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March 12, 2022 10 min read

If you’re at all serious about gaining muscle and looking your best, protein is the name of the game.

This macronutrient provides the building block of our muscles and ligaments, making it an essential part of diets—especially for those who hit the gym regularly.

And as we all know, there’s nothing like a steak or a chicken breast to give our body its fill of muscle juice. Getting enough dietary protein throughout the day is absolutely critical for optimizing lean body mass with weight loss and resistance training.

Whether it’s pre- or post-workout, whey protein supplementation is unparalleled.

This is also why whey protein powder is the closest supplement to being considered “necessary.”

However, animal-based proteins aren’t the end-all and be-all of protein. Although animal sources of protein are largely considered to be high-quality, “complete” proteins, there’s a whole world of plant-based proteins that offer unique benefits. The largest issue is that none of these are considered entirely complete. Well—except for soy protein. 

So, how does it measure up to the golden standard set by whey?

Products rich in amino acids. Protein sources and food for bodybuilders

What Makes a Good Protein Source?

What differentiates soy protein from other plant-based proteins is its “completeness.” But what exactly is a complete protein? 

There are over 20 different amino acids that act as the building blocks for thousands of different types of proteins that are found just in our bodies.

Although most of these amino acids can be produced by our bodies, nine of these cannot.

These are known as the essential amino acids (EAAs), and they consist of:

  • Histidine
  • Isoleucine
  • Leucine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Valine

For a protein to be considered complete, it not only has to contain all nine of the EAAs, but it also has to have them in sufficient ratios. For example, pea protein is low in methionine, which prevents it from being considered complete. 

Amino acids can be further subdivided into branched-chain amino acids (BCAAs) which are primarily used in the muscles. They’re therefore believed to play a particularly important role in muscle development.

The Effects of Whey Protein Intake

Whey protein powder comes from cow’s milk, as a by-product of the cheese-making process. That watery liquid that sits on top of yogurt when you open a container is whey. The protein powder itself goes through different levels of processing to create the substance that we all know and love.

Because whey protein is an animal product, it offers a complete amino acid profile.

Not only are there high levels of all the amino acids, but the protein is also extremely bioavailable, allowing it to be quickly absorbed by our bodies. These factors make whey protein powder the golden standard of protein supplementation.

It’s a relatively cheap and plentiful source of protein that’s accessible and easily implemented within diets and workout routines.

However, whey protein obviously doesn’t work with a lot of different dietary restrictions. Even looking past vegan diets, some people also have lactose intolerance.

Whey protein can sometimes not agree with peoples’ digestive systems, leading to gassiness and stomach issues. 

Due to these reasons, plant-based alternatives have been seeing increasing popularity. Chief among these is soy protein powder—but is it a worthy alternative?

The Effects of Soy Protein Intake

Soy protein powder comes from the legume called soybean.

Having been eaten by humans for over 5,000 years now, it is one of the only plant-derived proteins that is considered a complete protein. 

Other than the protein itself which makes up roughly a third of soy, another 30% is carbs, 19% is oil, and 14% is water (moisture). It’s especially unique in that it’s very fatty, and therefore it finds uses in a surprising amount of foods.

Soy milk, tempeh, edamame, and tofu are all directly made from soybean.

However, soy is also found in a ton of different baked goods, candies, and soap. It’s even sometimes found in whey protein powder because it contains an emulsifier that makes the whey easier to mix. 

This plant protein also contains a significant amount of zinc, potassium, iron, phosphorus, vitamin E, and vitamin B complex.

It can also help to keep cholesterol under control in those whose cholesterol is within normal levels.

On paper, this sounds great. However, there is a reason why soy has attracted so much scandal over the years. This can be generally encompassed by looking at soy’s effects on muscle building, on estrogen and testosterone, and its antinutrients.

Comparing Training Benefits

The big question that it comes down to, is whether soy is comparable to whey when it comes to the benefits it has on our training and body composition. Whey has been consistently shown to increase muscle size and training capacity when used with a training regime.

There are two factors that opponents of soy bring up: 

The first argument is that although soy contains all nine EAAs in sufficient quantities, it contains much less leucine which is important for muscle protein synthesis (the process that builds our muscles to be larger and stronger). 

