Vitamin D and bacterial pneumonias: the question of causality

[...]we cannot determine whether naturally low vitamin D levels increase risk of pneumonias through randomised controlled trials as these vitamin D levels are not determined by study investigators. [...]alternative approaches to test for causation are required. The paper crucially also uses control diseases to show that the analysis is not generating spurious associations. [...]although the results do not provide conclusive evidence of causality, this paper does add to the debate and supports previous studies that suggest vitamin D may play a role in bacterial pneumonia risk. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data.