International Journal of Sport Nutrition and Exercise Metabolism. 2018;28(6):674-685. doi:10.1123/ijsnem.2018-0071
- Katharine E. Reed, Juliana Camargo, Jill Hamilton-Reeves, Mindy Kurzer, Mark Messina, Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies, Reproductive Toxicology, Volume 100, 2021, Pages 60-67, ISSN 0890-6238, https://doi.org/10.1016/j.reprotox.2020.12.019.