If you're a young adult just starting strength training, drinking a shake with either whey or soy protein—both with the same amount of a key muscle-building amino acid (leucine)—will help you gain...

For young people new to lifting weights, taking soy protein or whey protein daily seems to have the same effect on thigh muscle growth.

If young adults who aren't used to working out eat a bit more protein—especially from whey or soy—and lift weights 3 times a week for 3 months, they’ll likely gain more muscle and get stronger, even if they weren’t eating enough protein to start with.

If young adults who aren't used to working out do strength training 3 times a week for 12 weeks, they’ll gain about 1.5 kg of muscle and get stronger legs — and it doesn’t matter if their protein shake is made from soy or whey, as long as both have the same amount of leucine (2 grams per day).

Older women around 69 years old who drink a whey protein shake every day and lift weights three times a week for three months may gain more muscle, lose belly fat, and improve their overall metabolic health more than those who just do the workouts without the protein.

If you're a healthy young adult, eating a plant-based protein shake with extra leucine can build muscle just as well as a whey protein shake, as long as both have the same essential amino acids — the leucine seems to be the secret ingredient.