Whey or Soy Protein for Building Muscle?

As most protein supplement connoisseurs know, whey protein and soy protein are two of the most popular choices on the market. Of course, they share similarities and differences. Athletes and fitness enthusiasts who want a convenient source of protein to aid in their training efforts use them both.

We know that getting enough protein is crucial for building muscle and improving your body composition. However, if you eat too little protein, your efforts in the gym, while not wasted, will not give you optimal results.

Which is the best option for strength training individuals looking to improve their body composition? Should you use whey protein or soy protein if you want to build muscle and lose fat? It’s the battle of the ages! Well, not really, but it’s an interesting enough question. Interesting enough for a brand new meta-analysis to sift through the available scientific research and, hopefully, give us an answer.1

In this article, we’ll take a look at said meta-analysis and dissect their conclusions. Read on!

Whey Protein Vs. Soy Protein

Both proteins are “complete proteins”. That means that they provide you with enough of all the amino acids you need to build muscle. However, whey protein is an animal protein, while soy protein is a vegetable protein. 

A comparison between the two is extra relevant because of the rise in popularity of veganism. If you eat a purely plant-based diet, you don’t have the option of using whey protein. 

While whey and soy share some similarities, there are also several differences between the two, apart from animal-based and plant-based. For example, they differ in how fast you absorb the protein and how fast you digest it. Also, their amino acid composition, meaning how much of the 20 amino acids they contain, differs quite a bit.

Whey Protein

Whey protein is one of the two proteins in milk. The other one is casein. The milk from all animals that produce breast milk, including humans, contains various amounts of whey protein. So when you pour milk on your cereal, you add a small amount of whey protein to your breakfast.

Whey protein in supplement form is a byproduct of the manufacture of cheese. This process separates the whey from the casein. The protein left in the cheese is entirely casein, and the whey can be used for other things. Whey was pig feed until somone came up with the idea to market it for athletic purposes. It still is, but now we also get gains in a plastic tub.

When you buy a whey protein supplement, you get whey protein from cow’s milk. There might be exceptions, like goat or camel whey protein, but that’s a niche product if ever there was one. Regular cow’s milk contains 20% whey protein, the rest being casein.

Whey protein contains plenty of the amino acids most crucial for stimulating protein synthesis. In particular, whey is the protein with the most leucine of all the proteins you can find. Leucine is the amino acid that triggers the whole process of building new muscle tissue. 

Many studies prove beyond a shadow of a doubt that whey protein stimulates muscle protein synthesis. Also, some suggest an association between whey protein and reduced body fat.

Soy Protein

Unlike most other plant-based proteins, soy protein provides you with enough of all the essential amino acids to support muscle protein synthesis. That means that your body can use soy protein by itself to build muscle to the fullest extent. That is unusual in plant-based proteins. Almost without exception, they contain too little of one or several of the amino acids you need to build muscle. Soy protein is that exception.

Not very surprisingly, you make soy protein supplements from soybeans. It is the most popular protein supplement for anyone who can’t or doesn’t want to use animal-based protein powders.

In addition to the protein, soybeans also contain a fair amount of both carbohydrates and fat. When you manufacture the protein powder, you filtrate the carbs and fat away. So while you get the same high-quality protein from eating soybeans, a supplement gives you the option of getting the protein and only the protein.

Let’s Look at the New Meta-Analysis

This was a meta-analysis of randomized clinical trials (RCTs) investigating the effect of whey protein supplementation and soy protein supplementation on body composition in adults. The researchers looked at how using a whey or soy protein supplement affected lean body mass, fat mass, total body mass, and body fat percentage compared to a placebo.

After the first search for relevant papers, the researchers ended up with a digital pile of 662 seemingly relevant articles. However, most of these were duplicates or turned out to not be eligible for inclusion after all. 

The researchers excluded observational studies, review articles, letters to the editor of a journal, case reports, animal experiments, and protocol studies. In addition, they also removed studies without a control group, studies that didn’t account for body composition changes, and studies where the participants had diseases that could affect the outcome.

All the studies were randomized and blinded. Unfortunately, the quality of the included studies was pretty low. After a quality assessment, the researchers concluded that only four studies qualified for a “fair” or “good” quality status. The rest were of poor quality.

Meet the Subjects

When all was said and done, the researchers were left with a handful of relevant studies. Ten randomized controlled trials with human subjects with a total of 596 subjects. 356 were in the intervention group, meaning they received the whey or soy protein supplements. The other 240 subjects were control subjects who received a placebo instead of protein.

Out of these ten studies, seven had normal-weight subjects, while three had overweight or obese subjects. The participants were both male and female between the ages of 18 and 65. The studies varied in length, from only two weeks to 36 weeks. 

The subjects in the whey groups received between 22 to 56 grams of supplemental protein per day. The dosage of soy protein varied from 22 to 70 grams per day.

The subjects in seven of the ten studies combined protein supplementation with exercise.

Whey or Soy Protein: Results

This is the first meta-analysis or review to compare the effects of whey protein and soy protein supplements on body composition.

In short, whey was the winner in the body composition department.

The researchers saw no association between either whey protein or soy protein and any changes in total body mass, body fat, or body fat percentage. 

However, they saw an association between whey protein and an increase in lean body mass, meaning muscle.

Not so with soy protein.

The combination of exercise and whey protein was associated with more muscle mass. Again, no such association for soy protein, even with exercise in the mix.

Neither whey nor soy protein was associated with body fat. However, two earlier meta-analyses covering whey protein and soy protein, respectively, both saw a reduction of body fat when using a protein supplement.2 3

Why the different results?

