Effects of 14 days of head‐down bed rest with or without exercise, and subsequent recovery on bone turnover, density and structure in older adults

Bed rest accelerates bone loss and may exacerbate skeletal fragility. This study examined the effects of 14 days of head‐down tilt bed rest (HDBR) with or without exercise, and subsequent recovery, on bone turnover, density and structure in older adults. Twenty‐two healthy older adults (55–65 years) completed the HDBR protocol. Participants were randomized to a control group that received passive physiotherapy (CON, n = 11) or a group that performed daily exercise (EX, n = 11). Serum biomarkers of bone formation (procollagen type 1 N‐terminal propeptide (P1NP) and bone‐specific alkaline phosphatase (BSAP)), resorption (N‐terminal cross‐linked telopeptide of type I collagen (NTX) and C‐terminal cross‐linked telopeptide of type I collagen (CTX)), and osteocalcin were measured at baseline (BDC4), day‐9 (HDT9), and immediately (R1), 4 weeks (4W), and 4 months (4M) post‐HDBR. Bone mineral density (BMD) was assessed via dual‐energy X‐ray absorptiometry (DXA) at BDC4, R1, 4W and 4M. Femoral bone structure was measured via peripheral quantitative computed tomography (pQCT) at BDC4 and R1. CTX and NTX increased at R1 vs. BDC4 (time: P < 0.001), while P1NP and BSAP increased at 4W and 4M (time: P < 0.001). No DXA‐derived BMD changes occurred. pQCT revealed reduced femoral trabecular volumetric BMD at 4% (EX: 342.3 ± 7.8 to 337.9 ± 8.0 mg/cm 3 ; CON: 329.3 ± 8.1 to 326.4 ± 8.4 mg/cm 3 ; time: P = 0.05) and cortical volumetric BMD at 25% (EX: 1089.5 ± 6.7 to 1087.5 ± 7.1 mg/cm 3 ; CON: 1102.7 ± 6.7 to 1095.7 ± 7.1 mg/cm 3 ; time: P = 0.05). However, changes were within the precision error of pQCT measurements. Fourteen days of HDBR, with or without exercise, increased biomarkers of bone resorption but did not alter BMD or bone structure.