22 Trials: Soy Protein Unlikely to Boost Muscle Gains

Our analysis of clinical trial data addresses the following question:

“How does the use of soy protein powder affect muscle mass, size, and strength in response to weight training?”

To answer this question:

We analyzed 792 data points from 22 relevant clinical trials (941 participants) conducted over the past 33 years. Given this data, in individuals who already consume a well-balanced diet:

Compared to placebo, soy protein supplements are unlikely to significantly enhance muscle growth and strength in response to weight training. Additionally, whey is likely not more effective than soy.

There are two possible exceptions here.

Let’s take a closer look.

Last Literature Review: Sep. 2023. Last Content Update: Apr. 30, 2024. Content Policy & Disclaimer.

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This image contains a table that compares soy protein vs. placebo and soy protein vs. whey. It shows that for the general population, soy protein powder is unlikely to increase lean muscle mass, muscle size, or strength better than placebo. It also shows that whey protein powder isn’t more effective than soy protein at building muscle mass, enhancing growth in muscle size, or improving gains in muscle strength. The image also shows that soy protein may be more effective than placebo for increasing lean muscle mass and strength in athletes and postmenopausal women.

Key Stats

  • 57.1% of relevant clinical trials conclude that soy protein supplements are neither better nor worse than placebo at enhancing muscle mass, size, or strength in response to weight training. (Stat Image)
  • 87.5% of relevant clinical trials conclude that whey supplements are neither better (nor worse) than soy protein supplements at enhancing muscle mass, size, or strength in response to weight training. (Stat Image)
  • Clinical trials conducted outside the U.S. and Canada were 3.9 times more likely to find that soy protein was superior to placebo (or that whey was superior to soy) at enhancing muscle mass, size, or strength in response to weight training. (Stat Image)
  • Compared to independent research, clinical trials sponsored by the dairy industry were 3.3 times more likely to find that whey was better than soy protein at enhancing muscle mass, size, or strength in response to weight training. (Stat Image)

1 Minute Summary

Soy-based foods, like tofu and soy milk, contain proteins and many other nutritional factors.

These nutritional factors might affect your muscles in different ways.

Therefore:

As much as possible, our review isolated the effect that soy protein (in supplemental powder form) has on muscle growth and strength.

For that, we found clinical trials that matched two criteria.

First:

The people in these trials engaged in one form of resistance (weight) training or another.

Second:

These individuals consumed soy protein powder at the same time.

After reviewing the data from 22 relevant trials, here is what we found:

6 of these clinical trials focused exclusively on “lab results”. That is, laboratory approximations of the way protein supplements might affect a person’s muscles.

Data from these “lab trials” implies (but doesn’t prove) that soy protein powder might be superior to placebo at enhancing muscle mass, size, and strength.

The other 16 trials provided data on “gym results”. That is, changes in real-world muscle mass, size, and strength.

Overall, these 16 trials tended to show three things.

First:

Weight training alone helps increase muscle mass, size, and strength.

Second:

Soy protein powder is unlikely to significantly enhance these gains in most people.

Third:

Whey protein powder doesn’t appear to be significantly better than soy protein powder at enhancing these gains.

These three findings may be especially true for generally healthy individuals who consume a well-balanced diet and are new to weight training.

There might be two exceptions:

Athletes and postmenopausal women.

For these two groups:

Soy protein powder might enhance the gains in muscle mass, size, or strength seen in response to weight training.

But the evidence for this is currently weak.

Results

General Public

This image shows that 57.1% of clinical trials found that soy protein is unlikely to boost muscle gains better than placebo in the general adult population. This image also shows that 87.5% of clinical trials found that soy protein powder is likely as effective as whey protein at enhancing muscle gains in response to weightlifting in the general adult population.

For individuals engaged in resistance-based exercise (ex: weightlifting, bodybuilding, etc):

57.1% (4 of 7) clinical trials found that soy protein powder is unlikely to boost muscle mass, size, or strength significantly better than placebo in the general adult population. (351 total participants.) [Weaker Evidence]

87.5% (7 of 8) clinical trials found that whey protein powder is unlikely to boost muscle mass, size, or strength significantly better than soy protein powder in the general adult population. (365 total participants.) [Moderate Evidence]

Novice Athletes

This image shows that 66.7% of clinical trials found that soy protein is unlikely to boost muscle gains better than placebo in novice athletes. This image also shows that 80.0% of clinical trials found that soy protein powder is likely as effective as whey protein at enhancing muscle gains in response to weightlifting in novice athletes.

