Correlations Between Changes in Undercarboxylated Osteocalcin and Muscle Function in Hypoparathyroidism

BACKGROUND Muscles and bones are interconnected. Recent studies suggest that undercarboxylated osteocalcin from bone may affect muscle mass and strength. There are, however, no prospective human data on this relationship. METHODS We previously treated patients with hypoparathyroidism with intact Parathyroid Hormone (PTH) or placebo in a six-month randomized, placebo-controlled trial and demonstrated a marked increase in undercarboxylated osteocalcin (ucOC) in the PTH-treated group. We therefore investigated if this increase correlated with changes in muscle mass, strength or function. Primarily, the muscle mass using Dual energy X-ray Absorptiometry (DXA) was measured and the maximal voluntary isometric muscle strength at the upper and lower extremities, using dynamometry, was assessed. Furthermore, repeated chair stands test, Timed Up and Go test were performed and postural stability using a stadiometer was assessed. Finally, the relationship between change in ucOC or the ratio of the changes in ucOC and total OC (ucOC%/OC%) and different measures of muscle function were analyzed, using regression analyses. RESULTS The findings indicated that ucOC%/OC% was positively and significantly associated with percentage change in max force production during elbow extension (β = 0.28, P = 0.034), however, all other associations were non-significant. CONCLUSIONS Given the number of statistical tests that were carried out, our one significant finding may represent a false positive. Thus the results do not support the role of ucOC in muscle function in humans with hypoparathyroidism. Our results are inconsistent with previous data from a human cross-sectional study; however, cross-sectional studies, do not allow for inference of causality. The analyses should be repeated in larger, randomized trials including healthy individuals.