Even if there is no sunlight for half the year (and this completely equalizes vitamin D levels in winter), various confounders of sun exposure will still affect

Even if there is no sunlight for half the year (and this completely equalizes vitamin D levels in winter), various confounders of sun exposure will still affect a study on vitamin D levels, assuming the study collects some samples during the period when there is sunlight and vitamin D levels vary. Also, the confounders of vitamin D supplementation I mentioned would apply even if you never get exposed to the sun. I think your comment is assuming the vitamin D supplementation is effective, when that hasn’t really been established. We don’t even know that vitamin D has a direct effect on most of the correlated health measures.