For generally healthy adults new to resistance training, or generally healthy adults back from a prolonged absence from resistance training:

66.7% (2 of 3) clinical trials found that soy protein powder is unlikely to boost muscle mass, size, or strength significantly better than placebo in novice athletes. (165 total participants.) [Moderate Evidence]

80.0% (4 of 5) clinical trials found that whey protein powder is unlikely to boost muscle mass, size, or strength significantly better than soy protein powder in novice athletes. (251 total participants.) [Moderate Evidence]

Experienced Athletes

This image shows that 100% of clinical trials found that soy protein is likely to boost muscle gains better than placebo in experienced athletes.

For generally healthy athletes engaged in regular resistance-based exercise:

100% (2 of 2) clinical trials found that soy protein powder may enhance gains in lean body mass better than placebo in experienced athletes. (80 total participants.) [Weaker Evidence]

100% (1 of 1) clinical trials found that soy protein powder may enhance gains in muscle strength better than placebo in experienced athletes. (66 participants.) [Weaker Evidence]

Female (Postmenopausal)

This image shows that 75% of clinical trials found that soy protein is likely to boost muscle gains better than placebo in postmenopausal women.

For postmenopausal women engaged in resistance training:

75.0% (3 of 4) clinical trials found that soy protein powder may enhance gains in muscle strength or muscle mass better than placebo in postmenopausal women. (198 total participants.) [Weaker Evidence]

Female (Frail)

This image shows that 100% of clinical trials found that soy protein is unlikely to boost muscle gains better than placebo in elderly frail women. This image also shows that 100% of clinical trials found that soy protein powder is likely as effective as whey protein at enhancing muscle gains in response to weightlifting in elderly frail women.

For frail female individuals, or female individuals at risk of becoming frail, engaged in resistance training:

100% (1 of 1) clinical trials found that soy protein powder is unlikely to boost muscle mass, size, or strength significantly better than placebo in frail women. (66 participants.) [Weaker Evidence]

100% (1 of 1) clinical trials found that whey protein powder is unlikely to boost muscle mass, size, or strength significantly better than soy protein powder in frail women. (66 participants.) [Weaker Evidence]

Male (Overweight, High Cholesterol)

This image shows that 100% of clinical trials found that soy protein is unlikely to boost muscle gains better than placebo in overweight men with high cholesterol. This image also shows that 100% of clinical trials found that soy protein powder is likely as effective as whey protein at enhancing muscle gains in response to weightlifting in overweight men with high cholesterol.

For overweight males with hypercholesterolemia (high cholesterol), engaged in resistance training:

100% (1 of 1) clinical trials found that soy protein powder is unlikely to boost muscle mass or strength significantly better than placebo in overweight men with high cholesterol. (28 participants.) [Weaker Evidence]

100% (1 of 1) clinical trials found that whey protein powder is unlikely to boost muscle mass or strength significantly better than soy protein powder in overweight men with high cholesterol. (28 participants.) [Weaker Evidence]

Type 2 Diabetes

This image shows that 100% of clinical trials found that soy protein is unlikely to boost muscle gains better than branched chain amino acid (BCAA) powder in adults with type 2 diabetes mellitus.

For individuals with type 2 diabetes mellitus engaged in resistance training:

100% (1 of 1) clinical trials found that branched-chain amino acid (BCAA) powder is unlikely to boost muscle mass or strength significantly better than soy protein powder in people with diabetes. (36 participants.) [Weaker Evidence]

Vegans

This image shows that 100% of clinical trials found that an omnivorous diet supplemented with whey protein powder is unlikely to boost muscle gains significantly better than a vegan diet supplemented with soy protein powder.

For generally healthy young men engaged in weight training:

100% (1 of 1) clinical trials found that an omnivorous diet supplemented with whey protein powder is unlikely to boost muscle mass, size, or strength significantly better than a vegan diet supplemented with soy protein powder. (38 participants.) [Weaker Evidence]

Discussion

Lab Results vs. Gym Results

This image shows two graphs. These graphs show that whey protein may be better than soy protein at improving laboratory-based measures of muscle growth. However, this same image shows that researchers found that whey and soy protein are equally effective at boosting muscle gains in response to weightlifting. This means lab results don't always correlate to real-world gym results.

Our review of 22 clinical trials reveals three specific tendencies.

Tendencies related to the gains seen in muscle mass, size, and strength in response to weight training.

First:

Biomarker (“lab test”) studies in people tend to show that soy protein powder may help enhance these gains.

Second:

Biomarker studies in people tend to show that animal protein is generally more effective than soy protein at enhancing these gains.

Third:

The results of these biomarker studies appear to have little to no practical consequences “at the gym”.

Here’s what we mean:

A lab test may show an increase in muscle protein synthesis in response to supplementation with soy protein powder.

However:

This increase doesn’t appear to correlate to any significant gains in muscle mass, size, or strength.

Here’s another example:

Many biomarker studies in people suggest that whey protein powder is in many ways superior to soy protein powder.

