Soy Protein vs Whey Protein: Which One Is Better?

Discover the key differences between soy protein and whey protein to better understand their effects on muscle growth, weight loss, and testosterone.

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By Staff Writer
Lucia Gcingca
Edited by Lucia Gcingca

Updated February 6, 2024.

a man sitting at a kitchen table drinking from a bottle

Whey and soy are two of the most popular protein powders out there. But which one is the better choice for you?

Research shows us that while whey protein is a powerhouse for muscle gain, soy protein holds its own in maintaining body composition. [1]​​ Let's dive further into the soy protein vs whey protein debate so you make the best choice for your fitness goals.

Soy vs whey protein for body fat reduction

Whey protein and body fat reduction

Study findings

A 5-month research study administered 56 grams/day of whey protein to 30 men, leading to an average loss of 2.3 kilograms, including 1.8 kg of body fat. [2] This group also saw significantly reduced belly fat.

Comparative group

In contrast, another 30 men received a carbohydrate supplement instead of protein powder and did not experience significant weight loss.

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Soy protein and body fat reduction

Study findings

The same study involved another group of 30 men taking 56 grams/day of soy protein who didn't show notable weight loss, akin to the carbohydrate group. Studies on soy are mixed - some indicate that it might decrease body fat, while others indicate that it has no effect. [4, 5] Thus, the USDA has stated that soy “does not have a clear advantage over other protein sources for weight and fat loss.”

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a bar graph shows the number of fat changes in the body
Soy vs. Whey Protein: Body Fat Reduction | Modified from Baer et al.

Whey protein's broad effects

Further research

Another study over three months indicated that consuming 21 grams of whey and casein protein daily led to 2.6 pounds of body fat loss and 3.3 pounds of lean body mass gain. [3]

Key takeaway: Strong evidence suggests whey protein is better at reducing body fat and increasing lean body mass than other proteins, including soy.

Soy protein vs whey protein for muscle building

Our bodies need protein to build and maintain muscle mass. Let's compare soy protein and whey protein in this respect.

Study background

In 2009, a study evaluated the effects of whey, soy, and casein protein on muscle growth. This was measured by the protein fractional synthesis rate (FSR).

Whey protein's effectiveness in muscle building

  • Muscle growth at rest: The study observed that men consuming whey protein experienced the most significant muscle growth at rest, more so than those consuming soy or casein.[6]
  • Muscle growth post-weight-lifting: Following weight-lifting, whey protein's impact on muscle growth was even more pronounced, surpassing that of the men who consumed soy or casein.[6]
  • Study's conclusion: Researchers concluded that whey protein stimulates muscle growth more effectively than either casein or soy.[6]
muscle gain and protein type
Soy and whey protein supplements and muscle gains | Data from Volek et. al

Comparing whey and soy proteins over longer periods

Another nine-month study showed that subjects who consumed whey protein gained 83% more lean body mass (i.e., muscle and bones) than those who ate soy. Thus, the researchers concluded that daily supplementation with “whey was more effective than soy promoting gains in lean body mass.” [7]

Key takeaway: While combining soy protein and resistance training can help you build muscle., research shows that whey is the stronger choice.

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Can these protein supplements boost testosterone?

Testosterone's role in health

Optimal levels of testosterone are important for both men and women. In addition to stimulating muscle growth, optimal testosterone levels increase muscle recovery, maintain libido, and improve mood. So, how do whey and soy affect your testosterone?

Study on whey and soy protein's impact on testosterone

Study background

A study involving 10 male athletes examined the effects of whey and soy protein on testosterone levels during weight training. Over 14 days, the participants received a portion of either whey or soy protein supplements.

Study findings

  • Soy protein group: The men who consumed soy protein had lower testosterone levels post-weight training than those who consumed whey. [8]
  • Whey protein group: This group showed lower cortisol levels, a hormone that can hinder muscle recovery and weaken immunity.
  • Estrogen levels: Contrary to some beliefs, the study observed no increase in estrogen levels in the soy-consuming group.

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Further research and considerations

A 2008 study suggested that whey might boost testosterone. However, more extensive research is needed to confirm these findings and fully understand the relationship between these protein supplements and testosterone. [9]

Key takeaway: Research suggests that whey might increase testosterone. While soy in moderation doesn’t increase estrogen, it doesn’t increase testosterone either.

» Are your testosterone levels low? Find out how you may be overtraining

Soy protein vs whey protein: the winner is...

The latest research says whey protein is a better choice to shed fat, pack on muscle, and boost testosterone. Soy protein isn’t necessarily a bad choice if you want a plant-based alternative. The FDA states soy is better for heart health than whey. [10] But there are also many excellent plant-based proteins ranging from pea to rice powder that are better for your overall health and athletic performance.

Whichever protein you choose, you should carefully monitor how it affects you on the outside and inside. InsideTracker can give you all the critical data from your blood and all the cool, research-backed ways to improve your health and performance for the long haul.

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