An association between seasonal fluctuation 'cycling' of 25(OH)D and increased bone resorption but not BMD or BMC in UK South Asian and Caucasian women living at 51on

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. This is an important issue, because if ‘cycling’ is detrimental for bone, then winter only rather than year round vitamin D supplementation may be useful for bone health to ‘blunt’ the rhythm. In the D-FINES study, n = 373 women (South Asian/Caucasian) had repeated measurements in four seasons for serum 25(OH)D and PTH, as well as a DXA scan in autumn and spring. Serum C-telopeptide (sCTX) was also measured in a random subset (n = 66). Cosinor regression analysis was used to identify individuals showing a significant rhythm (p 0.05) between ‘cyclers’ and ‘non-cyclers’ for any of the bone indices in either ethnic group. However, there were trends for a higher CTX and PTH in ‘cyclers’ versus ‘non-cyclers’ in both ethnic groups in every season, but no differences for BMD or BMC (Figs. 1–4). This suggests tentatively that ‘cycling’ could be associated with changes in bone metabolism but may not translate into structural changes. In summary, there is no clear evidence here to suggest that ‘cycling’ is detrimental to bone health, although there are trends in PTH and CTX that warrant further investigation with a larger sample