to the editor: We would like to thank the scientists for their comments (and agreement) on our Viewpoint article (see Ref. 3). Originally, we put forward the idea that physical inactivity may be a critical factor underpinning age-related differences in postprandial muscle protein synthesis rates (MPS) (1). In our view, this idea remains a strong one. For instance, exercise induces a substantial carryover effect on the feeding-induced stimulation of MPS that can last for days (2, 4). We know very little about whether simply getting out of bed in the morning, walking down/up a flight a stairs, or going for a casual stroll around the neighborhood block will also confer a nutrient sensitizing effect on MPS (an effect observed after higher intensity exercise). However, if the threshold of physical activity that induces a sensitizing effect on MPS is relatively low, than our view may need much more consideration. Specifically, if it is not the aging muscle tissue per se but rather its recruitment in a daily life setting that contributes to the proposed "anabolic resistance" (and its curiosity) then it is not entirely surprising that comparisons between different aging populations from different geographical locations do not seem to line up.
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