The second argument is that soy contains phytoestrogens which “feminize” men by increasing estrogen levels and decreasing testosterone levels. While most of the evidence argues against this, it is worth looking at the studies that have shown otherwise. But first, let’s compare soy and whey when it comes to building muscle mass.

Whey or Soy for Building Muscle?

Both soy and whey share all nine of the essential amino acids, so that means they should be equally as good at eliciting gains—right? 

Well, we don’t conclusively know. But there is good evidence that suggests soy might be slightly less optimal because it contains less of the EAA, leucine.

What is Leucine? 

Leucine is believed to be the most important amino acid in muscle protein synthesis (MPS). Muscle protein synthesis is a critical process that tells our muscles when to develop and grow, so it follows that it’s critical for optimizing our gains. This isn’t to say that all you need is leucine—it’s only one part of a myriad of compounds that help your muscles grow. However, it still holds a special place.

Leucine in Whey vs Soy

Whey protein, when compared to many other proteins, has a very high amount of leucine.

Because of this, the absorption rate of the protein content is much higher. For every 100 grams of whey protein isolate, there should be between 11 and 13 grams of leucine. The same amount of soy protein isolate will net you 8 to 9 grams of leucine.

When converting down to scoops, whey will have about 3 grams of leucine while soy will have just over two.

This difference is small and it only really matters if you’re concerned primarily with muscle protein synthesis. Which, to be fair, is an important concern if you’re trying to eke out every improvement you can in your training.

It’s an even more important concern when you consider that leucine has a threshold it has to reach before triggering muscle protein synthesis. Although studies in this area are pretty inconclusive, it’s believed that this threshold is 3 grams.

What this means is that you need to take slightly more soy protein to match the leucine content of whey.

And when leucine content is controlled between the two, there is  no significant difference in muscle growth and strength development. For example, eating 30 grams of either protein will likely mean that there’s no difference between the two in muscle protein synthesis.

And this very tentative “conclusion” only deals with one aspect of growing bigger and stronger (albeit an important one)—muscle protein synthesis. Whether you believe this is a big deal or not is going to largely come down to the individual, and it’s also part of the reason why soy has seen so much debate around it.

Soy Protein and Phytoestrogens

The next biggest part of the debate is whether soybeans feminize men by messing with levels of estrogen and testosterone. The argument is that the phytoestrogens in soy (namely, isoflavones) act like estrogen in the human body.

Before we continue, it should be mentioned that many different foods contain phytoestrogens, including apples, carrots, rice, and coffee. Furthermore, there have also been a lot of health benefits linked to phytoestrogens.

But the question still remains—does soy mess with our sex hormones?

As with most good debates, there’s evidence on both sides of the argument.

However, there’s definitely more evidence that shows that soy doesn’t have any effect on testosterone levels in males.

And the studies that do suggest that soy messes with hormonal balances also have subjects consuming a ton of soy—even up to 3 liters of soy milk daily. 

The truth is that the phytoestrogens in soy are very likely not going to make any detrimental difference in your training or your physique.

Even if there were definitely negative effects, these effects would be so small that dozens of other factors would come into play before. Your routine and diet would have to be seriously tuned in for phytoestrogens to be the deciding factor in your success. However, it’s still good to stray on the side of caution.

While soy is likely not going to have any detrimental effects, it’s also a good idea to be aware of them and change your routine accordingly.

Importance of Age

For both the question of gains and the question of testosterone, age was one of the biggest factors that most studies saw a difference in. 

Older people are going to need more leucine to get past the muscle protein synthesis threshold, so either eating more soy protein or supplementing with whey can be a good idea in this scenario.

Older people also seemed to be more affected by phytoestrogen, albeit, when soy intake is extremely high. If you’re young, then there’s an even better chance that these factors don’t matter as much.

Other Considerations

Having addressed the biggest points of contention when it comes to soy protein as compared to whey, we can address some of the other questions that make up another large part of the discussion. 

The biggest factor is the unique nutrient profile of soy. As we’ve seen already, it has both its alleged benefits and alleged drawbacks. However, there’s more to be said about anti-nutrients and isoflavones. 

