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Soy Protein and Muscle Gain Good or Bad?

Soy has recently become one of the most popular protein products with soy protein, consisting of isoflavones and saponins, being the most valuable part of it. The contents of the primary nutrients in soybeans is 42% proteins, 33% carbs, 20% fats, and 5% fiber. The amount of protein in soybean flour (the product of soy processing) is slightly more substantial and accounts for half of the total weight of the product. Soy protein is 90-92% pure protein.

soy protein

The major property that makes soy products preferable is the digestibility of their protein. The index of relative digestibility of food proteins, compiled by researchers for 2-5 years old children 2-5, indicates that pure soy protein is a leader here, followed by egg white protein and casein. These products have a high degree of digestibility equal to one and are superior in this regard to other sources of protein, like beef, kidney beans, pea flour, oats, etc. 

Many facts prove the effectiveness of soy protein. Researchers report that soy protein consumption in the amount of 1.5 g per kilo of body weight for three months helps increase muscle volume, burn excess fat, increase the amount of hemoglobin, and receive other beneficial effects.

Why is soy effective? It turned out that soy products can stimulate anabolism in the body.  Isoflavones and flavonoids contained within them make it possible. Their anabolic effect is reached through growth hormone, endorphin, and testosterone. 

Of course, the content of anabolism-stimulating factors is not quite great. However, the progress is always in motion, and the work is conducted already to find ways to make products that can contain previously mentioned factors in larger quantities. 

The anabolic effect of protein supplements combined with the physical training of a particular orientation leads to muscle hypertrophy. The process of hypertrophy itself is associated with the synthesis of intracellular protein. However, hypertrophic changes are possible in the body with low protein intake and shown by some experiments’ data. So the truth in these matters is not fully established. Nevertheless, it is undeniable that the use of large amounts of protein in strength training contributes to the growth of strength and volume.

Soy Protein In Bodybuilding

Soy protein is the worst type of protein for bodybuilding, both in gaining muscle mass and burning fat, as research states. It has a little cost and is widely used for feeding livestock. Due to economic reasons, it’s getting actively advertised and included in products of sports nutrition. According to most experts, soy protein is low in popularity and should not be used for the following reasons (relevant for isolates and concentrates):

  • low biological value (74);
  • insufficient amino acid Composition;
  • low rate of assimilation;
  • potential estrogenic activity (not applicable to isolates).
soy protein

Biological Value

Today, a lot of different types of protein can be found on the market. Their efficiency is measured by an indicator called biological value (BV). BV is the most accurate indicator of the biological and anabolic activity of a protein. The protein biological value index is calculated as the ratio of nitrogen remaining in the body to the amount of nitrogen obtained from a given protein, i.e., its digestibility is taken into account.

The biological value of different types of protein:

  • Whey – 130
  • Whole Egg – 100
  • Egg White – 88
  • Casein – 77
  • Soy – 73

To further prove the point, here are some data of studies:

Mass gain: Phillips SM and colleagues compared the anabolic properties of milk (whey and casein) and soy protein in 2009. Analysis data indicate that milk protein stimulated muscle growth significantly more than soy. In 2009, Tang JE and Moore DR reported that the stimulation of muscle growth in young people is more pronounced with the use of whey protein than soy protein. The conclusion is clear – for muscle growth, it is better to purchase whey protein supplements. Only whey protein must be used, to be more precise, because other kinds will generally reduce the anabolic effect (by reducing the overall absorption rate). It is allowed to take casein or complex products before bedtime and before long periods without eating.

Fat burning: When studying the benefits of various types of protein for weight loss, the following data were obtained: the thermogenic effect is higher with whey protein (14.4 ± 0.5%), then with casein (12.0 ± 0.6%) and soy protein (11.6 ± 0.5 last) %). Whey peptides possess the maximum ability to increase the rate of fat oxidation. Even though soy is absorbed at an average rate, it causes almost the same increase in insulin levels as whey, which is an additional negative factor when burning fat. Veldhorst MA concluded in his experiment dated 2009, that whey protein suppresses hunger more effectively than soy. Thus, it becomes evident that soy is not the preferred choice for fat burning. As a slow source of amino acids, it is better to use casein or complex additives.

Amino Acid Composition

High BV proteins are superior for maintaining a positive nitrogen balance, immunity, stimulating IGF-1 (insulin-like growth factor), as well as saving muscle on a diet. Therefore, products with high BV have a more pronounced anti-catabolic effect than those having a low BV. One of the reasons for the low BV of soy protein is the lack of sulfur-containing acid methionine. Methionine is an essential amino acid. It is not synthesized in the human body. Amino acids of this kind play a particularly important role in the synthesis of proteins, the normal functioning of the immune system, and glutathione production. Glutathione is one of the most significant amino acids in the body. It protects cells and detoxifies several harmful compounds, such as hydrogen peroxide, carcinogens, reactive oxygen intermediates, etc. In particular, glutathione is partly responsible for preventing the oxidation of LDLs («bad» cholesterol). More importantly, there is a low level of BCAA, the three vital bodybuilding amino acids that activate muscle growth and protect muscles.

Poor Assimilation

Soy proteins have a low absorption rate and contain components that prevent the digestion and absorption of many nutrients. The two most impairing anti-nutrients found in soy are lectins and protease inhibitors.

Lectins are plant elements that lead to various problems – from impaired absorption of essential nutrients to damage to the gastrointestinal tract.

Proteases are enzymes involved in the digestion of protein. Soy contains several protease inhibitors that interfere with healthy digestion and absorption of proteins in the gastrointestinal tract.

Estrogenic Activity

Soy is rich in phytoestrogens (isoflavones), which act in the body as female sex hormones – estrogens. As every bodybuilder knows, a change in the ratio of testosterone and estrogen in favor of the latter leads to the female-type fat deposition, suppression of libido, and other adverse effects that impede the achievement of the goals of strength athletes.

Sports supplements with soy are usually well rectified and contain a minimal amount of phytoestrogens.Studies have been conducted where, when men used soy isolates, estrogenic activity was not manifested. However, the degree of purification can vary greatly. In 2010, a meta-analysis of data showed that soy does not negatively affect testosterone levels in men, but this does not refute its estrogenic activity. A 2011 scientific article reported on male hypogonadism and erectile dysfunction associated with the consumption of soy products.

Pros & Cons Of Soy Protein

It must be stated right away that positive qualities are attributes of soy protein isolates only. Manufacturers of high-quality soy protein isolates remove or significantly reduce the activity of anti-nutrients when processing the raw materials. Besides, the addition of methionine to soy isolates significantly increases their biological and nutritional value. However, the BV of the final product is still inferior to that of whey or whole egg protein. Some studies indicate that soy protein isolate affects the production of thyroid hormones in humans. However, this may not be a significant advantage for bodybuilders, since changes in the level of these hormones are negligible. Soy protein has some antioxidant properties.


No matter the case or situation, it is quite evident already that soy protein is not the best choice.

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