Effect of age-related chronic immobility on markers of bone turnover

Introduction: Accelerated bone turnover and rapid bone loss caused by acute immobilization is well recognized, but the effects of age-related chronic,reduction in mobility on bone turnover have been less well studied. We assessed the associations between bone turnover and measures of mobility in a cohort of elderly subjects. Materials and Methods: We measured serum levels of the aminoterminal propeptide of type I collagen (PINP), a marker of bone formation, and serum concentrations of the carboxyterminal telopeptide of type I collagen (CTx), a marker of bone resorption, as well as serum intact PTH, serum 25 hydroxyvitamin D (25OHD), mobility, and static balance in a well-characterized sample of 1064 elderly men and women living in residential aged care facilities. Serum creatinine, phosphate, albumin, and calcium were measured in a randomly selected subgroup of 447 subjects. Results: The subjects were elderly and frail; their mean age was 86.0 years (range, 65-101 years); 69% used a walking aid; and 77% were vitamin D deficient (serum 25OHD level < 39 nM). Both serum PINP and CTx increased with age in both sexes. Elevated PINP or CTx was significantly correlated with high PTH, creatinine, and albumin in both genders, except for albumin in women. Age- and gender-adjusted serum CTx and PINP were significantly higher in those with poorer mobility and those with worse static balance. In multivariate analyses, higher serum PINP but not CTx was associated with poorer mobility and worse static balance. Conclusions: Our findings suggest that poor mobility contributes to the state of accelerated bone turnover usually seen in the elderly. The effect of chronic relative immobility may be more marked on bone formation than bone resorption.