[Vitamin D supplementation for all older people with COVID-19?]

Vitamin D supplementation has been suggested for many diseases and symptoms that are frequent in older people: loss of bone mineral density, falls, loss of muscle mass, infections (including COVID-19) and mortality. In observational studies, vitamin D concentration has been associated with many poor outcomes, but in most Randomized Controlled Trials, there is no beneficial effect of vitamin D supplementation on these outcomes, except for bone mineral density. An alternative explanation of the observed association may be 'confounding': older people with poor mobility have lower vitamin D, and irrespective of that also have poor outcomes. The recent COVIT-TRIAL seems to suggest there is benefit of high dose vitamin D supplementation compared to low dose on mortality in older people with COVID-19, but on closer methodological look this conclusion does not hold. Clinical guidelines rightly do not advise vitamin D supplementation for a whole range of diseases, including COVID-19.