Pea Protein vs. Soy Protein: How Do They Compare?

With the myriad of options on the market, choosing a plant-based protein powder can be overwhelming! 

Two of the most common vegan protein powders are pea protein and soy protein. Which of these is the best for building muscle? Which is the healthiest? Are there any risks to either of these?

My goal with this post is to discuss pea and soy protein powders, their amino acid profiles, their effects on muscle strength and hypertrophy, considerations for choosing the best option, and to clarify any misconceptions about their safety.

Scoops of various protein powders on a table

What is pea protein powder? 

The main ingredient in pea protein powder is generally pea protein isolate. Pea protein isolate is produced from yellow split peas using a wet processing method, in which proteins are separated from the other pea components (1). 

Pea protein powder is sold as a dietary supplement and often marketed as a convenient way to add extra protein to the diet, especially for the purpose of building muscle.

What is soy protein powder?

The main ingredient in soy protein powder is usually soy protein isolate. The process of producing soy protein isolate begins with soybean flour or flakes, which have had the natural fat removed, and is processed with water and alkaline compounds until the proteins have been separated from other components. Soy protein isolate must contain at least 90% protein (2).

Similar to pea protein, soy protein powder is also sold as a dietary supplement for the purposes of adding more protein to the diet and promoting muscle repair and growth. Pea and soy protein powders are sold as a vegan, plant-based alternative to animal-based protein supplements, such as whey and casein.

Nutrition profile and ingredients

Protein quality

Protein quality is usually determined using the combination of a protein’s amino acid content and how easily digestible it is.

A widely used measure of protein quality is the Protein Digestibility-Corrected Amino Acid Score, or PDCAAS. With this metric, egg, meat, and dairy-based proteins score the highest because they are most easily absorbed by the body and contain amino acids at levels that match what is required by the human body (3).

The Food and Agriculture Organization of the United Nations now recommends using the Digestible Indispensable Amino Acid Score (DIAAS) instead of PDCAAS.

DIAAS is considered more helpful because it considers the digestibility of individual amino acids rather than the protein as a whole. It also focuses on amino acid absorption in the ileal portion of the digestive tract instead of the entire digestive tract (4).

Pea protein contains all nine essential amino acids, but is lower in the amino acid methionine than is considered optimal for building muscle. It is typically considered to be a lower quality protein than soy, since soy protein contains all nine essential amino acids in amounts closer to what is required by the body.

However, these definitions of protein quality are limited by the fact that they do not consider the impact of different protein sources on chronic disease development or the environment. Eating more plant-based protein is associated with lower risk of many chronic diseases, and they have less of an impact on climate change than animal proteins (3).

Ingredients 

Pea protein powders may be sold with pea protein isolate as the only ingredient, or may be combined with another protein source due to it being lower in methionine.

For example, protein concentrates or isolates higher in methionine made from brown rice, hemp seeds, or chia seeds are often added to pea protein in order to provide all nine essential amino acids required for muscle growth and repair in proportions required by the human body.

Soy protein powder is typically not combined with other protein isolates, since soy is a complete protein containing all the essential amino acids in optimal amounts for building muscle.

Allergens

Soy is one of the 8 most common allergens, and therefore pea protein may be a safer choice for individuals with a soy allergy.

However, peas and soy are both legumes. So are peanuts, another common allergen. Although individuals with an allergy to one legume are not usually allergic to every other legume, they may experience an allergic reaction to one or two others, which could include peas (5).

If you’re allergic to a legume like soy or peanuts, be cautious when trying products with pea protein. Consult a licensed healthcare provider if you aren’t sure what products are safe to try.

Reasons for using protein powder 

Some reasons for using protein powder in your diet may include:

  • Building muscle. There are many plant-based foods with protein, but depending on the intensity of your training regimen, training frequency, your fitness goals, or your schedule, protein powders can provide a convenient and easy way to meet your protein needs for optimal muscle growth.
  • Poor appetite. Sometimes despite a person’s best attempts to eat enough protein-containing foods, they simply are unable to do so due to a poor appetite. This can happen especially in older adults, where appetite naturally goes down while protein needs go up. People with cancer, depression, gastrointestinal conditions, dementia, or morning sickness during pregnancy may also have a lower appetite and struggle to get enough protein in their diet.
  • Making meals more balanced. While including whole plant-based sources of protein in meals is optimal, sometimes hectic mornings and busy schedules make this challenging. Adding protein powder to a smoothie or having a protein shake along with a bagel or piece of fruit in the morning can make these foods more filling and nutritionally balanced, and therefore may help prevent overeating later in the day.

