Eating whey protein during extreme dieting might reduce the muscle’s fat-burning response to leptin, compared to eating sugar—but this isn’t proven yet.

Even though drinking whey protein makes more building-block amino acids show up in the blood during extreme dieting, those amino acids don’t get the muscle to grow like they normally would.

Even when people eat whey protein during extreme dieting and lots of exercise, their muscles stop responding to it like they normally would — the protein doesn’t trigger the usual growth signals.

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.

When rats exercised and then ate a small amount of whey protein hydrolysate, their muscles built protein faster than when they ate regular whey protein at the same dose.

For young guys who work out regularly, drinking a shake with 20g of any kind of milk protein (like whey or casein) plus 45g of carbs after a combo of strength and cardio exercise helps build muscle just the same — it doesn’t matter which protein they pick.