Bed rest decreases resting skeletal muscle O ₂ uptake and resting energy expenditure in young and elderly subjects

A decrease in resting muscle O₂ uptake ( V ̇ O 2 m V_O₂m ) 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 V_O₂m 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 V_O₂m (-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 V_O₂m 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 V_O₂m 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.