Figure 1.
There are many possible pathways that could explain how low 25(OH)D and poor cardiovascular outcomes can be correlated, but only a causal relationship would mean that vitamin D supplementation can improve outcomes for cardiovascular disease (CVD). Unaccounted confounding and reverse causation can also result in associations in observational studies, but this would mean that improving vitamin D status through supplementation does not improve cardiovascular health. These correlative relationships aren't necessarily mutually exclusive, and even all can be present at the same time, making them hard to control for in observational settings and reinforcing the need for randomized controlled trials to establish causal effect.