Whey or Soy Protein: Is One Better?

Dive into the world of whey and soy protein to find out which one reigns supreme. Make an educated decision on the best protein source for you.

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By Staff Writer

Published June 9, 2024.

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Whey and soy are two of the most popular protein powders out there. But which one is the better choice for you? Below we look at the research on how these two proteins stack up against each other in helping you meet three common goals: shedding body fat, building muscle, and boosting testosterone.

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Is whey or soy protein better for reducing body fat?

Whey protein may be more beneficial than soy in reducing body fat. In a study lasting 5 months, researchers provided 30 men with 56 grams/day of whey protein and 30 men with 56 grams/day of soy protein. Another 30 men took no protein powder supplement and took a carbohydrate supplement instead. After the 5 months, the men eating whey lost an average of 2.3 kilograms of body weight and 1.8 kg of body fat. 1 In contrast, the men that consumed soy and carbohydrate experienced no notable weight-loss. The men who consumed whey protein were also the only ones who lost a significant amount of belly fat. Because of their findings, the researchers concluded that “…different sources of dietary protein may facilitate weight loss and affect body composition.” 1

a bar graph shows the number of fat changes in the body

Modified from Baer et. al. 

A three-month study showed that subjects consuming 21 grams of whey and casein protein per day lost 2.6 pounds of body fat and gained 3.3 pounds of lean body mass. 2 Studies on soy are mixed: some indicate that it might decrease body fat while other indicate that it has no effect. 3,4 Thus, the USDA has stated that soy “does not have a clear advantage over other protein sources for weight and fat loss.”

Key takeaway: Strong evidence suggests that whey helps reduce body fat and increase lean body mass when compared to other proteins including soy.

Is soy or whey protein better at building muscle?

Protein is essential for building and maintaining muscle mass. In a study from 2009, researchers tested the effects of whey, soy, and casein protein on muscle growth by evaluating something called the protein fractional synthesis rate (FSR). At rest, men that ate whey protein had the greatest muscle growth followed by the men who ate soy and then those that ate casein. 5 After weight-lifting, the researchers noted that muscle growth was even more pronounced in the men who ate whey protein compared to men who consumed soy or casein. 5 The researchers concluded that “the consumption of whey protein stimulates [muscle growth] to a greater extent than either casein or soy.” 5  

Another nine-month study showed that subjects that consumed whey protein gained 83% more lean body mass (i.e. muscle and bones) than those that ate soy. 6 Thus, the researchers concluded that daily supplementation with “whey was more effective than soy protein...in promoting gains in lean body mass.” 6

a table with a measuring spoon next to a pile of protein

Data from Volek et. al

Key takeaway: Soy protein coupled with resistance training can help you building muscle. However, research show that whey is the stronger choice.

Can these protein supplements boost testosterone?

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

In a study on 10 male athletes, researchers provided subjects with whey or soy protein while they continued weight-training. After 14 days of supplementation, the researchers observed that the men that ate soy protein had lower levels of testosterone after weight-training than the men who ate whey. 7 The group that ate whey also had lower levels of cortisol, a hormone that decreases muscle recovery and weakens immunity. One important thing to mention: the researchers saw no increase in estrogen in the men who ate soy.  

A study from 2008 involving some intense biochemistry also suggests that whey may boost testosterone. That being said, more research needs to be done to evaluate the association between whey and soy with testosterone.

Key takeaway: A small amount of research suggests that whey might increase testosterone. While soy in moderation doesn’t increase estrogen, it doesn’t increase testosterone either.

What's the best one to chose? 

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

Whichever protein you choose, you should carefully monitor how it affects you on the outside and inside. We can give you all the critical data from your blood and all of the cool research-backed ways to make long-lasting improvements in your health and performance. 

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1. Baer, David J., et al. "Whey protein but not soy protein supplementation alters body weight and composition in free-living overweight and obese adults." The Journal of nutrition 141.8 (2011): 1489-1494.

2. Soenen, Stijn, et al. "Protein intake induced an increase in exercise stimulated fat oxidation during stable body weight.Physiology & behavior101.5 (2010): 770-774.

3. Liao, Fang-Hsuean, et al. "Effectiveness of a soy-based compared with a traditional low-calorie diet on weight loss and lipid levels in overweight adults." Nutrition 23.7 (2007): 551-556.

4. Cope, M. B., J. W. Erdman Jr, and D. B. Allison. "The potential role of soyfoods in weight and adiposity reduction: an evidence‐based review."Obesity reviews 9.3 (2008): 219-235.

5. Tang, Jason E., et al. "Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men." Journal of Applied Physiology 107.3 (2009): 987-992.

6. Volek, Jeff S., et al. "Whey protein supplementation during resistance training augments lean body mass.Journal of the American College of Nutrition 32.2 (2013): 122-135.

7. Kraemer, William J., et al. "The effects of soy and whey protein supplementation on acute hormonal responses to resistance exercise in men." Journal of the American College of Nutrition 32.1 (2013): 66-74.

8. Hulmi, Juha J., et al. "Androgen receptors and testosterone in men—effects of protein ingestion, resistance exercise and fiber type." The Journal of steroid biochemistry and molecular biology 110.1 (2008): 130-137.

9. Food and Drug Administration. "Food labeling health claims; soy protein and coronary heart disease." Fed Regist 64 (1999): 57699-57733.