Influence of physical mobility and season on 25-hydroxyvitamin D-parathyroid hormone interaction and bone remodelling in the elderly

Abstract

Objective: To investigate influences of physical mobility and season on 25-hydroxyvitamin D-intact parathyroid hormone (iPTH) interaction in the elderly. Design: We examined 188 frail institutionalized elderly at the expected nadir of their serum vitamin D concentrations (winter). This group was compared with 309 healthy ambulatory elderly at the expected time of maximum vitamin D repletion (summer), to accentuate the influences of season and physical activity

Methods: Serum concentrations of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, iPTH and urinary deoxypyridinoline (DPD) were measured.

Results: Vitamin D metabolites were significantly lower in the institutionalized elderly (P<0.0001), with an 82.5% prevalence of vitamin D deficiency (25-hydroxyvitamin D <12 ng/ml) in institutionalized elderly in wintertime and 15.5% in ambulatory elderly in summertime. Overall, median iPTH did not differ between groups. However, median iPTH secretion in the presence of low vitamin D serum concentrations (5-30 ng/ml) was greater in ambulatory elderly. This could be explained by lower mobility status being correlated with greater serum calcium concentrations (r=0.24, P=0.02, in women; r=0.35, P=0.001 in men) and greater urinary excretion of DPD (r=0.41, P=0.0001 in women: r=0.42, P=0.0003 in men), independent of 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D and iPTH.

Conclusions: These data support the hypothesis that immobility, even in the presence of vitamin D deficiency acts as an overriding influence on bone metabolism by promoting bone resorption (measured as urinary DPD) and increasing serum calcium independent of iPTH. Therefore mobility status may substantially affect 25-hydroxyvitamin D threshold values and the degree to which patients benefit From vitamin supplementation.