What's the deal with soy? It gets so much hype, both positive and negative. Is it ultra healthy, or is it the fastest way to wreck your gains? And what's the deal with soy estrogens?

What Is Soy?

Soy protein is extracted from the annual leguminous soybean plant, which has been in the food chain for over 5,000 years. It is the only plant-based protein considered to be a high-quality protein, containing all nine of the essential amino acids in the ratios needed to support growth and development.[2]

Soy is comprised of 30 percent carbohydrate, 36 percent protein, 19 percent oil, and 14 percent moisture. Soy's other nutritional functions include providing a respectable amount of potassium, zinc, iron, vitamin E, and phosphorous, as well as the full B complex.[1]

The Controversy Over Soy

Soy protein has received a lot of negative press over recent years. Why then, would one replace their tried and tested whey with the much maligned, supposedly inferior soy?

As is so often the case, when a particular scientific issue is debated, there is another side to the story. In recent years, scientists have been looking closely at the effects soy consumption really has on testosterone and muscle gain.

Soy Is Bad for You (Maybe)

Does Soy Raise Your Estrogen?

Some studies have suggested that the phytoestrogens contained in soy protein lead to both unwanted decreases in testosterone and increases in estrogen.[3]

Soy is rich in estrogenic compounds, or phytoestrogens, such as genistein and daidzein. There are over 300 plant-derived phytoestrogens that vary substantially in their physiologic effect and potency in animals and humans. A change in the testosterone-estrogen ratio in favor of estrogen can lead to increased body fat and other ill effects as it relates to a strength athlete's goals.

The isoflavones (a phytoestrogen) found in soy are thought to upset hormone balance, and result in the aforementioned unwanted testosterone and estrogen changes, in addition to thyroid problems.

Biological Value of Soy

Soy protein has a low BV score of 74. What does that mean? There are several ways of assessing protein quality. You have the protein efficiency ratio (PER), the net protein utilization (NPU), and the biological value (BV). The PER is an outdated measure of protein quality and is not used much anymore by most supplement manufacturers or nutritionists "in the know" about protein quality.

The NPU is a little better than the PER, but fails to take several important factors into account, such as absorption and digestibility, so it is not used much, either.

That brings us to the BV. The BV is the most accurate indicator of biological activity of a protein and measures the actual amount of protein deposited per gram of protein absorbed. As a rule, high BV proteins are better for nitrogen retention and IGF-1 stimulation, and are superior for reducing lean tissue loss during various wasting states than their low BV counterparts. That is, as a general rule, high BV proteins are more anti-catabolic than low BV proteins.

The highest BV protein available is whey protein, with whole egg a close second, which is why bodybuilders and other athletes rely heavily on these two protein foods and tend to avoid soy and other proteins with low BV scores.

Biological Value of Common Proteins

  • Whey: 104
  • Whole egg: 100
  • Egg white: 88
  • Casein: 77
  • Soy: 74

Soy, Methionine, and Cholesterol

In addition to its low BV score, soy has several other nutritional drawbacks that make bodybuilders wary. One reason soy is so low on the BV scale is because it's lacking in the sulfur-containing amino acid methionine.

The sulfur-containing amino acids (cysteine being the other one) are particularly important for protein synthesis and growth, proper immune system function, and the body's production of glutathione (GSH). GSH is one of the most important antioxidants found in the body. It protects cells and serves to detoxify a variety of harmful compounds such as hydrogen peroxide, carcinogens, reactive oxygen species, and many others. In particular, GSH is also partly responsible for keeping low density lipoproteins (LDL) from oxidizing and clogging our arteries.[4]

Several studies have shown soy protein to be inferior to whey for the production of GSH. Though soy has a reputation for reducing cholesterol, in one study, rats fed soy protein that was not fortified with methionine as 13 percent of total calories, had an increase in cholesterol and an increased susceptibility of LDL cholesterol to peroxidation. So not only did the rats' cholesterol go up, the LDL fraction oxidized more easily, potentially leading to clogged arteries. It is well established that an increased susceptibility of LDL to peroxidation is an essential step for the development of atherogenesis. These rats were found to have low levels of GSH and did not grow as well as another groups of rats fed casein.[5]

What Are Antinutrients?

If that were not bad enough to convince you to avoid soy, it gets worse. Soy protein contains something known as "antinutrients" that block the digestion and absorption of many nutrients. Two of the more important antinutrients found in soy are lectins and protease inhibitors.

Lectins are nasty constituents of various plants and can cause all sorts of problems, from interfering with the absorption of important nutrients to intestinal damage. Proteases are enzymes that assist in the digestion of proteins. Soy has several protease inhibitors that interfere with the enzymes trypsin and chymotrypsin, both of which are important for the digestion and absorption of proteins in the gastrointestinal tract.

