If you lift weights and drink a protein shake after your workout, one made only with fast-digesting whey might give your body a quicker boost of key muscle-building amino acids than a mix with mostly slow-digesting casein.

If you're a guy who lifts weights, drinking a protein shake after your workout with mostly fast-digesting protein gives your body more of a key muscle-building amino acid—leucine—than a shake with mostly slow-digesting protein.

If you're a guy who lifts weights, drinking a protein shake after your workout with different mixes of fast- and slow-digesting milk proteins — like whey and casein — gives you about the same muscle and strength gains, even if your body processes the amino acids differently.

If you're a guy who lifts weights, drinking a protein shake after your workout with different mixes of fast- and slow-digesting milk proteins — like whey and casein — gives you about the same muscle and strength gains, even if your body absorbs the amino acids differently.

When rats ate a larger dose of regular whey protein, their blood had more leucine than when they ate the same amount of hydrolyzed whey protein an hour later.

Even though both protein shakes had the same amount of protein, the whey one made the key muscle-building amino acid (leucine) in the blood much higher than the collagen one.