Bed Rest Decreases Skeletal Muscle O2 Uptake At Rest In Young And Elderly Subjects

A decrease in resting muscle O2 uptake (̇VO2 m ) described during bed rest (BR) coulddetermine a decreased whole-body resting energy expenditure (REE), potentiallyuseful during prolonged spaceflights. Two groups of recreationally active men (young[Y], n = 8, age 23 ± 5 years; elderly [E], n = 10, age 69 ± 3 years) underwent a 21-day (Y) or a 10-day (E) horizontal BR without countermeasures. Measurements wereperformed before and post-BR. Fat-free mass (FFM) was measured by bioimpedanceanalysis; REE was measured by open-circuit spirometry; restinġ VO2 m was measuredby time-domain near-infrared spectroscopy in the vastus medialis during a trans-ient ischaemia; citrate synthase (CS) activity (estimate of mitochondrial volume) wasmeasured on a vastus lateralis muscle biopsy sample. FFM decreased significantly bothin Y (−8%, P < 0.001) and in E (−5%, P = 0.009). Both restinġ VO2 m (−45%, −2.1% day−1in Y, P = 0.025; −29%, −2.9% day−1 in E, P = 0.001) and REE (−15%, −0.7% day−1 inY, P = 0.012; −12%, −1.2% day−1 in E, P = 0.012) decreased during BR. CS activitydecreased in E (−31%, P = 0.005), whereas the decrease in Y (−12%) did not reachstatistical significance (P = 0.38). After resting,̇ VO2 m data normalized for CS activityvalues in post- versus pre-BR were lower in Y (P = 0.021) but not in E (P = 0.99).The decreased restinġ VO2 m and REE may represent a ‘recalibration’ of ATP supplyto a reduced ATP demand, aimed at preventing excessive reactive oxygen speciesproduction and muscle atrophy. It would mitigate biological and logistic challenges ofprolonged spaceflights, but could negatively impact the health status of the subjects.