Settling The Debate: Does Soy DESTROY Your Muscle Gains?

A common worry among bodybuilders is that soy is high in protein but can also increase estrogen and decrease testosterone, negatively impacting muscle gain. But is that ACTUALLY true? Or, can you eat soy safely and enjoy a wide range of benefits? Here is my breakdown. 

Key Takeaways

• Soy is a high-quality protein that offers all nine essential amino acids your body needs for growth and recovery. Unlike most plant proteins, you don’t have to combine soy with other foods (e.g., beans or rice) to get a ‘complete’ protein.

• Although soy has plant compounds (isoflavones) with characteristics similar to estrogen, research suggests that it doesn’t decrease testosterone or increase estrogen levels, even when consumed regularly.

• Soy provides fast-digesting protein, which makes it a good pre-workout snack to limit muscle breakdown during sessions. It’s also a good post-workout food, but be sure to get 0.3-0.5 grams of protein per kilogram of bodyweight.

Overview: Forms of Soy

Soy comes in many forms. You can get soy protein from soybeans but also from foods like edamame, tofu, tempeh, miso, soy sauce, and soy milk. 

Here are three popular soy protein supplement options:

1. Soy Concentrate

Soy concentrate is made by removing part of the carbs in soy and all the water. 

It is 70% protein, while the rest is fats and carbs. You find it in powdered form, usually added to foods to thicken them or add more protein. 

It is found in soups, sausages, cheeses, dressings, frozen desserts, and pet food. 

2. Soy Isolate

Soy isolate is further processed from the soy concentrate. 

Most carbs and fats are removed, making it the purest form of soy protein with over 90% protein. 

It is often used in the food industry to increase the protein content of certain foods, including protein bars, dairy alternatives, and protein powders. 

3. Textured Protein

Textured proteins made from soy concentrate to create imitation meat like chicken. Although it depends on the brand, it is composed of 50-70% protein. 

It is often used to create vegan or vegetarian recipes. 

Why Does Soy Have A Bad Reputation With Building Muscle? 

One of the main concerns among bodybuilders is the possible interference with testosterone levels. 

Soy has natural substances called isoflavones, with properties similar to those of estrogen (the female sex hormone). 

Here is some insight from Alina Petre, MS, RD (NL):

“Soybeans are especially rich in isoflavones, a subclass of polyphenols referred to as phytoestrogens due to their ability to attach to and activate estrogen receptors in your body.”

Because of the similarities, some people have speculated that eating soy may lead to more body fat and less muscle mass––a bodybuilder’s worst nightmare. 

Two studies conducted in 2001 and 2005 initially caused concern for those eating soy. 

However, as you’ll see later, more current research has debunked these studies.

2001 Study: Dietary Soy Decreased Testosterone

In a 2001 study, rats were given 600 mcg of isoflavones daily over five weeks. 

The results? These rats had significant drops in testosterone levels and 35 times higher levels of isoflavones in their blood. 

Notably, these rats had a substantial intake of isoflavones, equivalent to a human consuming 1.5 mcg per gram of body weight. 

For an average human who weighs 70 kilograms, it would mean consuming 105,000 mcg (105 mg), or 17.5 cups of soy milk daily. 

2005 Study: Soy Protein Effects Hormones Of Young Healthy Men 

In 2005, a study was done on 35 men who consumed soy products for 57 days. 

They were divided into three groups where they consumed a milk protein isolate beverage, a low isoflavone soy protein isolate, or a high isoflavone soy protein isolate. 

Their testosterone levels were measured at the start and end of the experiment. In both groups (low and high isoflavone groups), testosterone levels had decreased, and estrogen levels had increased. 

Thus, they concluded that soy intake significantly adversely impacts hormone levels in men. 

• Related: Tofu vs Chicken: Which Is Better?

Is Soy REALLY That Bad For Bodybuilding? 

The science of soy has changed since those early studies, and the most up-to-date research broadly concludes that soy is not inherently bad for bodybuilding, nor does it impact hormone levels in men.  

41 Studies From 2010 to 2020 Show That Soy DOES NOT Decrease Testosterone Levels 

A 2021 meta-analysis review gathered information from 41 studies on soy and testosterone. 

The results found that eating soy didn’t affect testosterone levels. In eight of those studies, men consumed more than 100 mg of isoflavones daily, and testosterone levels were not affected (1)(2)(3)(4)(5)(6)(7). 

Consuming 100 mg of isoflavones is a very high dose. It would mean eating more than three cups of edamame or 15 ounces of tofu daily, which would not be sustainable. 

One Potential Con of Eating Soy From Bodybuilding

Soy Can Reduce Nutrient Absorption

Soy protein contains phytate, which is often called an ‘antinutrient’ because it prevents the absorption of certain nutrients. The compound is in soy, legumes, whole grains, nuts, and seeds. 

Phytates can reduce the absorption of nutrients like iron and zinc. Soy consists of 1-2% of phytates per 100 grams, which represents a reduction of 5-15% in the availability of iron and zinc. 

Poor iron absorption could lead to anemia, interrupting the oxygen supply to your muscles. Additionally, an insufficient supply of zinc can weaken your immune system. 

This doesn’t mean that eating soy will impair all iron and zinc absorption (only 5-15% will not be absorbed). 

