Vitamin D and Cardiorespiratory Fitness

Here is a study just published showing relationship between cardiorespiratory fitness and vitamin D levels. The study adjusted for various confounders and the relationship was independent of these confounders. The study is cross-sectional and hence the direction of causal effect is difficult to determine. That is, whether greater cardiorespiratory fitness is associated with more time spent outside and hence more sunlight exposure and vitamin D formation or whether vitamin D, through its effects on muscles, nerves, bones, results in higher cardiorespiratory fitness