Yet clinical trials consistently show that neither protein is better than the other.

At least not in terms of enhancing gains in muscle mass, size, or strength.

Let’s cover the details related to “lab results” vs. “gym results”.

Type II Fibers

Type II muscle fibers are a key part of the strength and power seen in bodybuilders and strongmen. (R)

Research shows that animal milk may significantly increase the size of type II fibers in response to weightlifting.

In fact, animal milk may do so better than soy protein according to biomarker studies in people. (R)

However:

It appears this “lab test” finding does not translate to any real-world differences between the two proteins.

In other words, under the microscope:

Milk protein appears to increase type II muscle fiber area more so than soy protein powder.

But “at the gym”:

Neither type of protein is better than the other at increasing muscle strength as measured by weight lifted.

This finding comes to us from a McMaster University clinical trial. (R)

Three groups of young male participants underwent the same three month, whole-body, weightlifting program.

  • Group 1: supplemented with a soy protein powder drink that was matched for energy, nitrogen concentration, and macronutrients to group 2.
  • Group 2: supplemented with fat-free milk.
  • Group 3: supplemented with placebo.

There was no significant difference in the amount of amino acids in the bloodstream between soy and milk.

All three groups experienced a significant increase in the size of their type II muscle fibers.

This increase was statistically larger for group 2 (milk) than group 1 (soy protein) or group 3 (placebo).

Despite this difference in lab tests, there was no significant difference in actual muscle strength between any of these groups.

Researchers also noted that all three groups increased their lean muscle mass (LMM).

The average increase in LMM was significantly greater for the milk group compared to the placebo group.

But the presence or absence of a significant difference between the milk and soy protein groups depends on the exact statistical analysis you’d like to believe. As such, we remain neutral on whether any true difference in LMM between soy protein and milk was demonstrated by this study.

There’s one other thing to note:

This was a de-facto open-label trial as there was potentially inadequate subject masking.

That means the participants may have known which substance they consumed in this trial.

Inadequate masking can alter the results of a trial in unforeseen ways.

Bioavailability

As a shorthand, we use the term “bioavailability” to refer to the following:

The relative amount of amino acids in the bloodstream after the consumption of a protein.

The bioavailability of milk proteins appears to be generally higher than soy protein powder.

Especially for specific amino acids, like leucine. (R)

But that’s not always the case.

That’s because the soy protein formula may influence whether such a difference even exists. (R)

Moreover, a difference in bioavailability may not have any significant real-world consequences.

At least not with respect to the way muscle mass, size, and strength changes in response to weight training.

In other words:

“In the lab”, milk protein may have a higher bioavailability than a comparable amount of soy protein powder.

But “at the gym”, milk protein may not enhance gains in muscle mass, size, or strength any better than soy protein powder.

That may be because there are only so many supplemental amino acids the body needs, or can use, at any given time.

Therefore, more is not always better in terms of real-world outcomes. Especially if your diet contains enough protein to begin with. (R)

Here’s a closer look at this.

One clinical trial compared three groups of similar individuals. (R)

They underwent the same whole-body resistance training program for 9 months.

  • Group 1: supplemented with soy protein isolate.
  • Group 2: supplemented with whey protein concentrate.
  • Group 3: placebo.

All three groups increased lean body mass (LBM) and strength at the end of the study.

The bioavailability of leucine (an amino acid) was higher in response to whey than soy protein isolate. Leucine is often thought of as being particularly important for muscle protein synthesis. (R)

Despite the higher bioavailability of leucine in whey:

There was no difference in muscle strength between the three groups at the end of the study.

This implies that neither soy nor whey are better than placebo at increasing muscle strength.

Please note:

The p-values in this trial make it appear that whey-supplemented individuals increased LBM more so than people who took soy or placebo.

We strongly caution against jumping to this conclusion.

The confidence intervals provided by this trial make it clear there was actually no significant difference in lean body mass between whey and placebo.

Nor any significant difference between soy and placebo.

(There isn’t enough clear data to be sure about the difference between whey and soy.)

As such, we rate the abstract’s conclusions as questionable based on the data provided.

An abstract’s conclusions often fail to fairly reflect the totality of data in the trial. (R)

And this particular trial was also sponsored by the dairy industry.

Moreover:

The results section of our review consistently shows that whey is not superior to soy protein powder for increasing lean body mass in response to weight training.

This underscores the fact that differences in amino acid bioavailability do not always lead to differences in practical (patient-centered) outcomes like gains in muscle strength, size, and mass.

Muscle Protein Synthesis

Lab tests in people show that whey increases muscle protein synthesis more so than soy protein powder.

But this doesn’t appear to lead to any practical benefit effect “at the gym”.

