Effect of artificial gravity on calcaneal bone marrow adipose tissue and mineral content in female and male participants in 60 days of bed rest

Modulation of bone marrow adipose tissue (BMAT) with prolonged inactivity was reported in haemopoietic but not in non-haemopoietic bones. This prospective randomized controlled trial submitted 16 men and 8 women to 60 days of 6° head-down-tilt bed rest. They were assigned to control, continuous or intermittent artificial gravity (AG) interventions. The AG consisted of daily centrifugation at 2g for 30 min. The serial foot pain questionnaire, MRI and dual-energy X-ray absorptiometry of the calcaneus were performed at baseline, during bed rest and at reambulation. At baseline, all groups had comparable calcaneal BMAT (P = 0.581) and bone mineral density (BMD) (P = 0.574). After bed rest, 83% of participants reported foot pain. Calcaneal BMAT was not significantly modulated after 60 days of bed rest (control, +0.2% ± 0.8%; continuous AG, +0.5% ± 1.1%; and intermittent AG, +0.1% ± 1.5%; P = 0.368). Calcaneal BMD was reduced at reambulation days 3 and 11 after 60 days of bed rest (-0.05 ± 0.06 and -0.06 ± 0.12 g/cm², respectively; P = 0.008 and P = 0.020). The AG interventions did not significantly alter calcaneal BMAT or BMD. Sex-based analyses demonstrated calcaneal BMD loss in men but not in women. Calcaneal BMAT and BMD were inversely correlated in women and in men (Spearman's ρ, -0.40 and -0.28, respectively; both P = 0.020). Sixty days of bed rest caused foot pain and calcaneal demineralization not rescued by AG interventions. Although inversely correlated with BMD, calcaneal BMAT was not statistically increased by 60 days of head-down-tilt bed rest, possibly owing to a ceiling effect, and no bone marrow reconversion was measured at reambulation. These results have clinical relevance when returning to activities after prolonged bed rest or returning from space.