High levels of vitamin D associated with less ischemic heart disease – A nested case-control study among rural men in Sweden

Introduction and objective. Low levels of serum vitamin D have been associated with increased occurrence of ischemic heart disease (IHD), but findings are inconsistent, e.g. seasonal variation is not always considered in the analyses. The aim of this study was to investigate whether vitamin D is associated with IHD among rural middle-aged men in Sweden when seasonal variation is taken into account. Materials and method. A nested case-control study was performed within a population-based cohort of Swedish farmers and rural residents followed for 20 years. Outcomes were from national health registers. For 276 cases (IHD at any time during follow-up), and for 276 aged-matched controls free of cardiovascular diseases or diabetes, serum levels of vitamin D (25-hydroxyvitamin D₃) were analyzed with liquid chromatography/tandem mass spectrometry (LC/MS/MS), in stored blood samples drawn at the baseline survey in 1990–91. The odds ratio (OR) with 95% confidence intervals (CI) of IHD for vitamin D-quartiles (based on measured as well as adjusted for month of sampling) was evaluated in logistic regression models; potential confounders were taken into account. Results. The measured mean vitamin D levels were 27.6 ng/ml(standard deviation (sd)8.2 ng/ml) and 28.8 ng/ml (sd 9.0) among cases and controls, respectively. Compared to the quartile with the lowest month-adjusted vitamin D levels, the risk of IHD was significantly lower in the highest quartile (OR=0.46, 95% CI 0.25–0.84, p=0.012) in the fully-adjusted model. Conclusion. Rural Swedish men in the highest quartile of serum vitamin D had halved relative risk of IHD, compared to those in the lowest quartile.