The Health Benefits of Soy Protein

Soy Protein Supplements in Weight Training


Verywell / Alexandra Shytsman

The soya bean is a widely grown crop throughout the world. Its value is derived from its high protein content, which is higher than all other beans grown for human consumption. The soya bean is also a complete protein, which means it contains all of the essential amino acids (EAA) your body can't make itself and needs to obtain from food.

"Soy" and "soya" are interchangeable, although some cultures tend to use one more than the other.

Soy products are food staples in some parts of Asia, and soy protein is used extensively in manufactured products in many countries. These products include soy milk, meat substitutes tofu, and fermented products miso, tempeh, and soy sauces.

Bodybuilders and weight trainers like to take commercial protein supplement powders to support their training and, hopefully, build muscle. That hope is generally realized. According to the International Society of Sports Nutrition (ISSN), which issued a review of the intake of protein for healthy exercisers in 2017, results from many studies indicate that in both men and women protein supplementation exerts a small-to-modest impact on strength development. And pooled results of multiple studies consistently indicate that protein supplementation—specifically 15 to 25 grams over four to 21 weeks—exerts a positive impact on performance.

Most protein powders are derived from either the milk proteins casein and whey, egg protein, or the extracted soy protein from soya beans. Soy protein is particularly well suited for vegetarians and vegans.

Of course, there is great debate among purists on the best type of protein powder and also the best commercial brands for athletes and weight trainers. Many varied formulas of protein powder supplements exist. Whether you actually need to take special protein powder supplements is another discussion, as some believe skim milk powder will do a satisfactory job at less cost.

Health Benefits

Boost Muscle Growth

Of all the plant proteins, soy overwhelmingly has the most research. However, that research has shown that soy is considered a lower-quality complete protein when it comes to increasing muscle mass and strength gains, according to the ISSN report.

One key point from the report is that while soy is considered a complete protein, research shows that products containing animal- and dairy-based proteins contain the highest percentage of essential amino acids and result in greater gains following resistance training when compared to vegetarian proteins like soy.

Soy contains lower amounts of "branched chain amino acids" (BCAAs) than cow milk as well. BCAAs refer to a trio of EAAs considered key when it comes to maintaining muscle: leucine, isoleucine, and valine. Of the three, leucine is one that's considered the switch that stimulates protein synthesis.

Research also finds that dietary soy phytoestrogens inhibit "mTOR expression," which regulates numerous components involved in protein synthesis, in skeletal muscle. In short, not only does soy contain lower amounts of the EAAs and leucine, but it may also be responsible for inhibiting growth factors and protein synthesis.

Clinical research bears out these findings. In one study, participants that consumed milk protein increased lean mass and decreased fat mass more than the control and soy groups—and the soy group was not significantly different from the control group. Similarly, another study found that the ability of soy to stimulate muscle protein synthesis both at rest and in response to a single bout of lower-body resistance training was greater than the dairy protein casein but less than whey.

Possible Side Effects

The Estrogen Effect

Soy contains plant estrogens called "soy isoflavones." These chemical compounds have biological effects similar to human estrogen hormones but are mostly of lesser potency. Soy-based foods are often recommended to women to blunt the effects of menopause.

Bodybuilders don't like excess estrogen because, they theorize, too much may inhibit the effects of testosterone or make them store fat. They want to maximize muscle and strength and minimize fat. As a result, many bodybuilders, weight trainers, and athletes won't use soy protein foods or supplements because they fear this negative effect on body and performance.

This concern is controversial and the research shows mixed results. One small study on 10 resistance-trained men published in 2013 demonstrated that 14 days of supplementation with soy protein did appear to partially blunt testosterone following an acute bout of heavy resistance exercise. But a 2018 study on 47 previously untrained men that resistance trained for 12 weeks found that soy supplementation didn't appreciably affect either male or female hormones. Two earlier studies reported similar findings.

In light of the fact that soy doesn't seem to be as effective as other types of protein supplementation, if you're concerned about effects on testosterone, you may want to consider supplementing with a different form of protein.

Thyroid Disease

Anyone with thyroid disease or a predisposition to thyroid dysfunction should limit his or her intake of soy-based protein, due to its potential to affect hormone balance and inhibit the absorption of thyroid hormone medication. No soy product should be consumed within 2-4 hours after taking thyroid medications. If you choose to eat a soy product it, should be done as far apart from thyroid hormones as possible.

Dosage and Preparation

Recommendations regarding the optimal protein intake per serving for athletes to maximize muscle protein synthesis are mixed and are dependent upon age and recent resistance exercise stimuli, according to the ISSN report. General recommendations are 0.25 grams of a high-quality protein per kilogram of body weight or an absolute dose of 20 to 40 grams. Higher doses, approximately 40 grams, are likely needed to maximize muscle boosting in elderly people.

To recover after exercise, an athlete or avid exerciser should try to consume protein within an hour of a workout. That’s when your muscles are most responsive to the use of protein for the repair and growth process.

What to Look For

It can be difficult to be sure you get what you think you're getting when you purchase protein supplements: In a recent ConsumerLab test of 15 protein products, three failed because they contained more cholesterol, sodium, and carbohydrates than indicated. The one soy product it tested, Vitacost Soy Protein Isolate, passed.

Some tips when buying include to avoid products with added sugars or dextrins/maltodextrins if you're trying to lose weight. Also, conventionally-grown soy is genetically modified (GMO) and heavily sprayed by pesticides. So conventional soy in the U.S. is contaminated with glyphosate and other herbicides. If consumers choose to consume soy, they can look for non-GMO organic soy products.

To save money, buy tubs of protein powder instead of pricier ready-to-drink protein shakes. But keep in mind that you often get what you pay for: Low-cost proteins may blend inexpensive proteins that your body can't readily digest.

Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

By Paul Rogers
Paul Rogers is a personal trainer with experience in a wide range of sports, including track, triathlon, marathon, hockey, tennis, and baseball.