Some people have digestive issues with dairy proteins. Like whey and casein, soy is a complete protein containing all of the essential amino acids. Because it is plant-based, soy doesn’t contain the lactose found in cow’s milk, which is the source of both whey and casein protein. A study published in the Journal of the American College of Nutritioninvestigates the hormonal impact on resistance trained men.

Ten weight lifters in their early 20s consumed 20 grams of whey protein isolate or soy protein isolate every morning for 2 weeks. Afterward, they did 6 sets of squats for 10 reps each using 80% of their one rep max. There was a lower testosterone response to exercise in the soy group. Also, whey protein tended to blunt production of catabolic cortisol during recovery.