First, let’s take a look at a non-randomized, open-label, clinical trial from 2009. (R)

This trial was sponsored by the dairy industry.

It compared three groups that underwent a single session of lower body resistance exercise:

  • Group 1: supplemented with whey hydrolysate
  • Group 2: supplemented with soy protein isolate
  • Group 3: supplemented with micellar casein

Here’s what the researchers found:

Whey protein generally resulted in a significantly larger increase of amino acids in the blood than casein or soy.

Soy generally resulted in a significantly larger increase in amino acids than casein.

Mixed muscle protein synthesis (MMPS) was significantly greater with whey than with either casein or soy. And it was significantly greater with soy than with casein.

Leucine is an amino acid that is often thought of as playing a key part in muscle protein synthesis. It’s generally more abundant in whey than in soy. (R)

The study’s authors speculated that the difference in MMPS may relate to the different leucine content in each type of protein.

Is this actually the case?

To gain a clue, let’s turn to a newer, randomized, double-masked clinical trial from 2020. (R)

This trial lasted for 3 months.

It investigated the rate of protein synthesis in muscle cells (myofibrillar protein synthesis).

It also measured the rate of protein synthesis in the mitochondria (energy centers) of muscle tissue (mitochondrial protein synthesis).

This trial split the participants into 3 similar groups that underwent the same training program:

  • Group 1: supplemented with soy protein isolate (SPI)
  • Group 2: supplemented with whey protein concentrate (WPC)
  • Group 3: supplemented with soy protein isolate with an amount of leucine that matches the amount of leucine in the WPC (SPI+LEU).

In this trial, the researchers took blood samples and biopsies from the participants.

The people who took WPC or SPI+LEU had much higher amounts of leucine in their blood.

You would think that this would mean that the WPC and SPI+LEU would have higher rates of protein synthesis as well.

That wasn’t the case in this trial.

The rates of both types of protein synthesis were no different between the three groups.

As importantly, we know from the results section of our literature review that there is likely no practical difference between whey and soy protein powder.

This study’s result highlights what is sometimes true in medical studies:

The results of lab tests don’t always have practical, real-world consequences.

Resting Metabolic Rate

One clinical trial compared the effect of whey, soy protein powder, and placebo on people’s resting metabolic rate (RMR). (R)

The trial lasted 9 months and participants engaged in total body weight training.

As previously described, the results and conclusions found in trial abstracts are often unreliable. (R)

So after closely analyzing this study’s data, here’s what we found:

Weight training with protein supplementation did not significantly alter the RMR of people supplemented with whey protein, soy protein, or placebo

There was no significant difference between these three groups either.

In other words, soy protein powder doesn’t appear to influence changes in RMR in people who weight train.

Additionally:

This trial found that individuals supplemented with whey gained significantly more lean body mass than individuals supplemented with soy or placebo.

However:

This trial’s design and analysis focused on measuring RMR. It was not adequately designed to carefully measure nor analyze differences in lean body mass.

As the results section of our literature review indicates:

Trials that focused on measuring changes in lean body mass have consistently failed to find a difference between whey, soy, and placebo in response to resistance training.

Soy vs. Whey

This image shows that 87.5% (7 of 8) of clinical trials found that whey protein powder is not better than soy protein powder at enhancing muscle mass, size, or strength in response to weight training.

We filtered the clinical trials in our literature review in order to figure out how soy protein compares to whey.

We focused on real-world results: changes in muscle mass, size, or strength.

8 clinical trials compared soy protein powder to whey protein in the context of resistance training. These trials included a total of 364 participants.

So, is soy protein better than whey protein for building muscle?

Results from 87.5% (7 of 8) clinical trials suggest that soy protein isn’t better (nor worse) than whey protein at enhancing muscle mass, size, or strength in response to weight training. The 1 trial that found that whey was superior to soy was funded by the dairy industry. This same trial didn’t provide adequate statistical information to be certain of its results.

Moreover, there is no evidence to suggest that soy reduces gains. There is also no evidence to suggest that soy makes it harder to build muscle. In fact, quite the opposite.

This image shows that soy protein powder doesn't reduce muscle gains. Soy protein doesn't make it harder to build muscle. 42.9% of clinical trials suggest that soy protein helps build muscle. 57.1% of clinical trials suggest that soy protein does nothing to help or hurt build muscle.

42.9% (3 of 7) clinical trials suggest that soy enhances gains or helps build muscle better than placebo. At worst, 57.1% (4 of 7) clinical trials suggest that soy is not better (nor worse) than placebo at enhancing gains or building muscle.

No clinical trials found that soy was “worse” than placebo.

In other words, no clinical trials found that soy hindered any gains in muscle mass, size, or strength.

At worst, it appears that soy “does nothing” (good or bad) for most weightlifters and bodybuilders.

