The accredited laboratory from the Icelandic Heart Association conducted 25OHD measurements using the Liaison chemiluminescence immunoassay (DiaSorin Inc., Stillwater, Minnesota). The inter-assay coefficient of variation was < 6.5% when calculated data are from measurements using a frozen serum pool as the control sample and < 12.7% when calculated data is from measurements using liaison quality controls. Existing serum 25OHD levels were then standardized according to the international Vitamin D Standardization Program (VDSP) as previously described [19 (link), 26 (link)]. For the analyses, standardized 25OHD were categorized into three groups based on Guidelines for Health Professionals from the National Institutes of Health (2014): deficient (≤ 30 nmol/L), insufficient (31–49 nmol/L), normal–high levels (≥ 50 nmol/L).