Vitamin D and chronic disease prevention

Vitamin D “deficiency” (circulating 25-hydroxyvitamin D concentration <30 nmol/L) has been linked to a remarkable array of chronic diseases, including bone mineral disease, autoimmunity, cancer, diabetes, and cardiovascular outcomes.1 So plentiful are vitamin D’s putative mechanistic actions that it has been whimsically invoked as an explanation for why good triumphs over evil in JRR Tolkien’s The Hobbit.2 Parody aside, the vitamin D literature comprises a minefield of observational data and mixed quality evidence from predominately small trials. Appropriate interpretation of the data is further muddied by seemingly endless media reports suggesting vitamin D as a panacea for chronic disease.3 4

Against this backdrop, two new papers bravely attempt to make sense of the existing data. Theodoratou and colleagues (doi:10.1136/bmj.g2035) highlight differences between observational data (relating circulating 25-hydroxyvitamin D to outcomes) and randomised controlled trials of supplementation.5 Of a remarkable 137 different outcomes reportedly linked to 25-hydroxyvitamin D, only 10 had also been tested in trials, and only one (birth weight) had apparently concordant evidence of “benefit” from observational studies and trials. This pattern of findings should ring alarm bells; observational epidemiology extolled the virtues of antioxidant vitamins, …