As men get older, their bodies don't absorb protein from food as well after eating — specifically, older guys get less use out of the amino acids from a protein meal compared to younger guys.

When older guys drink a whey or pre-digested casein protein shake, the protein shows up in their blood faster than when they drink regular casein — meaning it gets absorbed more quickly.

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.

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.

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.