Wait! Soy Is Good for You (Maybe)

Some nutritional counselors have begun to suspect that soy may be to blame for the low energy, digestive disturbances, hypothyroidism, infertility, and other ailments they see in clients.

However, on the other side of the coin, many scientists believe soy to be relatively innocuous, and in fact beneficial for a whole range of physical ailments. The previous section does not paint a very pretty picture of soy proteins, but it's not the entire story.

Soy Protein Isolate: Processed to Be Healthier

The problem of the antinutrients found in soy protein has been taken care of as the manufacturers of high-quality soy protein isolates remove them or dramatically reduce their activity during processing, so this is not a big point of concern anymore. Also, the addition of methionine to soy isolates greatly improves its BV and nutritional value, though it still does not reach the BV of whole egg or a good whey protein.[6] Rats fed soy protein enriched with methionine grew at a similar rate as those fed casein.[7]

In other words, some of the problems with whole soy are not an issue with soy protein isolate, which gives you the good part without some of the bad parts.

OK, But What About the Estrogenic Compounds in Soy?

While some studies have shown the phytonutrients in soy to affect hormones levels, others suggest they do not decrease testosterone or raise estrogen.

Though soy proteins contain these estrogenic compounds, it appears that they are "tissue specific." One study that used rhesus monkeys found that soy proteins had no effects on the reproductive hormones of these animals.

Testosterone, DHEAS, sex hormone binding globulin (SHBG), testicular weight, prostatic weight, and other measurements were taken. The study found no difference between male animals who ate soy protein that contained the plant estrogens and those who ate soy with the estrogens removed, leading researchers to conclude the isoflavones (genistein and daidzein) in soy protein have no apparent negative effects on the reproductive system.[8]

"Our data supports an interpretation that soybean estrogens have tissue specificity in part because of their mixed estrogen agonist and antagonist properties," the scientists concluded.

From this and other data, it seems the phytoestrogens in soy don't, in fact, have systemic estrogenic effects, such as body fat increases.

More Good News About Soy: Thyroid Hormone

Soy protein has been found to raise thyroid output in a wide range of animals, from rats to pigs. Studies done with human subjects have been harder to quantify (what else is new?), but several studies suggest an effect on thyroid hormones in people eating soy protein isolate.[9]

Soy protein has been shown to raise thyroid hormone output, which could be a real advantage to bodybuilders trying to shed some fat. The intake of various high-quality proteins has been associated with higher levels of thyroid hormone, but soy appears to have thyroid hormone raising abilities unique from those of other proteins.

Though some research has shown changes in the thyroid hormone triiodothyronine (T-3) and thyroid stimulating hormone (TSH), the real effect appears to be with thyroxine (T-4), which is elevated consistently in the studies done using animals—and to a lesser degree people—eating soy proteins.

At this time, exactly how soy proteins have this effect on thyroid output is not well understood, but they're working on it.

Why Should I Care?

So, what does soy's relation to thyroid hormones mean to bodybuilders? Two points are the most relevant to strength athletes:

  1. Though thyroid hormones are considered catabolic hormones, they are actually more catabolic to fat and carbohydrates, but stimulate protein synthesis if adequate calories are eaten and the amounts of thyroid hormones are not too high. This could be useful for increasing protein synthesis and reducing body fat.
  2. When a person diets, the success of that diet is often brought to a screeching halt when the body figures out what you are up to and reduces the output of thyroid hormones. This is a reaction by the body brought on by reduced caloric intake, which reduces metabolic rate and sets a new caloric set point.

Additional Benefits of Soy Protein

Soy has many other potential benefits that could make it a super-healthy addition to your supplement stack.

  • Soy protein increases the nutritional value of other foods due to its complete amino acid profile.
  • Soy has been shown to help maintain a healthy cholesterol level if yours is already in the normal range.[1]
  • Soy could enhance athletic performance.[11,12] The isoflavones found in soy protein produce antioxidant effects, which speed recovery and reduce muscle soreness and inflammation following exercise.
  • In addition, soy does supply a full complement of amino acids for the exercising muscles, meaning muscles will become larger and stronger with soy protein. In fact, athletes who incorporate both soy and whey protein into their nutritional regimens may benefit from the different rates of digestion and amino acid absorption.

Soy Versus Whey: Which Protein Is Better?

How does soy compare to whey when it comes to maximizing your results in the gym? The short answer is they're similar, but different.

Soy Is as Good as Whey for Building Muscle: What the Science Says

Some studies have suggested soy to be as effective as whey, if not more so, in terms of its ability to promote gains in lean muscle mass.

