Hypovitaminosis D and morbidity in critical illness: is there proof beyond reasonable doubt?

Vitamin D is recognized to have important actions outside its well-recognized role in musculoskeletal health. These include antimicrobial action, anti-inflammatory, and cardio-protective properties. A high prevalence of vitamin D deficiency and its association with adverse clinical outcomes have now been widely documented in observational studies in the critically ill. These studies of association, however, do not necessarily imply causation, as vitamin D deficiency may be merely a marker of higher illness severity and consequently poorer outcomes. This issue can be clarified only by undertaking high-quality randomized controlled trials of vitamin D supplementation in this vulnerable population.