Future directions in vitamin D and cardiovascular research. 
Evidence is accumulating that vitamin D status may influence the risk of cardiovascular disease (CVD).  Final confirmation for a causal relationship between vitamin D and CVD is however still lacking.  The present viewpoint article outlines several future research directions to close this gap.  Future directions include the need of performing large randomised controlled supplementation trials with vitamin D in specific risk groups.  In addition, large register sets of data on vitamin D supplementation can be used, provided that adequate statistical methods such as propensity score modelled analysis are applied.  To better understand vitamin D-mediated effects on CVD risk, the routine measurement of circulating levels of the hormonal vitamin D form, 1,25-dihydroxyvitamin D, is also necessary, in addition to the determination of its precursor 25-hydroxyvitamin D. Further, genetic association studies may help in clarifying the contribution of vitamin D to the development of CVD.  Finally, the interrelationship of vitamin D with physical activity should be considered when studying CVD risk.  Overall, it can be expected that the next 10-15 years will provide an increased clarity concerning the role of vitamin D in CVD.