If young guys drink a protein shake with micellar casein, their bodies show lower levels of key amino acids after eating compared to when they drink whey or a mix of whey and casein — based on blood tests over 6 hours.

If a young guy drinks a shake with 25 grams of whey protein, his body absorbs more essential amino acids over 6 hours compared to drinking the same amount of casein protein.

If a young guy drinks a protein shake with a mix of whey and casein, his body absorbs the important amino acids about as well as with pure whey—and better than with pure casein—over the next 6 hours.

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.

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 healthy guys drink more casein protein, more amino acids show up in their blood overall—even though a smaller percentage gets absorbed—probably because their bodies haven’t finished digesting it within 5 hours.