Novices vs. Athletes

The results section of our review suggests a difference between athletes and novices.

With respect to building muscle mass, size, and strength:

Clinical trials show that people that are relatively new to weight training are unlikely to benefit from soy protein powder.

This image shows that 66.7% of clinical trials found that soy protein is unlikely to boost muscle gains better than placebo in novice athletes. This image also shows that 80.0% of clinical trials found that soy protein powder is likely as effective as whey protein at enhancing muscle gains in response to weightlifting in novice athletes.

But bodybuilders, elite athletes, or individuals who work out at the gym on a regular basis might benefit from soy protein supplementation.

This image shows that 100% of clinical trials found that soy protein is likely to boost muscle gains better than placebo in experienced athletes.

For brevity’s sake, we propose only two of many possible explanations for this difference.

The first one is quite ordinary.

The data from clinical trials conducted on athletes is relatively unreliable. The difference between novices and athletes may therefore be a statistical fluke. More reliable data on athletes may show soy protein powder doesn’t help them either.

The second explanation, proposed by researchers, is a bit more intricate. (R)

For argument’s sake, we’ll say that novices start at “0” when it comes to muscle mass, size, and strength (MSS).

Therefore, resistance training alone will lead to large gains in MSS over time.

Let’s pretend that soy protein powder does enhance these gains, but by a tiny amount.

A trial may compare novices given a placebo to novices given soy protein powder.

Both groups undergo a resistance training program.

The trial finds there is no difference between the two groups.

It could be that the large gains in MSS from the resistance training simply overshadow the tiny (but very real) enhancement in gains from the soy protein powder.

As a result, statistically speaking, we say there is no difference between placebo and soy.

Now let’s turn to elite athletes.

We’ll pretend the trial design is exactly the same as before.

Elite athletes are at or near the peak of their physical capabilities.

Additional resistance training is likely to add very little in terms of MSS. Therefore the effect of resistance training alone, on gains in MSS, is very small. So too, the effect of soy protein powder on the enhancement of gains is very small.

But now, the effect from the soy is no longer massively overshadowed by the effect of resistance training alone.

This, then, reveals the (possible) small effect soy protein powder has on gains in muscle mass, strength, or size more clearly.

Postmenopausal Women

This image shows that 75% of clinical trials found that soy protein is likely to boost muscle gains better than placebo in postmenopausal women.

We found 22 clinical trials relevant to this literature review.

5 of them provided data exclusively on women.

4 of these trials focused on postmenopausal women.

The general results of this literature review state that most people are unlikely to benefit from soy protein powder supplementation. At least with respect to enhancing gains in response to weight training.

An exception seems to be postmenopausal women.

Postmenopausal women appear to enhance their muscle strength and mass with soy protein supplementation.

There can be many explanations for this exception.

We propose two such theories.

The first one is very simple.

The data from studies on postmenopausal women is not highly reliable.

For instance, one study didn’t control properly for the effect of exercise alone. It also used less reliable statistical techniques to reach its conclusions. (R)

Another study didn’t properly control for the amount of dietary (vs. supplemental) protein. (R)

Less reliable data may simply reflect statistically permissible random chance. Random chance that makes it appear like soy protein powder is effective. When it’s not.

Better data is less subject to the whims of random chance.

So more reliable trials in the future may find that postmenopausal women do not benefit from soy protein powder supplementation in this respect.

There may be another explanation.

Perhaps postmenopausal women really do stand to gain more from resistance training combined with soy protein powder supplementation. As opposed to resistance training alone. (R)

Some older women may consume insufficient amounts of dietary protein. (R)

This may play a part in the lower muscle strength and mass seen in older women compared to men or younger individuals. (R)

Owing in part to this lack of dietary protein:

Supplementation with soy protein powder may therefore synergistically and significantly enhance the beneficial effects of resistance training in postmenopausal women. (R)

Again, this is a hypothesis that hasn’t been proven.

But it does have some basis in fact worth exploring in future trials.

Does Dose Matter?

This is an image of a graph. This image shows that 66.7% of clinical trials found that soy protein powder is unlikely to boost muscle gains better than placebo at a dose of less than 26 grams per day. This image also shows that 50% of clinical trials found that soy protein powder is unlikely to boost muscle gains better than placebo when taken at a dose of 35 or more grams per day.

For this section of our review, we found 10 clinical trials that matched three criteria:

  • Soy protein powder was compared to placebo.
  • There was enough information to calculate the daily dose of supplemented soy protein consumed.
  • The trial measured gains in muscle mass, size, or strength (as opposed to lab test results related to this).

We then sorted these ten trials in order of the daily dose of soy protein.

There was a natural split in these trials.

The dose was less than 26 grams of protein per day in 6 trials.

