Soy Protein Guide: Benefits, Dosages, & FAQ (2024)

Soy protein powder is a rather esoteric supplement to many physique competitors, strength trainers, athletes, and even just general health/fitness enthusiasts. Soy protein generally gets the short end of the stick since it has lesser bioavailability and essential amino acid (EAA) content (per gram) than whey, casein, and egg proteins. That being said, soy protein is still a useful “alternative” source of protein and has a place in almost any person’s (especially a vegan’s) diet in some capacity.

Research continues to provide evidence that protein demands are generally greater in active individuals and especially those who lift weights regularly. Naturally, many people find supplemental protein powders useful for meeting their protein quota.

1. What Is Soy Protein?

“Soy” generally refers to the soybean, a common legume/plant originating from eastern Asia. Soy protein powders are derived from de-fatted soy flakes/soybean meal, and come in four forms: concentrate, isolate, hydrolysate, and soy flour. Soybeans contain a variety of protein fractions, but primarily globulins.¹ For simplicity’s sake, the term “soy protein” throughout the rest of this guide will remain singular and encompass the variety of specific fractions/bodies it contains.

Unlike other vegetable proteins, soy protein is a complete protein source, which denotes that it contains all 9 essential amino acids (EAAs). Soy protein is extremely heat stable and is readily soluble in warm water. Due to its neutral taste and physical properties, it may be used as a replacement or additive in a variety of food products.

Common foods that contain soy protein include:

• Soy flour
• Soy sauce
• Soybeans/soynuts
• Soy milk
• Tofu
• Edamame
• Tempe
• Miso

If you are unsure if a food contains soy, check the label and it should be listed as an ingredient or say that the food “contains soy”.

2. Production of Soy Protein

The production of soy protein from soybeans may be accomplished via several extraction and filtration methods depending on the desired protein content. Like other protein powder supplements, the extracted product is spray-dried with soy lecithin to give the desired powdered product which may then be utilized by the supplement manufacturer for further modifications like flavoring, coloring, etc.

3. Types of Soy Protein

3a. Soy Protein Concentrate (SPC)

This refers to soy powders that contain ~70-80% protein concentration with the rest being comprised of fats and carbohydrates. SPC is made by defatting coarse soy grits and extraction via heat treatment or mixing with organic solvents.²

SPC is generally the most economical soy protein supplement and may be used in a variety of settings. Depending on the protein concentration of the powder (generally around 70-80%), SPC can be a great option for individuals on a budget who don't mind a bit more fat and carbohydrate in their soy protein supplement. SPC is also a good high-protein substitute in baking and cooking applications.

3b. Soy Protein Isolate (SPI)

SPI is produced via alkaline extraction of defatted soy flakes, with the goal of reaching >90% protein concentration and removal of most fats and carbohydrates.

SPI is highly bioavailable, easy to digest, low in fat and carbohydrates, and contains a high protein concentration. The main disadvantages to SPI are that it is more costly than SPC and can be a bit “grittier” in texture. Like SPC, SPI may come in handy as a high-protein ingredient substitute in various recipes.

3c. Soy Flour

This flour is comprised of defatted and coarsely ground soybean flakes. Soy flour contains the least amount of protein per gram of the 4 soy protein types. Typical protein content ranges from 40-60% in soy flour.³

3d. Soy Protein Hydrolysates (SPH)

A relatively new technique in soy protein production is enzymatic hydrolysis of either SPCs or SPIs. Essentially, this acts as a method of “pre-digesting” the protein by separating (i.e. lysing) peptide bonds. In turn, SPH is readily absorbed by the small intestine due to the truncated amino acid chain.⁴

While SPH is certainly the most “pure” form of soy protein, it does come with a higher price tag and is not as readily available for purchase as SPC/SPI. The main advantage of SPH is that is very well tolerated and easy to digest. In fact, many baby formulas include SPH since it is so readily absorbed. If you’re willing to pay the extra money for a pure SPH supplement, it might be worth your while, especially if you don’t tolerate SPC/SPI well.

4. Benefits of Soy Protein

Proteins are essential macromolecules that play a critical role in muscle development and maintenance (as well as many other physiological processes). Proteins are composed of amino acids peptide bonded to one another, thus amino acids are basically the building blocks of proteins.

Proteins then act as the building blocks of muscle tissue since muscles serve as the richest reservoirs of amino acids in the human body.⁵ Amino acids may take on a plethora of roles physiologically, such as neurotransmission, energy production, brain metabolism, cardiovascular function, immune system function, and several others.⁶

There is a multitude of benefits from ingesting soy protein that stems from the biological role of essential amino acids and other phytochemicals it contains.

As previously mentioned, soy protein is a complete protein (i.e. contains all 9 EAAs) with a somewhat nominal amount of L-leucine which is critical for stimulating the mammalian target of rapamycin (mTOR) pathway, which regulates muscle protein synthesis, among other things. Thus, soy protein plays a role for individuals looking to improve their musculature, fitness, and even just overall bodily function.⁷

Moreover, it is suggested that a nominal dose of soy protein (~25g) per day soy may play a role in reducing the risk of heart disease, reducing cholesterol levels, reducing the risk of cancers, lowering blood pressure, and preventing osteoporosis.^{8, 9, 10} These are likely due to the activity of the isoflavones provided by soy.

