When total daily protein intake and resistance training regimens are matched, slow-digesting and fast-digesting proteins—including plant-based sources such as soy—are equally effective at increasing lean body mass in humans.

Study: No Significant Differences in Muscle Growth and Strength Development When Consuming Soy and Whey Protein Supplements Matched for Leucine Following a 12 Week Resistance Training Program in Men and Women: A Randomized Trial

The study found that soy and whey protein worked equally well for building muscle when people did the same workouts and got the same amount of key nutrients.

Study: High-Protein Plant-Based Diet Versus a Protein-Matched Omnivorous Diet to Support Resistance Training Adaptations: A Comparison Between Habitual Vegans and Omnivores

The study found that people who ate only plant protein (like soy) gained just as much muscle as those who ate animal protein, as long as they ate the same amount of protein and did the same workouts.

Study: Effects of Postexercise Protein Intake on Muscle Mass and Strength During Resistance Training: Is There an Optimal Ratio Between Fast and Slow Proteins?

The study found that it didn't matter if the protein was fast-digesting or slow-digesting; people gained the same amount of muscle and strength as long as they did resistance training.

Study: Effects of Postexercise Protein Intake on Muscle Mass and Strength During Resistance Training: Is There an Optimal Ratio Between Fast and Slow Proteins?

The study found that whether people drank fast- or slow-digesting protein after workouts, they gained the same amount of muscle over time, as long as they ate the same total protein and trained equally.