The dose was 35 grams or more per day in the other 4 trials.

Here is what we found with respect to gains in muscle mass, size, or strength.

For trials with a dose of less than 26 grams per day:

  • 67% (4 of 6 trials) did not find a difference between soy and placebo.

For trials with a dose of 35 grams or more per day:

  • 50% (2 of 4 trials) did not find a difference between soy and placebo.

It’s hard to draw any firm conclusions given such a small sample size.

We caution that the dose may strongly interlace with the training program’s frequency, intensity, and duration in a way that makes it difficult to conclude anything based on dose alone.

Does Duration Matter?

This is an image of a graph. This image shows that 75% of clinical trials found that soy protein powder is unlikely to boost muscle gains better than placebo when taken for up to 3 months. This image also shows that 50% of clinical trials found that soy protein powder is unlikely to boost muscle gains better than placebo when taken for 4 or more months.

With respect to building muscle mass, size, or strength in response to weight training:

A lot of clinical trials have failed to show that soy protein powder is better than placebo.

This might be because many clinical trials are too short to show a difference.

Of the trials with adequate data that measured gains in muscle mass, size, or strength in response to weight training:

8 lasted for 3 months or less.

6 lasted for 4+ months (up to a maximum of 9 months).

  • Of the trials that lasted 3 months or less, 75% (6 of 8) found no difference between whey and soy, or soy and placebo.
  • Of the trials that lasted 4+ months, 50% (3 of 6) found no difference between whey and soy, or soy and placebo.

It appears that longer duration trials might favor soy over placebo, or whey over soy.

However, the sample size is too small and many of the trials provide unreliable data.

We can’t draw any firm conclusions as a result.

Soy Isolate vs. Concentrate

This image shows that soy protein isolate, soy protein concentrate, and whey protein are likely equally effective at boosting muscle gains in response to weight training.

Does the type of soy protein powder influence how effective the powder is?

With respect to gains in muscle mass, size, or strength in response to weight training:

4 clinical trials clearly stated they used soy protein isolate (SPI).

2 clinical trials clearly stated they used soy protein concentrate (SPC).

  • For trials that used SPI:
    • 67% (2 of 3) trials found no difference between whey and soy.
    • 100% (1 of 1) trials found that soy was superior to placebo.
  • For trials that used SPC:
    • None found a difference between whey and soy.
    • None found a difference between soy and placebo.

Overall, these clinical trials suggest that soy protein isolate may be better for building muscle than soy protein concentrate. At least when compared to placebo.

However, we believe that the sample size is too small to draw any firm conclusions.

Effect of Leucine

Leucine is an amino acid.

It might play a potentially outsized role in muscle protein synthesis. (R)

There’s usually more leucine in whey than soy protein. (R)

Researchers have speculated that this might be the reason why some (but not all) trials found that whey is better than soy protein for building muscle size and strength. (R)

What does the evidence actually say?

When it comes to muscle growth and strength in response to weight training:

Our review found 2 clinical trials that provided the same amount of leucine in the whey and soy protein supplements.

Here is what these trials found:

  • Neither whey nor soy protein powder were better than placebo.
  • Whey protein was neither more nor less effective than soy protein powder.

Overall, it appears that the addition of leucine to soy protein powder is unlikely to change the results outlined in this review.

In other words:

With or without leucine supplementation, whey is likely not superior to soy (and soy is likely not better than placebo) for most people.

Admittedly, the sample size is quite small for leucine-matched trials.

Therefore, no firm conclusion can be made.

Side Effects

6 clinical trials reported on adverse events (side effects) related to the ingestion of soy protein powder.

4 of these trials stated there were no adverse events.

1 trial did not identify which participants (those taking soy or those taking placebo) experienced which adverse events.

1 trial stated that the only adverse event related to soy protein powder was constipation (1 participant; 6.67% of those who received soy protein powder).

Misleading Abstract Data

The results section of our analysis shows that soy protein powder is unlikely to enhance the gains seen in muscle mass, strength, and size (MSS) in response to weight training.

Moreover, whey protein powder doesn’t appear to be more effective than soy protein powder in this regard either.

The idea that whey protein doesn’t significantly enhance gains in MSS is not new.

It has been documented in studies on whey protein powder many times in the past. (R)

But a quick skim of the abstracts of all trials can paint a very different picture.

A picture that soy protein is effective.

Or that whey protein is better than soy protein powder.

We urge caution.

Our literature review identified 22 highly relevant clinical trials.

If you compare the numerical conclusions in our review’s results section to the data provided in the abstracts of these trials, you might believe we are misleading you.

We found numerous instances where the results and conclusions of an abstract were strongly or entirely misleading.

Misleading as compared to the actual data presented within each respective study.

This is nothing new.