Overview of soy protein’s benefits:

• Is a complete, vegan-friendly protein source
• Is readily absorbed/digested
• Is easy/convenient to add to one’s diet and use in cooking/baking
• Increases anabolic response to resistance training
• Enhances immune system functioning, especially in those who are physically active¹¹
• Helps maintain muscle mass and prevent age-related muscular atrophy
• May reduce risk of heart disease, cholesterol, and various forms of cancer^{8, 9}
• May prevent degeneration of bone tissue/osteoporosis¹⁰

4a. Who Can Benefit from Soy Protein Supplements?

The most obvious beneficiaries of soy protein supplementation will be those who are physically active and looking for an alternative protein source to animal-derived sources. Soy is definitely a useful adjunct to whey and casein protein supplements as well. Here is a quick list of individuals who should consider supplementing with soy protein (*allergies notwithstanding):

• Bodybuilders & strength trainers
• Competitive athletes
• Vegetarians/vegans
• Those with whey/milk allergies
• Recreational exercisers and those new to weight/strength training
• Anyone else who is looking for a simple way to get more protein in their diet

5. Side Effects of Soy Protein

Soy protein is generally a safe, well-tolerated supplement/food (assuming you don’t have a soy allergy). However, in some circumstances, there is the risk for certain side effects such as:

• Bloating/cramping/upset stomach
• Nausea
• Increased bowel movements/passing gas
• Allergic reactions
• May increase estrogen receptor activity and lower testosterone levels¹⁴

Note: The significance of the hormonal impact from chronic soy protein ingestion is still a topic of investigation

These side effects can generally be easily alleviated by monitoring your total soy protein intake and making sure you are aware of any possible food allergies that you may have. If a nominal dose of soy protein consistently causes stomach/GI issues, consider trying a different soy protein supplement or possibly another source of protein if need be.

6. Assessing The Quality of Soy vs. Other Protein Sources

Taken from the whey protein expert guide.

The two most popular indications of determining a protein source's overall quality/efficacy are the biological value (BV) and protein digestibility corrected amino acid score (PDCAAS). The BV of a protein source refers to a practical measurement that assesses the degree to which an animal is able to utilize that protein. It is computed by analysis of nitrogen retention in an animal after ingesting the intended protein source to be tested.¹²

The PDCAAS is a number between 0 And 1 that evaluates protein quality based on its amino acid contents in relation to human’s requirements for them. Essentially, the higher the BV and PDCAAS of a protein the more efficiently it is utilized by the respective animal (yes, humans are animals too). Below is a chart that summarizes the BV and PDCAAS of a variety of common protein sources for humans:¹³

7. Using Soy Protein Supplements

It should be noted that like other protein sources, soy protein itself is a food source (technically), yet it is still oft referred to as a supplement since the FDA does not regulate supplements currently. Choosing the right soy protein supplement will come down to a few factors including budget, quality, flavor, desired macronutrient content, and intended uses.

7a.How and When to Use Soy Protein?

Soy protein supplements are generally a bit more bitter and gritty than whey protein supplements. That being said, with a little effort and creativity it is possible to create some delectable hi-protein shakes and foods using a soy protein supplement. For some great ideas, check out the Muscle and Strength protein shake and protein bar recipe pages.

Soy protein supplements, like any other protein source, don’t have to be restricted to certain times either. Soy protein is just that, a protein; it can and should be utilized whenever you are looking for a source of protein to add to your diet.

7b. Doseage of Soy Protein

There is no black and white answer to this since everybody’s protein needs will vary. The first thing to do is determine your caloric and macronutrient allotments. A general nutritional calculator can be found here.

Furthermore, your soy protein intake will likely vary based on what type of diet you are following. For a vegan, they may need more soy protein in their diet just to meet their total protein quota for the day.

For other people, especially males, it appears that the FDA recommended value of ~25g of soy protein per day remains a safe, healthy dose and shouldn’t significantly interfere with endocrine functioning.

8. Soy Protein FAQ

Does soy protein really lower testosterone and raise estrogen in males?

As noted earlier in the guide, the significance of soy protein intake on the hormonal status of males is still under investigation. Most studies have analyzed individuals ingesting 50-60g of pure soy protein per day, which is double what the FDA recommends. Err on the side of caution and keep your intake of soy protein in the 20-30g per day range if you’re afraid of any possible endocrine disruptions.

I’m lactose intolerant, can I still use a soy protein supplement?

Soy protein powder does not contain milk sugar and is a suitable protein source for individuals who are lactose intolerant.

I’m allergic to milk, is a soy protein supplement safe for me?

Soy does not contain milk ingredients and should be a safe alternative to dairy protein sources.

Is it true that high-protein diets are bad for the kidneys?

This fallacy stems from the issue of renal impairment in individuals who have chronically superfluous amounts of protein intake in their diet. Assuming you’re not greatly exceeding your protein needs on a daily basis, there is reason to fear debilitation of the kidneys.

Can I combine soy protein supplements with my other powdered supplements like creatine, glutamine, etc?

Yes, that’s absolutely fine.

Does it matter what liquid I mix my soy protein with?

Nope, but I would try and match the flavors unless you plan on concocting some sort of protein “frankenshake” (like mixing grape juice with cinnamon roll-flavored protein)

Doesn’t cooking/baking denature protein?

Yes, but this has little ramifications in regards to how your body utilizes the protein since denatured protein is essentially “hydrolyzed” protein; you’re still ingesting all the amino acids that were originally there, to begin with.