Post-absorptive muscle protein synthesis is higher in outpatients as compared to inpatients.

Abstract

Several factors affect muscle protein synthesis (MPS) in the post-absorptive state. Extreme physical inactivity (e.g., bedrest) may reduce basal MPS, whereas walking may augment basal MPS. We hypothesized that Outpatients would have a higher postabsorptive MPS than Inpatients. To test this hypothesis, we conducted a retrospective analysis. We compared 152 Outpatient participants who arrived at the research site the morning of the MPS assessment to 350 Inpatient participants who had an overnight stay in the hospital unit before the MPS assessment the following morning. We used stable isotopic methods and collected vastus lateralis biopsies ~2 to 3 hours apart to assess mixed MPS. MPS was ~12% higher (p<0.05) for Outpatients than Inpatients. Within a subset of participants, we discovered that after instruction to limit activity, Outpatients (n=13) took 800 to 900 steps in the morning to arrive at the unit, seven times more steps than Inpatients (n=12). These reduced steps were correlated with a reduced basal MPS. We concluded that an overnight stay in the hospital as an Inpatient is characterized by reduced morning activity and causes a slight but significant reduction in MPS compared to participants studied as Outpatients. Researchers should be aware of physical activity status when designing and interpreting MPS results.