The first of these studies, presented at the American College of Sports Medicine (ACSM) Meeting in 2004, and sponsored by the U.S. National Dairy Council, found the post-workout consumption of an isonitrogenous and isoenergetic soy drink to be statistically significant in hastening mass, fat-bone free mass (FBFM), and increases in strength, when compared to other post-workout formulations (skim milk and maltodextrin beverage), both of which had a similar degree of efficacy.[6]

This study demonstrated that intact proteins from both soy and milk protein are effective in supporting muscle hypertrophy, lending a degree of support to soy as a legitimate post-workout nutritional beverage.

This study's method involved randomizing 34 subjects to milk, soy beverage, or maltodextrin beverage, using a double-blind allocation process.

Participants trained 5 days per week on a whole-body split resistance-training program and consumed 500 milliliters of their assigned drink, both immediately and 1 hour post-workout.

Another study that reinforces soy as an effective aid to muscle building, as well as showing its safety for testosterone levels, was presented in 2005, and underwritten by the Solae Company (which sold soy, but the dairy-sponsored study had similar results).

The study found daily supplementation of soy protein, whey, or a soy-whey blend all resulted in an increase in lean body mass and did not negatively affect testosterone or estradiol levels in 20 male athletes engaging in a weight-training program.[13]

As Greg Paul, Ph.D., former director of nutrition for the Solae Company, explained,

"The results of this study show that soy protein is just as effective as whey protein in building lean muscle mass as part of a dedicated exercise and nutrition regimen, while contradicting the myth that soy protein may negatively impact testosterone levels in men."

Paul continues, "These results are consistent with prior studies which have compared the effect of whey protein and soy protein on lean muscle mass, and supports the notion that dedicated athletes may benefit from a nutrition regiment that includes both soy and whey proteins."

Douglas S. Kalman, M.S, R.D., Director of Nutrition & Applied Clinical Research at Miami Research Associates, backs him up:

"We believe that this study validates that soy protein is safe and just as effective as whey protein in helping exercising males achieve their fitness goals and supports the development of lean muscle mass," Kalman says.

The study's purpose was to compare the effect of supplementation of 50 grams per day of four different protein supplements in combination with resistance training on lean body composition and serum sex hormone changes in males.

The method of this study involved the subjects consuming protein shakes twice daily and participating in three hypertrophy-oriented sessions per week over a 12-week period.

The protein shakes consisted of either soy protein concentrate (SPC), soy protein isolate (SPI), a soy-whey blend composed of a 50/50 mixture of SPI with whey protein concentrate (WPC) and whey protein isolate (WPI), or whey protein composed of a 50/50 mixture of WPC and WPI.

The study found that all of the protein sources resulted in the desired effect of increasing lean body mass. In addition, there was no difference between the supplement sources on changes in testosterone levels. The lean muscle mass gains that were demonstrated in this study are consistent with prior research looking at how soy and whey impact lean muscle mass in conjunction with an exercise regimen.

How Soy Is Different from Whey

While soy and whey will both help you build muscle at the gym, the two types of protein are a little bit different. Soy has some unique benefits and it's worth including in your supplementation program, even if you rely on whey for most of your protein supplementation needs.

Athletes who incorporate both soy and whey protein in their nutritional regimens may benefit from their different rates of digestion and amino acid absorption. Whey protein digests more quickly, while soy protein digests more gradually. Together, they may provide a more prolonged, deliberate release of amino acids to key muscle groups.

The above study is consistent with other studies that have demonstrated that soy protein has unique benefits for exercising adults in improving antioxidant status. These findings indicate that soy protein can help combat free radical formation during exercise, which may help speed muscle recovery after a workout.

Soy and Whey Are Complementary

Used together in an exercise regimen, soy and whey proteins complement each other well. Whey protein is high in branched-chain amino acids, used as an important energy source by the body during exercise, while soy protein has high amounts of the amino acids arginine and glutamine. Arginine is well known as a stimulant of anabolic hormones that stimulate muscle formation.

What Forms Does Soy Protein Come in?

How Soybeans Are Processed

The soybean undergoes a number of stages before it can be used for foods and supplements.

  1. The soybeans are cleaned, conditioned, cracked, dehulled, and rolled into flakes.
  2. The soy oil is then removed from the flakes.
  3. The flakes are then dried, creating the defatted soybean flakes.
  4. This defatted product forms the basis of the three major soy product categories: soy protein concentrate, soy protein isolate, and textured soy protein.

Types of Soy Protein

Supplemental soy comes in three types: soy protein concentrate, soy protein isolate, and textured soy protein.

Soy Protein Concentrate: Soy protein concentrate is essentially what is left at the completion of the defatting process. In this moisture-free form (when all the moisture has been removed from the soy flakes), soy concentrate contains at least 65-percent protein and most of the carbohydrates contained in the soybeans.

Soy Protein Isolate: As with whey protein isolate, soy protein isolate is the most pure and refined form of this protein available. Soy isolates are made from the defatted beans, but with most of the other ingredients removed, leaving almost a pure protein source (at least 90 percent). Carbohydrates are removed from soy protein isolates so there is less of a soybean taste with this particular form.

