The question of which is better, soy or whey protein, generates a huge number of conflicting opinions: sellers praise both, depending on the circumstances, while pseudoscientists provide scientific arguments in favor of the product for which they are paid.
In search of objectivity regarding soy protein, its pros and cons, we compared the material from the popular English-language portal bodybuilding.com, dedicated to the unequivocal advantages of soy protein for muscle gain, with scientific research, where it is referred to as “a worthy alternative to whey.” The truth is discovered through comparison. Here’s what we found.
Main thoughts:
Soy protein affects testosterone and estrogen levels; this also applies to soy protein isolate
According to some studies, soy protein is NOT less effective for muscle gain than whey
For this article, we deliberately took material from bodybuilding.com to demonstrate the contradictions in the information on the pages of a large online store, whose goal is to sell, compared to the results of scientific research.
Originally, the article on bodybuilding.com is titled “Soy or Whey Protein. The Latest Research!”, however, it is difficult to call it relevant, as it relies on scientific studies from 10-20 years ago, and “objective,” as it is based on the results of only two experiments, one of which is funded by a major soy protein manufacturer.
Soy protein is indeed unique among all plant proteins; it is complete and well-absorbed. Serious concerns arise regarding the active substances contained in soy, particularly isoflavones.
Research indicates serious dangers associated with their consumption.
Overall, the material gives the impression of being written “on demand” to stimulate sales of the corresponding product in the online store.
Recommended: Soy: harm and benefits. Scientific facts
Today, whey protein is one of the best-selling types of sports nutrition.
It has long been one of the most popular proteins in bodybuilding, thanks to its high degree and speed of absorption (biological value) and its near-ideal amino acid profile.
At the same time, soy protein has a reputation as a feminizing protein (manifestation of female traits), suppressing testosterone levels, and low effectiveness in building muscle mass.
Note how this argument is commented on by the author of a popular portal:
Bodybuilding.com, “Which is better: soy or whey protein?”:
“If it is true that soy protein is low in effectiveness and has a feminizing effect, then it is unclear why an increasing number of professional bodybuilders are switching from a tried-and-true whey protein that has proven itself over generations as ‘the best’ to soy, the consumption of which seems to pose health risks?
In recent years, scientists have been diligently studying the impact of soy consumption on testosterone levels, which play an important role in muscle mass development. The conclusions they reached: soy protein is at least (and according to some data, possibly even more) effective in building muscle mass than whey.“
The names of well-known professional bodybuilders who have decided to switch to soy protein, unfortunately, remain undisclosed…
Let’s take a look at what scientists say about the pros and cons of soy protein as a source of protein in bodybuilding, and its advantages/disadvantages over whey.
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Pros and cons of soy protein
1 Pros of soy protein: absorption rate and amino acid profile
Bodybuilding.com: “Soy protein is the only plant protein characterized as high-quality and complete, as it contains all essential amino acids in optimal ratios for muscle growth and has a high biological value (high absorption rate) 4,5.
Soy protein contains all 9 essential amino acids in ideal ratios for health.
In addition, soy is rich in vitamins and minerals: potassium, zinc, iron, vitamin E, phosphorus, B vitamins 5.“
Soy protein is indeed unique among plant proteins, as it has a complete amino acid profile and contains all essential amino acids.
However, the question of “optimality and ideality” of the amino acid profile is under discussion.
Compared to whey protein, soy protein has a lower content of the essential amino acid methionine, which, due to its essentiality, performs critically important functions in the body, including muscle protein synthesis and immune support.
Knowing about this issue, manufacturers of soy-based sports nutrition, particularly soy protein isolate, enrich it with methionine.
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Thus, the amino acid profile of soy protein is close to ideal with a slight deficiency of one amino acid.
According to the popular Russian-language portal SportWiki, which published an expert review on the pros and cons of soy protein, another important argument against in the debate “soy protein or whey” is “significantly lower biological value (or degree of absorption) of soy protein compared to whey (73 vs. 130)”.
In understandable terms, this means that when the same amount of both types of protein enters the body, soy will be absorbed ~45% less.
This is bad.
However, the figure for whey protein is clearly inflated.
Journal of Sport Science and Medicine provides different values for biological value: 74 for soy protein and 104 for whey protein, thus reducing the difference in absorption rate by ~30 6. That’s better.
Furthermore, for a complete picture, it is worth noting that biological value is a not very accurate method of assessing the degree of absorption; the PDCAAS method (see more in our article on scientific methods for determining the best protein), which is recognized as the most reliable internationally and widely used due to its higher accuracy, gives a more encouraging result: identical absorption rates for soy and whey proteins 6.
