Fascinating, keep up the good work!

The study used intrinsically-labeled protein. That is protein that has stable isotope tracers in it. It allows you to follow the ingested protein throughout the body (how much is absorbed, how much ends up in muscle, etc). It takes a lot of time and money to make it (you cannot buy it somewhere).

In this case, they made the intrinsically-labeled protein for other research studies where they were specifically interested in milk protein. Since we had enough left, we used it for this study so we could do more fancy measurements. We did not choose milk specifically over whey.