Elderly individuals exhibit 'anabolic resistance' to amino acids, requiring higher leucine thresholds or faster-digesting proteins (like whey) to stimulate muscle protein synthesis compared to younger individuals.

Several studies have identified a so-called anabolic resistance of older muscle to amino acids... If a threshold of leucine is the 'trigger' for protein synthesis in muscle, this may explain the observations of Dangin et al. These researchers showed that the paradigm of how 'fast' and 'slow' proteins affect protein anabolism, however, differs in the elderly whereby whey proteins appear to be more effective in stimulating whole-body protein synthesis as opposed to casein.