Important: Thus, from the perspective of amino acid composition, soy protein slightly lags behind whey protein, while their absorption rates are approximately the same.
From the perspective of absorption and the completeness of the amino acid profile, we give soy protein a plus, slightly overlooking the lack of methionine, which often ceases to be a deficiency in most cases, as noted, since manufacturers often enrich soy products with this amino acid.
Soy protein is the best among all plant-based proteins: it has a very high absorption rate, close to whey, contains all amino acids, BUT in less optimal proportions than whey.
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2 Disadvantages of soy protein: impact on testosterone and estrogen
The question of the impact of soy protein on hormonal levels is one of the most important sticking points for this product. The essence of it is that soy, in addition to protein, contains a whole range of biologically active plant substances that, when entering the body, make changes to the functioning of our chemical laboratory.
In particular, isoflavones or phytoestrogens are said to potentially suppress the secretion of an important hormone for muscle growth – testosterone, and enhance the secretion of the hormone estrogen, which is responsible for the manifestation of female sexual characteristics (such as breast growth in men, fat accumulation in corresponding areas, etc.).
The answer to this question is one of the most important arguments in the dilemma of “soy or whey protein”.
Here’s what Bodybuilding.com says about this: “For a long time, researchers have suggested that the phytoestrogens found in soy protein lead to a decrease in testosterone levels (a hormone directly related to muscle growth) and an increase in estrogen levels (a female hormone responsible for the manifestation of female secondary sexual characteristics: breast growth, fat deposits in characteristic areas, etc.) 1.
For example, it was believed that isoflavones (a type of phytoestrogens) found in soy lead to the aforementioned hormonal imbalance, as well as problems with the thyroid gland.
Some dietitians also suspected soy of digestive disorders, lethargy, hypothyroidism (disruption of hormone secretion by the thyroid gland), infertility, and other deviations that they observed in their patients.
At the same time, many scientists today are convinced of the harmlessness of soy and even its positive influence on the healing of certain diseases.“
To confirm the lack of influence on testosterone and estrogen levels, two scientific experiments (described below) are also cited, the scientific integrity of which is questionable.
In fact, all the hormonal issues mentioned in the quote have scientific backing. See our material on the dangers of soy, as well as the research on vegetarianism in sports, which highlights the risks of strict vegetarianism for athletes.
These issues are characteristic of both soybeans themselves and products derived from them, including soy protein isolate.
Important: It is believed that the technological processing of soy during the production of isolate leads to the destruction of isoflavones. However, in reality, – not completely. Scientists state that even in residual amounts they pose serious health risks.
The technological procedure for their removal is quite costly, and many manufacturers neglect it.
This is a significant disadvantage NOT in favor of soy protein.
Soy protein affects testosterone and estrogen levels; this also applies to soy protein isolate
We recommend: Phytoestrogens: questionable benefits during menopause, content in products, harm. Scientific facts
3 Which is more effective for muscle mass gain: soy or whey protein?
Gaining muscle mass is the primary goal of using protein in bodybuilding. Countless scientific articles have been written about the benefits of whey protein for this, supported by experiments and the experiences of each of us.
Based on the completeness of the protein and a decent amino acid profile, soy protein should also be a good fuel for mass gain.
Bodybuilding.com, in support of the benefits of soy protein for mass gain, presents the results of two scientific experiments:
Experiment: Pros and cons of soy protein for mass gain
One of the studies was conducted by the American College of Sports Medicine. It involved 34 athletes: 12 on dairy protein, 11 on soy protein, and 11 on maltodextrin. All athletes trained under the same program (5 days a week) for 12 weeks and consumed 500 ml of their assigned protein shake immediately and 1 hour after training.
Result: consumption of soy protein shakes after training leads to a noticeable increase in muscle mass and strength, comparable in effect to the action of sports protein drinks based on dairy protein and maltodextrin with equivalent protein content and energy value 2.
Another confirming experiment was conducted by Solae Company (April 2005). The aim of the study was to confirm the effectiveness of soy protein in building muscle mass and to refute the claim that soy consumption leads to a decrease in testosterone levels.
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The essence of the study: 41 male athletes were included in a strength training program for 12 weeks (3 days a week); four types of protein shakes were used as special protein nutrition (soy protein concentrate, soy protein isolate, a blend of soy protein isolate and whey protein concentrate 50/50, whey protein isolate) with a total protein content of 50 g, twice a day 3.
