The purpose of this study is to assess the optimal protein intake to maximize whole-body protein net balance in healthy adults. In a randomized, double-blinded, crossover design, 12 young, healthy adult males and females aged 18-35 years, will consume a test diet consisting of free amino acid mixtures based on milk protein that vary in protein content 7 separate acute metabolic trials. The main study endpoint is optimal protein intake based on the lowest protein intake that maximizes whole-body protein net balance.