Textured Soy Protein: Made from soy protein concentrates, the textured soy proteins are what make up various soy products, such as imitation chicken, pork, and steak.

How Much Soy Should I Take?

All of the above information presents something of a dilemma for a fitness-conscious person. Soy has many potential benefits, but the one considerable drawback is its lower biological value compared to other proteins. Unfortunately, if a bodybuilder starts to replace too much of the other high quality proteins in their diet with soy, they run the risk of losing muscle due to this lower-quality protein.

This would be particularly noticeable during a reduction in calories, such as while dieting. The lower the calorie intake, the higher the quality of protein needs to be to maintain lean body mass. Make no mistake about it, soy protein does not have the nitrogen retaining, anti-catabolic, muscle building abilities of proteins such as whey, whole egg, red meat, etc.

Luckily, it appears that a person does not need to eat a great deal of soy protein isolate to get the benefits. Approximately 10-30 grams a day of a high-quality soy protein isolate should do the trick for most people. You can achieve this with a scoop of soy protein isolate once a day, or mix a partial scoop with whey protein in a 2:1 ratio and have that 2-3 times a day.


More research will need to be done, but if the studies presented here are anything to go by, soy protein could be an excellent anabolic aid, either used independently or in concert with whey protein.

This report is not intended to denounce whey protein or hail soy as the newest miracle product, but, rather, to present another side to the whey-versus-soy story, with the research to help people make more informed choices.

  1. (n.d.). Retrieved October 18, 2017, from https://ndb.nal.usda.gov/ndb/foods/show/4845
  2. Soy Protein. (n.d.). Retrieved October 18, 2017, from http://www.danisco.com/product-range/soy-protein
  3. Dwyer, J. T., Goldin, B. R., Saul, N., Gualtieri, L., Barakat, S., & Adlercreutz, H. (1994). Tofu and soy drinks contain phytoestrogens. Journal of the American Dietetic Association, 94(7), 739-743.
  4. Gotoh, N., Graham, A., Nikl, E., & Darley-Usmar, V. M. (1993). Inhibition of glutathione synthesis increases the toxicity of oxidized low-density lipoprotein to human monocytes and macrophages. Biochemical Journal, 296(1), 151-154.
  5. Moundras, C., Remesy, C., Levrat, M. A., & Demigne, C. (1995). Methionine deficiency in rats fed soy protein induces hypercholesterolemia and potentiates lipoprotein susceptibility to peroxidation. Metabolism, 44(9), 1146-1152.
  6. Friedman, M. (1994). Improvement in the safety of foods by sulfhydryl-containing amino acids and peptides. A review. Journal of Agricultural and Food Chemistry, 42(1), 3-20.
  7. Hajos, G., Gelencser, E., Grant, G., Bardocz, S., Sakhri, M., Duguid, T. J., ... & Pusztai, A. (1996). Effect of proteolytic modification and methionine enrichment on the nutritional value of soya albumins for rats. The Journal of Nutritional Biochemistry, 7(9), 481-487.
  8. Anthony, M. S., Clarkson, T. B., Hughes Jr, C. L., Morgan, T. M., & Burke, G. L. (1996). Soybean isoflavones improve cardiovascular risk factors without affecting the reproductive system of peripubertal rhesus monkeys. The Journal of Nutrition, 126(1), 43.
  9. Forsythe, W. A. (1995). Soy protein, thyroid regulation and cholesterol metabolism. The Journal of Nutrition, 125(3), 619S.
  10. Wilcox, J. N., & Blumenthal, B. F. (1995). Thrombotic mechanisms in atherosclerosis: Potential impact. The Journal of Nutrition, 125(3), 631S.
  11. Hartman, J. W., Bruinsma, D., Fullerton, A., Perco, J. G., Lawrence, R., Tang, J. E., ... & Phillips, S. M. (2004). The effect of differing post exercise macronutrient consumption on resistance training-induced adaptations in novices. Medicine and Science in Sports and Exercise, 36(5), S41-S42.
  12. Rossi, A., Disilvestro, R. A., & Blostein-Fujii, A. (1998, March). Effects of soy consumption on exercise-induced acute muscle damage and oxidative stress in young adult males. In FASEB Journal (Vol. 12, No. 5, pp. A653-A653). 9650 Rockville Pike, Bethesda, MD 20814-3998 USA: Federation of American Societies for Experimental Biology.
  13. Kalman, D., Feldman, S., Martinez, M., Krieger, D. R., & Tallon, M. J. (2007). Effect of protein source and resistance training on body composition and sex hormones. Journal of the International Society of Sports Nutrition, 4(1), 4.

About the Author

David Robson

David Robson

As an active martial artist, bodybuilder and accredited personal trainer, David employs the latest cutting edge research to enhance his own progress.

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