Abstract
Aim: This study aimed to assess whether seasonal cycling of 25(OH)D (25-dihydroxy vitamin D) is associated with bone health. Method: A subgroup of 65 South Asian and Caucasian women who took part in the 2006-2007 D-FINES study was analysed. During this study they had blood drawn in four seasons for determination of 25(OH)D and serum c-telopeptide (sCTX)and in autumn and spring they had a DEXA scan (Hologic). Cycling of 25(OH)D was assessed by calculating the difference between the winter (nadir) and summer (peak) 25(OH)D and for ease of interpretation, expressing all change as positive values. Dependent variables analysed were absolute values for autumn femoral neck and lumbar spine BMD, BMC and bone area, and absolute sCTX in each season. Also, change in sCTX from summer to winter and change in the DEXA bone indices from autumn to spring were analysed. Results: ANCOVA, controlling for summer and winter 25(OH)D status, age, BMI, socioeconomic status, physical activity, and dietary calcium showed no statistically significant association (p>0.05) between quartile of cycling of 25(OH)D and any bone measurement in either ethnic group except in the Asians for absolute autumn CTX (F=5.925, p=0.01, fig 1) and change in FNBMC (F=3.111, p=0.05, see fig.3). Also, in Asians only, absolute autumn lumbar spine BMD approached significance (F =2.780, p=0.07, see fig 2). Conclusions: It has been suggested that some findings of increased risk of some cancers in countries with high 25(OH)D could be due to slow adaption of CYP27B1 and CYP24 to fluctuating 25(OH)D (1). This begs the question as to whether seasonal cycling of 25(OH)D could be detrimental to bone. Indeed, a recent review discussed a correlation between 25(OH)D and bone indices (2). The lack of an association between cycling and most bone indices found here does not support this view that 25(OH)D cycling is detrimental to bone. However, in Asians only, the loss of femoral neck BMC during the year in the top and bottom quartiles but gain in the 3rd quartile, and the increased autumn sCTX in the third quartile warrants further investigation.