If you're a healthy young adult, your muscles might use amino acids from supplements better than from real milk—scientists tracked this by seeing how much labeled phenylalanine ended up in muscle over 6 hours.

If you drink a mix of free amino acids instead of whole milk protein, your body absorbs them faster and more of them—especially phenylalanine—show up in your blood over six hours.

If you're a healthy young adult, drinking a supplement with 30 grams of broken-down amino acids won't help your muscles grow more than drinking the same amount from real milk protein—even though your body absorbs the amino acids faster.

For healthy guys, drinking regular milk protein gives your body more usable protein building blocks over 5 hours than taking just whey or just casein alone.

When healthy guys drink milk protein, about half of the protein gets into their bloodstream within 5 hours — that's what muscles can use. The rest is either still being digested or gets used by organs like the liver before it ever reaches the rest of the body.

When healthy guys drink more whey protein, their bodies absorb more of an important amino acid called phenylalanine—about 10% more when they take a higher dose.