Unlike the meta-analysis covered in this article, the first one only looked at resistance-training subjects, which could partially explain the differences. Perhaps you need to lift to have any use of whey protein for fat loss.

The second meta-analysis only featured overweight or obese subjects. However, most of the subjects in this new meta-analysis were normal-weight, so the interventions might not be directly comparable. Also, some of the studies provided soy protein in the form of various soy products, like soy milk, not just as protein powder.


Whey protein is associated with muscle mass gains. However, it is not associated with any changes in total body mass or any body fat parameters. 

Soy protein is not associated with any significant changes in body composition at all, including muscle mass.

The meta-analysis in question has several weaknesses. 

The first is the overall low quality of the included studies. That is not the fault of the analysis itself but a reflection of the state of protein research.

Another weak point is that the whey and soy protein doses varied a lot from study to study. As a result, it’s difficult to conclude anything when comparing anything from 22 to 70 grams of added protein per day.

The limited number of studies means low statistical power. But, again, that’s not something the researchers behind this meta-analysis can influence. There simply aren’t any more studies comparing the effects of whey and soy protein on body composition.

And, of course, the results might be of limited use to dedicated lifters who are serious about their diet. If that sounds like you, you are not representative of the subjects in the analyzed studies. So don’t take the results as indisputable facts when approaching your own protein intake and supplementation.

Whey or Soy Protein: Our Take

It would have been nice to have an analysis of a large number of well-controlled studies featuring serious strength-training subjects eating enough protein overall to support their muscle-building efforts. Unfortunately, however, that research isn’t available.

We have to make do with what we’ve got, and what we’ve got are these ten studies, the majority of which aren’t of the highest quality. 

This analysis failed to find any support for soy protein for building muscle. However, if you’re a vegan or don’t use whey protein for some other reason, don’t despair. A recent study demonstrated that whey protein and soy protein supplementation results in the same amount of muscle growth and strength gains in strength-training men and women, as long as you match the leucine content of the proteins.

Read more:

>> The Best Protein Powder for Men and Women Over 50

What Does “Match the Leucine Content” Mean?

It means that you’d have to eat or drink more soy protein for comparable effects. Soy protein does not contain as much leucine as whey protein on a gram-for-gram basis. Therefore, you need to up the dosage slightly. In said study, the whey group received 19 grams of protein and the soy group 26 grams, and they both gained the same.4 That study did not have a control group and is not featured in this new meta-analysis. It is still highly relevant, though.

We believe that soy protein works just fine for building muscle, as long as your total protein intake is sufficient and you use enough soy protein per serving. Several studies show that 20 grams of whey protein stimulate muscle protein synthesis to the max. So if you make sure you eat, say, 30 grams of soy protein to compensate for the lower amount of leucine, you should be good.

Another new meta-analysis also concludes that plant-based proteins are less effective than animal-based ones for gaining muscle mass and strength.5 However, that does not mean that plant-based proteins are useless. Once again, eat a little more of it, and you should be fine. A meta-analysis partially financed by the soy industry found no difference between soy protein and animal proteins for muscle mass and strength gains.6 Take that for what you will, but the financing shouldn’t affect the statistical analysis of other studies.


Even if this meta-analysis found no effect of soy protein supplements on muscle mass, it is too general to necessarily apply to dedicated lifters eating plenty of protein. We believe that as long as your total protein intake is sufficient and you pay your dues in the gym, you will gain muscle and strength regardless of your choice of protein supplement. Or even if you use one at all.

Lift, eat, gain. You’ll get results. Whey protein and soy protein are both good options to supplement your diet. Or get the protein you need from your regular foods.

Further Reading

We have dedicated review articles diving deep into both whey and soy. Check them out!

Want to learn more about dietary supplements? Which ones are worth your money, and which are questionable or useless? Check our StrengthLog’s Supplement Guide, our free guide where I review 26 of the most popular supplements.


  1. Br J Nutr. 2021 May 11;1-27. Comparison of the Effect of Soy protein and Whey protein on Body Composition: A Meta-Analysis of Randomized Clinical Trials.
  2. Food Funct. 2019 May 22;10(5):2766-2773. Effect of whey protein supplementation during resistance training sessions on body mass and muscular strength: a meta-analysis.
  3. Nutrients 2019, 11(11), 2790. Soy Products Ameliorate Obesity-Related Anthropometric Indicators in Overweight or Obese Asian and Non-Menopausal Women: A Meta-Analysis of Randomized Controlled Trials.
  4. Int J Environ Res Public Health. 2020 Jun; 17(11): 3871. No Significant Differences in Muscle Growth and Strength Development When Consuming Soy and Whey Protein Supplements Matched for Leucine Following a 12 Week Resistance Training Program in Men and Women: A Randomized Trial.
  5. Nutrients 2021, 13(2), 661; Animal Protein versus Plant Protein in Supporting Lean Mass and Muscle Strength: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.
  6. Int J Sport Nutr Exerc Metab. 2018 Nov 1;28(6):674-685. No Difference Between the Effects of Supplementing With Soy Protein Versus Animal Protein on Gains in Muscle Mass and Strength in Response to Resistance Exercise.
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Andreas Abelsson

Andreas is a certified nutrition coach with over three decades of training experience. He has followed and reported on the research fields of exercise, nutrition, and health for almost as long and is a specialist in metabolic health and nutrition coaching for athletes. Read more about Andreas and StrengthLog by clicking here.