The fact that abstracts provide misleading information is an established problem in evidence-based medical practice. (R)

The fact that abstracts should never be relied upon for clinical decision making is just as well-known.

Therefore:

The results section of our review does not reflect a superficial overview of data in the abstracts.

Our results section reflects a closer examination of clinical trial data, methods, and statistics within each study.

Funding Bias

This image shows that clinical trials sponsored by the dairy industry are far more likely to conclude that whey is better than soy at boosting muscle gains. Independent clinical trials were more likely to find that whey protein is not superior to soy protein for building muscle in response to weighlifting.

Many of the clinical trials we uncovered were funded by for-profit entities.

Did such conflicts of interest potentially influence any results?

21 clinical trials in this review provided information that helped determine conflicts of interest.

We caution that a lack of a clear conflict of interest in a trial is not evidence for the absence of a conflict of interest. That’s because many authors don’t properly report such conflicts. (R)

Nevertheless, we found 10 clinical trials (397 total participants) that had strong potential for conflicts of interest. This represents 45.45% of all trials in our review.

We identified 6 trials as sponsored by the dairy industry. These were trials where whey protein was compared to soy protein powder.

We labeled a further 4 trials as sponsored or likely sponsored by the soy industry.

  • 66.67% (4 of 6) dairy-sponsored trials published results that favored whey protein over soy protein powder.
    • Compare: 20.0% (1 of 5) independent trials had results that favored whey protein over soy protein powder.
  • 50% (2 of 4) soy industry-sponsored trials published results that favored soy protein powder over placebo.
    • Compare: 33.3% (2 of 6) independent trials had results that favored soy protein powder over placebo.

Studies sponsored by the dairy industry seem to find more favorable results for whey over soy, as compared to respective independent studies. Likewise, studies sponsored by the soy industry seem more likely to favor soy over placebo.

Financial conflicts of interest often correlate to favorable clinical trial results. (R)

Geographic Bias

This image shows that clinical trials conducted outside the U.S. and Canada were far more likely to conclude that whey protein is superior to soy protein at enhancing muscle gains in response to weight training.

Evidence shows that falsified data or outright fake clinical trials are a problem in medical literature. (R)

Especially from specific countries. And even in critical medical fields and within important medical journals.

We are not implying any of the trials we identified are fake or contain fake data.

But it would be inappropriate for us to dismiss this possibility either.

Of the trials that provided data on gains in muscle mass, size, and strength in response to resistance training:

  • 14.3% (1 of 7) trials conducted in the U.S. and Canada found that soy was superior to placebo or that whey was superior to soy.
  • 55.6% (5 of 9) trials conducted outside the U.S. and Canada found that soy was superior to placebo or that whey was superior to soy.

The trials conducted outside the U.S. and Canada were conducted in Brazil, India, Japan, and Romania.

Admittedly, this is a somewhat simple comparison.

Trial design, industry funding, and other factors could also explain why this split occurred.

Methods

Clinical Question

Our analysis focused on answering the following clinical question:

“How does the ingestion of soy protein powder affect measures of skeletal muscle adaptations in response to resistance-based exercise?”

Primary intervention of interest: supplementation with soy protein powder in the context of resistance-focused exercise.

Primary outcomes of interest: patient-centered measures of skeletal muscle adaptations (mass, size, and strength).

Secondary outcomes of interest: comparison of the efficacy of soy protein powder to relevant alternatives, such as whey protein powder, on primary outcomes of interest.

Tertiary outcomes of interest: adverse events and correlation of surrogate endpoints (such as amino acid bioavailability or rates of muscle protein synthesis) to primary or secondary outcomes of interest.

Inclusion & Exclusion Criteria

As per the clinical question, the purpose of our literature review was to—as much as practically feasible—isolate the magnitude and direction of the effect of soy protein itself (as opposed to other nutritional factors found within soybeans or soy products) on outcomes related to bodybuilding, weightlifting, and other resistance-focused exercises.

This means we excluded studies that used tofu, tempeh, soy milk, and similar soy-based foods as the primary intervention, since these foods include a large variety of nutrients that may alter the effect of soy protein itself. Similarly, we further excluded any studies that did not provide adequate information for us to determine that the primary intervention was soy protein in powder form. This was important, as many studies mentioned they used an undefined “soy protein”. We know from experience that “soy protein” does not have to be a term for soy protein powder and may refer to various non-powder forms of soy such as textured soy protein, which is not equivalent to soy protein powder.

We also excluded any studies where the primary intervention was a soy protein powder that was blended with another form of protein; unless the non-primary intervention comparator arm(s) excluded the soy protein powder in an otherwise equivalent blend of protein, thereby allowing us to isolate the effect size of soy protein in such cases.