Benefits for muscle

Man performing a barbell deadlift

Unfortunately, there aren’t many research studies directly comparing pea to soy protein. 

To determine the effectiveness of different proteins, they are most often compared to whey protein, considered the “gold standard”. This is due to whey’s high leucine content and easy digestibility. Leucine is one of three branched chain amino acids (BCAAs) that is especially important for stimulating muscle protein synthesis.

Let’s see how pea and soy protein compare to whey for muscle strength and hypertrophy.

Strength

In an 8-week study of CrossFitters taking either a whey protein or pea protein supplement, there were no significant differences between the groups in strength gains for either maximal squats or maximal deadlifts (6). 

Additionally, a larger exercise study comparing the effect of whey vs. pea protein found no significant differences in biceps curl strength between the groups (7).

Although more research would be helpful, it appears that pea protein is as effective as whey protein for muscle strength, despite its lower PDCAAS score. 

Interestingly, the pea protein used in these two studies were pure pea protein isolate not combined with protein from other sources to make up for its lower methionine content. It would be interesting to see whether a pea protein combined with brown rice protein, for example, would lead to even better results.

Similarly, when comparing soy protein vs. whey protein in strength training programs, the strength gains are about the same regardless of the protein used. This was the case in young men and women (8) and in older men (9).

Hypertrophy (muscle size)

An exercise study comparing the effect of pea vs. whey protein found that both supplements increased the size of the biceps with no significant differences between the groups (7). Another similarly found no difference between pea and whey protein supplementation on the thickness of two quadriceps muscles after a CrossFit training program (6).

In a 2019 systematic review study comparing the effects of soy vs. whey protein during exercise, four research studies found that whey and soy protein equally increased muscle mass, while two studies showed that whey protein had the advantage. 

Overall, both pea protein and soy protein appear to be comparable to whey protein in increasing muscle hypertrophy despite both plant proteins having lower PDCAAS/DIAAS scores than whey. It seems the conventional protein quality score for a protein does not always correlate with its effects on muscle.

Potential risks

Heavy metal levels

A recent study by the Clean Label Project found high levels of heavy metals and BPA, an endocrine disruptor, present in 133 different protein powders. Overall, plant-based protein powders had the worst levels, although some whey products scored poorly as well (10). 

However, it is unclear how dangerous these levels are. One study attempted to determine whether these levels present health risks to humans. Even with up to 3 servings per day, the study found that the levels of heavy metals in these protein powders do not present a risk to human health (11).

However, if you’d like to be cautious and reduce your exposure to these heavy metals, choose a brand with lower levels. In the Clean Label Project study, the plant-based products highest in heavy metals were:

  • Garden of Life Organic Shake & Meal Replacement Chocolate Cacao Raw Organic Meal
  • Vega Sport Plant-Based Vanilla Performance Protein

Additionally, organic products had about 2.5 times higher heavy metals than non-organic varieties, so choosing a non-organic product may reduce your exposure. It is also theorized that chocolate flavored powders tend to have more heavy metals because cacao plants can easily absorb them from soil, so choosing other flavors may also help reduce exposure.

Isn’t soy unhealthy?

Some common concerns about soy include:

Genetic engineering

Most soy grown in the US is genetically engineered to be resistant to the herbicide glyphosate, and is most often used as crops to feed livestock and to make soybean oil (12). While genetically engineered foods are tested for safety before being approved for use in the food supply, studies on the long-term health effects of these foods are lacking.

If you wish to avoid genetically engineered soy, choose an organic brand of protein powder. Organic soybeans are not allowed to be genetically engineered or sprayed with glyphosate.

Glyphosate

Glyphosate (brand name Roundup) is a weed killer frequently sprayed on Roundup Ready soybeans which have been genetically engineered to survive it. Concerns about the health risks of glyphosate are widespread. 

After evaluating glyphosate for safety, the Environmental Protection Agency (EPA) has stated that glyphosate does not pose a risk to human health, is not carcinogenic, and does not act as an endocrine disruptor. They do state that glyphosate poses a risk to land and aquatic plants and birds (13). 

