Is soy protein really worse for muscle growth than milk protein?

“There’s a version of the protein debate that ends with the claim that ‘animal protein is better,’ and no one looks any deeper. And while earlier research suggested that plant-based proteins were inferior, more recent studies paint a much more detailed picture, especially when it comes to soy,” Schwarzenegger begins his recent post.

Arnold, a seven-time Mr. Olympia champion, may have long since stopped competing, but his passion for bodybuilding remains. Despite his busy schedule, he always finds time to analyze the latest trends in nutrition.

Is soy an inferior source of protein? Arnold answers

In his newsletter, Arnold’s Pump Club, Schwarzenegger recently analyzed data from dozens of studies and found that soy protein yields results comparable to those of dairy protein in terms of muscle building.

“A 2025 meta-analysis, which included 43 randomized controlled trials, found that soy protein produces comparable muscle gains to milk protein. If you’ve been avoiding soy because someone told you it’s a bad protein for building muscle, the evidence suggests otherwise,” he explains .

Although some earlier research suggested that plant-based protein generally leads to slightly less muscle mass gain, Arnold highlighted one very important caveat:

“When soy was compared directly to milk protein in 17 studies, the difference essentially disappeared. The discrepancy in the broader analysis was actually caused by single-source proteins, such as rice, oats, chia, and potatoes, which showed a greater disadvantage compared to animal proteins.”

Arnold also noted that for adults over 60, even this small difference ceased to be statistically significant. For this age group, it is simply much more important to meet the daily goal for total protein intake than to worry about the source of that protein. As for increases in strength and physical performance, no significant differences were observed across any groups, regardless of the type of protein.

What about whole foods like tofu?

The former governor pointed out that the study in question primarily examined dietary supplements (soy isolates in powder form) rather than whole foods themselves. Nevertheless, these foods play a huge role in the diet.

“Research on protein quality confirms that most soy products are highly digestible. A controlled study measuring amino acid absorption even found that tofu is about 95% digestible, which is fully comparable to animal sources,” Arnold notes, adding that soy is also a complete protein containing all nine essential amino acids.

According to him, the main functional difference compared to milk is thus only a slightly lower content of the amino acid leucine, not a fundamental deficiency in the overall quality of the protein.

Conclusion

Schwarzenegger, 78, who in recent years has personally discovered the benefits of a plant-based diet as a means of disease prevention, sees no reason why you should feel like you’re losing muscle if you choose soy.

“As with any dietary decision, what you eat is your choice. You don’t have to eat soy, and you can avoid it if you feel that’s best for you. But if soy protein is part of your diet—whether from supplements or foods like edamame and tofu—scientific evidence suggests that you’re definitely not losing muscle mass,” Arnold concludes.

How do you choose the right protein for maximum muscle growth in practice?

That’s exactly why modern athletes and people who want to live without prejudice have long since stopped letting themselves be limited by myths about the “inferiority” of plant-based proteins. They realize that a well-chosen and high-quality plant-based source can provide muscles with a completely comprehensive amino acid profile without burdening digestion with lactose or animal fats. It’s not about blindly rejecting traditional options, but about enriching your diet with smart nutrition that’s great for vegans, people with allergies, and anyone who wants to give their body more varied and gentler fuel.

After a tough workout, your muscle fibers are starved from microtraumas and crying out for immediate relief. This is when it makes perfect sense to reach for a top-quality plant-based protein from a premium selection.

Whether you choose a purely plant-based approach or opt for proven multi-ingredient blends, your body needs, above all, quality and purity in processing. Once you stop believing the old myths that only animal protein guarantees muscle growth, and start paying attention to the actual ingredients, you’ll discover that high-quality plant-based protein is the gentlest and most effective way to give your body exactly the fuel it needs to take it to the next level.