You've heard it a thousand times in the gym: animal protein is king for building muscle. Whey is the undisputed gold standard, the fastest-acting, most anabolic protein you can get. But what if that's not the whole story?
A brand-new, high-quality study just published in The American Journal of Clinical Nutrition put this dogma to the test in middle-to-older adults, a group where maintaining muscle is critically important. The results might just change how you think about your post-workout shake.
The Study Setup: Animal vs. Plant Protein Showdown
Researchers took 27 adults between the ages of 50 and 70 and put them through a rigorous 10-day protocol. They were split into two groups, both following a controlled diet and a resistance training program.
Here’s the clever part:
- The Diet: Both groups consumed a moderate, "typical" protein intake of about 1.0 gram per kilogram of body weight per day. This wasn't a high-protein bodybuilding diet; it was designed to reflect how many people actually eat.
- The Groups:
- Animal-Whey (AW-D) Group: Their diet was structured so that about 75% of their protein came from animal sources, supplemented with whey protein concentrate smoothies.
- Plant-Pea (PP-D) Group: Their diet was flipped, with about 75% of their protein coming from plant sources, supplemented with pea protein isolate smoothies.
- The Training: All participants performed supervised unilateral (one-legged) knee extensions every other day. This design is brilliant because each person's non-training leg acts as a perfect control, allowing for a direct comparison between a trained and an untrained muscle.
The goal was to measure the direct impact of these diets on the rate of new muscle creation, a process called myofibrillar protein synthesis (MPS).
The Main Event: What Happened to Muscle Growth?
After 10 days of controlled diets and hard training, the researchers took muscle biopsies from both legs to see what happened at a cellular level. The findings were crystal clear on two fronts.
First, and most importantly: Training is King.
Unsurprisingly, the leg that performed the resistance training showed significantly higher rates of muscle protein synthesis compared to the rested leg. This was true for both groups. It's a powerful reminder that the primary driver for muscle growth is, and always will be, the stimulus from lifting weights.
Second, the surprising part: Protein Source Didn't Matter.
When comparing the animal-whey group to the plant-pea group, there was no significant difference in muscle protein synthesis. Not in the rested leg, and not in the trained leg.
Let that sink in. The diet with a higher proportion of plant-based protein, supplemented with pea protein, supported the muscle-building response to resistance training just as effectively as the animal-dominant diet supplemented with whey.
But Whey Has More EAAs, Right?
This is where it gets really interesting. The researchers confirmed that the whey protein supplement provided about 37% more Essential Amino Acids (EAAs) per serving than the pea protein supplement. According to conventional wisdom, this should have given the whey group a clear anabolic advantage.
So why didn't it?
The study authors suggest a few potential reasons:
- Context of a Mixed Meal: The protein supplements weren't consumed in isolation. They were part of smoothies and taken alongside whole-food meals. Eating protein with carbs and fats slows down digestion and absorption, which may have blunted the famously rapid spike of amino acids from whey, leveling the playing field.
- Pea is a High-Quality Plant Protein: While not identical to whey, pea protein isolate is still a very good protein source. It may have provided enough EAAs to maximize the muscle-building signal when combined with the other protein sources in the diet.
- Cumulative vs. Acute Effects: While whey might cause a bigger, faster spike in MPS right after you drink it, this study measured the cumulative MPS over 10 days. It seems that over the long haul, the small, acute differences didn't add up to a meaningful long-term advantage in this context.
Practical Takeaways for Lifters and Coaches
This study provides some fantastic, evidence-based takeaways that you can apply to your own training and nutrition.
- Focus on the Big Rocks: The single biggest factor for building muscle in this study was the training itself. Before you stress about the minutiae of protein timing or type, make sure your training is consistent, progressive, and intense.
- Protein Source is Flexible: At a typical protein intake (~1.0 g/kg), the source of your supplemental protein likely matters less than you think. If you prefer pea protein for ethical, environmental, or digestive reasons, this study provides strong evidence that you are not compromising your ability to build muscle.
- Total Daily Protein Still Matters: This study controlled total protein to be equal between groups. The takeaway isn't that protein doesn't matter, but that at the same total intake, the source was interchangeable. Hitting your daily protein target remains a cornerstone of building muscle.
- A Potential Health Bonus: Interestingly, the plant-protein group saw a significant drop in non-HDL cholesterol, which is a positive marker for cardiovascular health. This suggests that incorporating more plant-based protein may offer benefits beyond the gym.
The Bottom Line
In this well-designed 10-day study, resistance training was the potent stimulus for muscle protein synthesis in adults over 50. A diet higher in plant-based protein, supplemented with pea protein, supported this process just as effectively as a traditional animal-based diet supplemented with whey.
This doesn't mean protein quality is irrelevant, but it strongly suggests that in the context of a mixed diet with adequate total protein, you have more flexibility than the old-school dogma would have you believe. Pick a quality protein source you enjoy, hit your daily targets, and most importantly, train hard.
Reference:
Korzepa, M., Quinlan, J. I., Marshall, R. N., Rogers, L. M., Belfield, A. E., Elhassan, Y. S., ... & Breen, L. (2025). Resistance training increases myofibrillar protein synthesis in middle-to-older aged adults consuming a typical diet with no influence of protein source: a randomized controlled trial. The American Journal of Clinical Nutrition. PMID: 40288581.