Abstract
Introduction: It has been hypothesised that the U shaped association between 25(OH)D and some health outcomes may be due to large seasonal fluctuations of 25(OH)D1. It is unknown whether such fluctuation of 25(OH)D (‘cycling’) influences bone health. Methods: In the D-FINES study, n=373 women (South Asian/Caucasian) had repeated measurements in four seasons for serum 25(OH)D and PTH. A random sample (n=66) were measured for serum C-telopeptide (CTX). Seasonal cycling of 25(OH)D was assessed as the absolute difference between winter (nadir) and summer (peak) 25(OH)D and was split into quartiles within ethnicity. Summer to winter change in CTX and PTH were calculated. Results and Discussion: ANCOVA showed no statistically significant association between quartile of cycling of 25(OH)D and CTX or PTH. However, in Asians, there was a trend for increased cycling to be associated with reduced PTH but not CTX, and for an increase in PTH from summer to winter. In Caucasians, there was a trend for increased cycling in all seasons to be associated with reduced CTX. However, increased cycling was associated with increased PTH in summer and spring, but lower PTH in other seasons, as well as a reduction in PTH from summer to winter (p=0.06). Therefore increased cycling in Caucasians was associated with lower bone resorption and was differentially associated with PTH depending on season. Further analysis of banked samples for urine CTX (n=1500) will enable these novel results to be explored further.