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Influence of physical mobility and season on 25-hydroxyvitamin D-parathyroid hormone interaction and bone remodelling
R Theiler1, H B Stähelin, M Kränzlin
1Departments of Rheumatology, University of Basel, Switzerland.
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 <12ng/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 (5-30ng/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=0.0002 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.