Results:
- all four types of protein shakes are equally effective in building muscle mass;
- no differences were found in the effects of all types of protein nutrition on testosterone and estrogen levels ;
- the rate of muscle mass gain is equivalent for all four types of protein nutrition.
Soy protein should indeed be effective in building muscle mass, as it is well absorbed and contains all amino acids.
However, it is important to note the following:
- The studies mentioned did not consider other important aspects of soy protein consumption, particularly the impact of active substances in soy (isoflavones), which we discussed earlier, on hormonal balance. They are recognized as toxic.
That soy protein is good is not disputed. But nothing is said about the possible consequences of long-term consumption of these substances, especially when soy is used as a sole source of protein by women, pregnant individuals, teenagers, and athletes.
- The scientific integrity of the second experiment is questionable.
Solae Company – the company that funded the second study – is the largest global producer of soy food products. It is hard to imagine the disclosure of negative facts about a product by employees of a specific company (even with a Ph.D. degree) who are receiving a salary there (follow the link, don’t hesitate, read about the company).
Here is a review of the effectiveness of soy protein from a representative of Solae Company, Dr. Greg Paul (who holds a Ph.D. degree):
“The results of our study indicate that soy protein is as effective in building muscle mass as whey protein and at the same time refute the myth of its negative impact on testosterone levels in men.”
“Our results are consistent with previous studies that compared the effectiveness of whey and soy proteins in building muscle mass and confirm the benefits for athletes of a diet consisting of whey and soy proteins.”
According to some studies, soy protein is NO less effective for muscle mass gain than whey.
We recommend: Which protein is better: sports or natural?
Soy or whey? Summary
So, let’s summarize the undeniable “pluses” of soy protein from the perspective of bodybuilding.com and dilute them with our “minuses”:
1 Bodybuilding.com: “Soy protein has the highest digestibility index of 1.0 according to the PDCAAS method.” (the digestibility index considering the amino acid composition. PDCAAS is one of the common methods for assessing protein quality, widely used today; see our material on methods for assessing protein quality).
According to the PDCAAS method, the digestibility index of soy is indeed 1, the same as that of whey protein. The value varies widely across different methods of assessing digestibility, but PDCAAS is recognized as the most effective internationally.
2 Bodybuilding.com: “Due to its complete amino acid profile, soy protein increases the nutritional value of other foods it is consumed with.“
From the perspective of amino acids – yes, it increases; soy protein is indeed of very high quality. But the overall effect of its consumption is important, especially the influence of isoflavones. There is a saying that a drop of tar spoils a barrel of honey…
3 Bodybuilding.com: “It has been proven that soy reduces the risk of cardiovascular diseases due to its ability to lower blood cholesterol levels.“
Doubtful. See our material on the harms of soy.
4 Bodybuilding.com: “Scientists believe that soy protein reduces the risk of other diseases, such as colon diseases, breast cancer, prostate cancer, and osteoporosis. Many studies confirm the ability of the bioactive compounds in soybeans (isoflavones) to reduce the risk of certain types of cancer.“
Sounds like a myth. See our material on the harms of soy.
5 Bodybuilding.com: “Soy contributes to increased workout efficiency: the same isoflavones have antioxidant effects, which aid in recovery and also reduce pain and inflammation in the muscles.“
Seems like a marketing ploy. Check out our material on the harms of soy.
6 Bodybuilding.com: “Soy protein is effective in increasing muscle mass and strength, thanks to its complete amino acid profile and high digestibility. Combining soy (“slow”) and whey (“fast”) protein allows for a flexible adjustment of the protein delivery mechanism to muscle tissue: soy protein is digested more slowly, whey protein faster, and their combination (multi-component protein) releases amino acids evenly over a prolonged period.“
In an alternative large-scale study on the harms of soy and isoflavones, it is specifically mentioned that soy isoflavones hinder the synthesis of steroid hormones in the body, and consequently, muscle growth.
7 Bodybuilding.com: “Together, soy and whey protein complement each other well in terms of amino acid composition: whey contains a greater relative amount of BCAAs, which are used as an energy source during workouts, while soy contains arginine and glutamine. Arginine is known as a stimulator of anabolic hormone release, which determines muscle growth, while glutamine is involved in regulating mental processes.“
8 Bodybuilding.com: “Soy protein is cheaper than whey protein.“
9 Bodybuilding.com: “Soy protein is beneficial for women’s health: alleviating symptoms associated with menopause, protecting bone health, and reducing the risk of breast cancer.“
Questionable. See our materials on the harms of soy, as well as on the harms of phytoestrogens from soy for women.
