Vitamin D deficiency and supplementation in critical illness—the known knowns and known unknowns

The burgeoning literature on vitamin D deficiency and supplementation over the past decade or so has generated a greater understanding of some areas but also an appreciation of the many areas of equipoise. This is particularly relevant in the field of critical care with the heterogeneous patient populations, the severity and duration of illness and the frequency of comorbid conditions. This review aims to summarise the current knowledge base of vitamin D deficiency within the context of critical illness—“the known knowns”—and also highlight the areas of recognised uncertainty—“the known unknowns”. It acknowledges the fact that there may well be other knowledge gaps of clinical relevance of which we are currently unaware—“the unknown unknowns”.