Lastly, practical things like diet and taste should also be taken into consideration when trying to decide between soy protein powder and whey.


Aside from the factors we’ve already looked at, there are three more main aspects that separate soy protein from whey. 

While there is no conclusive evidence yet, there have been studies that have shown how soy raises the output of the thyroid. What does this mean practically speaking?

It means that it could make it easier to shed some fat—something that’d be particularly useful for bodybuilders.

Although complete proteins overall have been shown to raise levels of thyroid hormone, it seems that soy has a unique effect that elevates it from other proteins. There has also been evidence that shows that the isoflavone in soy (which is a phytoestrogen) is beneficial for the human body. This is because isoflavones have an antioxidant effect on us, meaning that there’s less inflammation, for one.

When it comes to working out, this is especially important because it shows that soy can potentially help speed up recovery while reducing soreness.

What about Antinutrients?

Along with the questions around soy’s usefulness as a muscle-building agent and its effects on estrogen, another large question has been around the antinutrients found in soy. 

Antinutrients are compounds that actually block some of the absorption of different types of nutrients.

Lectin is the antinutrient that’s often cited when it comes to soy, but another one is protease inhibitors. This might sound like a nail in the coffin for soy, but things aren’t as dire as they seem.

Even ignoring the fact that humans have been eating soy for a very, very long time with seemingly no repercussion, the modern manufacturing process of soy protein takes into account a lot of these drawbacks. For example, high-quality soy protein often has its antinutrients either completely or mostly removed. This allows it to have all of the benefits without any of the drawbacks.

The manufacturing process even addresses soy’s relatively low biological value (the availability of nutrients for our bodies). What hurts soy’s low biological value is its relatively low level of methionine, but many companies add this amino acid into the powder to increase its bioavailability.

Dietary Considerations

Outside of the question of nutrients are also the dietary considerations that will persuade people for one or the other. 

If you’re vegan, soy protein powder can be an obvious choice if you’re looking for a plant-based protein powder with a full amino acid profile.

But vegans aren’t the only ones who might want to consider switching over to soy. Whey protein powder can be hard on some people’s digestive systems. It can lead to gassiness, bloating, and cramps.

A plant-based powder like soy can help a lot of people in this aspect, even if they might not necessarily be lactose intolerant. And even ignoring the environmental considerations between plant and animal-based proteins, there’s also the simple fact that variety is good for your gut.

You want to get your protein from different sources—people weren’t made to stick to one source of a nutrient for their entire life.

Non-dairy alternative Soy milk or yogurt in mason jar on white wooden table with soybeans in hemp sack

Taste and Texture

While this may be the least important aspect when it comes to the grand nutritional debate, it’s still important as a practical factor. Why does any of this matter if you find one or the other absolutely disgusting? 

In this regard, whey gets most of the points.

There’s a reason that whey is so accessible and easy to implement into any lifestyle. It has a relatively neutral taste and when mixed with water or milk, it has a creamy texture.

Although different manufacturers will produce slightly different textures, this is largely true for most of them. On the other hand, soy protein has a grainer texture that might take some getting used to. It also has a stronger, nuttier flavor than whey protein.

This will come down entirely to personal preference, but there are always a ton of flavors to experiment with for both whey protein and soy.

Is Whey the Way?

Whey has been the golden standard of protein supplementation for decades—and for good reasons. It provides a high-quality protein that’s extremely bioavailable and easily absorbed by the body. It’s very likely that nothing will ever topple it from the top. However, there will be increased interest in substitutes.

As we all know by now, it’s seen its fair share of infamy as both an ingredient to be avoided, and as a substitute that’s actually better for us. And as with most things in the health world, conclusive statements can’t be reached.

Nevertheless, most of the evidence we have seen points to it being another, healthy source of protein.

But at the end of the day, supplements should be an afterthought to a well-rounded and varied diet. Even if the highest quality protein powder with a ton of purported benefits isn’t going to (and shouldn’t) measure up to a balanced meal that includes a variety of nutrients.

Humans need variety, and leaning on one source of protein (or any other macronutrient) isn’t going to cut it. Experiment, try new things, and make mistakes, because variety is the spice of life and that’s the only way we’ll learn new things.