There does not appear to be worldwide consensus with this stance, however. In 2015, the World Health Organization International Agency for Research on Cancer classified glyphosate as being probably carcinogenic to humans (14). 

Given such conflicting information, it makes sense that some people prefer to avoid crops heavily sprayed with glyphosate. To best avoid soy sprayed with glyphosate, choose an organic soy protein powder.

Estrogen content 

Soy contains phytoestrogens (“plant” estrogens), also called isoflavones, which are commonly confused with the estrogen found in humans and animals. While similar in structure to mammalian estrogen, phytoestrogens do not always act the same way in the body. 

In fact, depending on the area of the body, phytoestrogens can either have estrogenic or anti-estrogenic effects. The ability of phytoestrogens to block estrogen in reproductive tissues like the breast and uterus may at least partially explain the association between soy foods and decreased risk of cancer in these areas (15).

But what about processed soy, as in protein powder? Isn’t that harmful?

The available research does not support this fear. While whole or minimally processed soy foods such as edamame, tempeh, tofu, and soy milk contain more vitamins, minerals, and isoflavones, this does not mean that soy protein powders are unhealthy. They simply serve a different purpose in the diet.

Environmental impact

It’s difficult to find studies directly comparing the environmental impact of peas to that of soy. 

Some negative impacts of soy on the environment include its high water requirement and role in deforestation in the Amazon rainforest. However, the negative impacts of soy come mainly from its role in animal agriculture. A surprising 80% of the world’s soybeans are grown to be fed to animals farmed for meat, which requires a huge amount of land (17).

But as plant-based proteins, both pea and soy protein have a lower environmental impact than animal-based proteins (18). This is due in part to the large amount of land it takes to raise animals and to grow crops for their feed.

What about organic? The information here is a little conflicting. 

Organic soybeans scored more favorably in one study for global warming potential, non-renewable energy use, and other environmental quality markers than non-organic soybeans, but had a worse impact on land use. Contrastingly, a study in China found that conventional farming of soybeans was significantly more energy efficient than organic farming.

How to choose a protein powder

When choosing a protein powder, whether pea protein, soy, or other plant-based protein, here are some things to consider:

  • Third party testing. Protein powders are considered dietary supplements, and therefore are not regulated by the FDA. So can you know that your supplement actually contains what it says it does, and if it is safe? It should say on the label whether the supplement has been tested by an independent third-party lab (not by the supplement company itself). Look for NSF-certified manufacturers that also follow Good Manufacturing Processes (GMP).
  • Serving size. Some protein powders list a serving as 1 scoop, while others may list it as 2 scoops. Be sure to check the protein content per scoop, since a bottle with a 2-scoop serving may not last you as long. Different brands may also have slightly different amounts of protein, with most brands containing between 18-25 grams per serving.
  • Leucine content. The amino acid leucine is essential for stimulating muscle growth, so if your goal with a protein powder is to gain muscle you should choose a protein powder with a high leucine content. Soy is slightly higher in leucine than pea protein (19), but if you prefer pea protein you can just use more at a time to get more leucine. And as mentioned above, both proteins have fairly equal effects on muscle strength and hypertrophy.
  • Flavor. Finding flavors you love and alternating between them will keep your tastebuds from getting bored, and may inspire you to be more creative in how you consume your protein powder. 
  • Texture. Some protein powders are chalkier or grainier than others, so try different brands until you find one you like the best.
  • Organic vs. non-organic. This is up to personal preference. If choosing soy protein, you may wish to buy organic to avoid genetically engineered soy. On the other hand, organic protein powders were found to be over 2 times higher in heavy metals than non-organic. 

Main takeaway: Is pea or soy protein better?

It’s up to you! In terms of improving muscle strength and hypertrophy, pea and soy protein don’t seem to be too different from each other despite the lower PDCAAS/DIAAS scores and leucine content of pea and soy protein compared to whey.

If you’re looking to improve your protein intake due to poor appetite, older age, or a hectic schedule with limited time for preparing meals, either would be a great choice. If you find yourself already eating a lot of soy products, you may wish to choose pea protein for more variety, and vice versa.

As plant protein sources, both pea and soy protein have less of an environmental impact than whey or other animal-based proteins. If you prefer to avoid genetically engineered soy sprayed with glyphosate, consider buying organic soy protein.

If you’re not sure whether you should incorporate protein powder into your routine or if you struggle with getting enough protein on a vegan diet, speaking with a vegan registered dietitian can help!

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