In order to gather the best available evidence for any potential cause-and-effect relationship pertinent to the clinical question, we focused on data solely from clinical trials and any meta-analyses thereof. This means we excluded observational and basic science literature and any meta-analyses thereof, any interim analyses, conference abstracts, whitepapers, and other non-peer-reviewed data.

All trials wherein study participants performed predominantly aerobic or endurance-based exercises (such as running) were excluded.

Trials where our primary outcome of interest was a primary, secondary, or surrogate (biomarker) measure were included. We excluded all trials where our primary outcome of interest was a tangential, indirect, or incomplete outcome measure (like the maintenance of lean body mass in free-living elderly adults).

Studies published in a non-English language were excluded.

Studies on individuals under the age of 18 years were excluded.

Search Strategy

An initial literature search was conducted in September of 2023, using three distinct searches to maximize the number of relevant clinical trials we could find.

(See the Last Literature Review date in the beginning of this article to see when we last searched for any new trial data on this topic.)

Search #1: Primary Intervention (high specificity, low sensitivity search); Primary Outcome (low specificity, high sensitivity search).

Using PubMed, search #1 used search terms focused solely on identifying published studies where the primary intervention was a soy-based protein product (high specificity, low sensitivity). No search terms were used for the primary outcome (low specificity, high sensitivity).

Filters for clinical trials on humans, practice guidelines, and errata/retractions were applied.

Search #1 yielded 1,322 results.

This image shows the results of the first PubMed search we undertook in our review.

Search #2: Primary Intervention (low specificity, high sensitivity search); Primary Outcome (high specificity, low sensitivity search).

Using PubMed, search #2 was (in terms of specificity/sensitivity) the opposite of search #1. We used very general terminology for the primary intervention (low specificity, high sensitivity) but specific terminology for the primary outcome (high specificity, low sensitivity).

Filters were applied to search #2 in the same manner as for search #1.

Search #2 yielded 577 results, of which 267 were duplicates from search #1, resulting in 310 additional results.

This image shows the results of the second PubMed search we undertook in our review.

Search #1 and Search #2 thereby yielded a total of 1,632 unique results.

Search #3 was a post-hoc “sweep” conducted after we identified trials of interest from searches #1–#2 but before we began their analysis.

For search #3, we used a free-form PubMed search, Google search, Google Scholar, as well as the citations within the identified trials and related published literature in order to find any potential trials of interest that we may have missed in the first two searches. The sweep is an acknowledgement that PubMed doesn’t contain all studies of interest (or may not at the time of our search) and that PubMed filters are imperfect and, on occasion, miscategorize or fail to categorize studies of interest.

After searches #1–#3 were completed, we found a total of 22 clinical trials that matched our inclusion/exclusion criteria and began data collection and analysis of said trials.

Limitations

Our clinical question sought to isolate the effect of soy protein (in supplemental powder form). As such, we excluded many clinical trials where the intervention was soy milk, textured soy protein, tofu, soybeans, and other soy-based products.

It’s possible that other nutritional factors in more complete soy products may enhance or impair the effect (or lack of effect) that soy protein powder has on gains in muscle mass, size, and strength seen in response to weight training.

We further excluded clinical trials where the exercise was more aerobic in nature, such as running. Naturally, aerobic exercises also confer gains in muscle mass, size, or strength.

Our literature review did not include studies published in a language other than English.

While we believe (based on our search criteria) we found most (if not all) relevant clinical trials at the time of the search, it is possible we failed to find trials with relevant secondary or surrogate endpoint data that was not mentioned in the focus of our keyword search (the titles and abstracts).

The studies we identified for this review had limitations of their own that may influence the results outlined in our review.

For example, not all studies matched soy and whey supplementation gram for gram and most studies did not adequately monitor for the influence of dietary protein intake. Many studies didn’t provide an adequate amino acid composition or nutritional information related to the protein supplements.

The extent of organoleptic blinding was often impossible to determine.

Results showing an effect or null effect of soy protein powder or whey routinely stemmed from post-hoc analyses. Some trials statistically compared the interventions of their trial (soy vs. placebo) to those of another trial (whey vs. placebo) in order to draw conclusions for whey vs. soy.

While many of the trials we reviewed supervised the consumption of each supplement or placebo, many other trials did not. While many trials supervised a participant’s exercise, other trials collected exercise data in survey format instead.

Some of the clinical trials we came across did not provide adequate information on the baseline comparability of participants in each arm.

A large number of trials focused on the effect of soy protein on untrained participants as opposed to more experienced athletes.

Importantly, virtually none of the clinical trials actively monitored for (nor reported) information about adverse events related to the soy protein supplementation.

Disclosure

We declare no conflicts of interest. We do not sell any supplements and this review hasn’t been sponsored by, nor affiliated with, any outside entity in any way.

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