If you consume soy, add other iron and zinc sources (chicken, meat, and fish being good options) at other meal times to compensate for this slight decrease. 

Is Soybean Effective As A Complete Protein?

Animal proteins are complete because they have all nine essential amino acids your body needs. 

On the other hand, plant-based proteins are often considered lower quality or ‘incomplete’ because they don’t have all the essential amino acids. 

For example, rice is high in cysteine and methionine but low in lysine, whereas beans are very high in lysine but low in methionine. Thus, when rice and beans are combined, you get a complete protein. 

With that said, soybeans are one of the few plant-based proteins with all the amino acids your body needs, which means you don’t need to mix it with another plant-based protein.  

Although some hardcore bodybuilders might tell you that soy is low in methionine, a study showed that it provides enough of each essential amino acid and supports muscle gain. 

• Related: Can You Mix Whey Protein With Soy Milk?

Is Soybean Enough For Gaining Muscle?

You need three things for muscle gain: muscle stimulus (exercise), enough calories (caloric surplus), and protein (1.6-2.2 grams per kilogram; 0.7-1 gram per pound).

Soy has been widely studied to determine if it is suitable for gaining muscle. 

Studies have shown that soy protein is adequate for muscle and strength gain compared to animal protein. 

In a study comparing soy isolate, soy combined with whey, and whey, the results showed no significant differences. 

Participants averaged 1.0-1.5 kilograms of lean weight gain over 12 weeks. 

• Check out other beans that are highest in protein.

Are There Other Benefits To Eating Soy As A Bodybuilder?

Weight Loss

For those in a cutting phase, soy protein has weight loss benefits, too. 

In a study, 30 overweight or obese men were put on a plant-based or animal protein diet. Both groups had the same macro breakdown: 40% carbs, 30% protein, and 30% fats. 

For both diets, they saw an average weight reduction of 2-2.5 kg in 3 weeks. Additionally, their satiety levels increased (i.e., they weren’t as hungry throughout the day), which prevented constant snacking.

This study shows that no matter the protein type  (plant-based or animal), they promote weight loss and appetite control, which are highly beneficial for a bodybuilder in a cutting phase.  

Increased Blood Flow

Another benefit of soy is its high content of the amino acid arginine. 

Arginine stimulates the production of nitric oxide, which increases your body’s blood flow. Increased blood flow means more oxygen and nutrients are carried to your muscles. 

In a study, 25 cyclists and triathletes were given a placebo or a 30-gram soy protein supplement before a 20-kilometer trial. 

Those who took the soy supplement had improved blood flow, which led to a faster completion time than those in the placebo group. 

High In Antioxidants

Soy has antioxidants that can help reduce the inflammation presented after exercise. 

Reducing the inflammation post-exercise can help with muscle recovery. 

In a study, men were given 33 grams of whey or soy protein supplement. Their muscle mass and antioxidant function post-exercise were measured. 

Both groups saw an increase in muscle mass, and researchers found that those who took soy had better antioxidant protection post-exercise.

Lactose-Free

It is said that over 65% of the population has some trouble digesting lactose. This is due to a decrease in the enzyme (lactase) that breaks down lactose. 

Although some people can take whey or casein without gastrointestinal issues, some can’t tolerate dairy. 

Given that soy is lactose-free, it can be a good dairy alternative for people with lactose intolerance.

When To Eat Soy?

Before A Workout

Soy protein digests quickly, which means you can have it before a workout without worrying about gastric problems like you would if you had beef or chicken. 

Thus, adding soy before a workout can supply a steady protein release to limit muscle protein breakdown during a session. 

Additionally, the high arginine content can increase your blood flow, which provides more oxygen and nutrients to your muscles. 

It’s worth noting that this last idea is speculative and hasn’t been tested in human studies.

After A Workout

Protein helps repair and grow your muscles after training, and soy is an excellent protein option to help with recovery. 

Also, thanks to its antioxidant properties, it helps reduce inflammation, potentially leading to better muscle recovery. 

Research recommends having 0.3-0.5 grams of protein per kilogram of body weight after training. A 70-kilo (154-lb) bodybuilder would need 21 to 35 grams of protein.

If you can’t cover that with soy protein alone, combine it with another source, such as beans (if you eat a plant-based diet) or meat.

As a Snack

Finally, a good moment to include soy is during snack time. 

A steady supply of protein every 2-3 hours keeps your satiety levels high (preventing snacking) and helps preserve muscle mass. 

Plus, research from Ph.D. Brad Schoenfeld and Alan Aragon suggests having a minimum of four protein meals/snacks throughout the day for optimal absorption.

In their words:

“Based on the current evidence, we conclude that to maximize anabolism one should consume protein at a target intake of 0.4 g/kg/meal across a minimum of four meals in order to reach a minimum of 1.6 g/kg/day.”

How Much Soy Should You Eat?

Currently, there is no upper daily limit for soy protein for men. As long as you don’t add it to every meal and have other protein sources varieties, you can have a couple of soy products during the day. 

For women, on the other hand, a study showed that having more than 100 mg of isoflavones could increase the risk of certain health issues. 

This represents around 125 grams of soy protein powder (around three scoops per day). 

What To Read Next

• Do Bodybuilders REALLY eat baby food? 

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