Analysing the effect of multiple sclerosis on vitamin D related biochemical markers of bone remodelling

The Irish population is at risk of vitamin D deficiency during the winter months, but the secular trend over the past 40 years is for marked improvement. Multiple sclerosis (MS) is common in Ireland with a latitudinal pattern favouring highest incidence in northern regions; MS is linked strongly with vitamin D status as a causal factor. We sought firstly to study the relationship between vitamin D status and vitamin D-related bone biochemistry, and secondly to evaluate if MS had an independent effect on vitamin D related markers of bone remodelling. Using a case-control design of 165 pairs (MS patient and matched control) residing in three different geographic regions during winter months, we measured serum 25-hydroxyvitamin D (25OHD), parathyroid hormone (PTH), C-terminal telopeptide of type I collagen (CTX) and total procollagen type I amino-terminal propeptide (PINP). Given the paired case-control design, associations were explored using mixed-effects linear regression analysis with the patient-control pair as a random effect and after log transformation of 25OHD. A two-way interaction effect was tested for vitamin D status (25OHD <30 nmol/L) and the presence of MS on PTH, CTX, and PINP. In the total group, just over one-third (34.5%) had 25OHD <30 nmol/L. PTH was elevated in 7.6%. CTX was not elevated in any case, and PINP was elevated in 4.5%. On mixed-effects linear regression analysis after adjusting for confounders (age, sex, renal function, and serum albumin), we demonstrated the principal determinant of 25OHD was geographical location (p < 0.001), of PTH was 25OHD (p < 0.001), of CTX was PTH (p < 0.001), and of PINP was PTH (p < 0.001). MS did not have an independent effect on PTH (p = 0.921), CTX (p = 0.912), or PINP (p = 0.495). As regards an interaction effect, the presence of MS and 25OHD <30 nmol/L was not significant but tended towards having lower PTH (p = 0.207). In conclusion, in Ireland in winter only a minority had any abnormality in the secondary indices of vitamin D deficiency, and MS had no independent effect on parathyroid status or bone remodelling activity.