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.

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.

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.

When people with fatty liver disease eat a lot of protein and lift weights at the same time while eating very little, they don’t get any extra muscle benefits — the two things don’t work together like they usually do.

Even though whey protein made muscles build faster, it didn’t change the usual ‘on/off’ switches in muscle cells that are supposed to control growth—suggesting other mechanisms are at play.

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.