Bed rest decreases resting skeletal muscle O2 uptake and resting energy expenditure in young and elderly subjects.

A decrease in resting muscle O2 uptake ( V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ ) described during bed rest (BR) could determine a decreased whole-body resting energy expenditure (REE), potentially useful 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 were performed before and post-BR. Fat-free mass (FFM) was measured by bioimpedance analysis; REE was measured by open-circuit spirometry; resting V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ was measured by time-domain near-infrared spectroscopy in the vastus medialis during a transient ischaemia; citrate synthase (CS) activity (estimate of mitochondrial volume) was measured on a vastus lateralis muscle biopsy sample. FFM decreased significantly both in Y (-8%, P < 0.001) and in E (-5%, P = 0.009). Both resting V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ (-45%, -2.1% day-1 in Y, P = 0.025; -29%, -2.9% day-1 in E, P = 0.001) and REE (-15%, -0.7% day-1 in Y, P = 0.012; -12%, -1.2% day-1 in E, P = 0.012) decreased during BR. CS activity decreased in E (-31%, P = 0.005), whereas the decrease in Y (-12%) did not reach statistical significance (P = 0.38). After resting, V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ data normalized for CS activity values in post- versus pre-BR were lower in Y (P = 0.021) but not in E (P = 0.99). The decreased resting V ̇ O 2 m ${\dot V_{{{\mathrm{O}}_2}{\mathrm{m}}}}$ and REE may represent a 'recalibration' of ATP supply to a reduced ATP demand, aimed at preventing excessive reactive oxygen species production and muscle atrophy. It would mitigate biological and logistic challenges of prolonged spaceflights, but could negatively impact the health status of the subjects.