Is high dose vitamin D harmful?

With the potential to minimize the risk of many chronic diseases and the apparent biochemical safety of ingesting doses of oral vitamin D several-fold higher than the current recommended intakes, recent research has focussed on supplementing individuals with intermittent, high-dose vitamin D. However, two recent randomized controlled trials (RCTs) both using annual high-dose vitamin D reported an increase, rather than a decrease, in the primary outcome of fractures. This review summarises the results from studies that have used intermittent, high doses of vitamin D, with particular attention to those finding evidence of adverse effects. Results from observational, population-based studies with evidence of a U- or J-shaped curve are also presented as these findings suggest an increased risk in those with the highest serum 25D levels. Speculative mechanisms are discussed and biochemical results from studies using high-dose vitamin D are also presented. Emerging evidence from both observational studies and RCTs suggests there should be a degree of caution about recommending high serum 25D concentrations for the entire population. Furthermore, benefit of the higher doses commonly used in clinical practice on falls risk reduction needs to be demonstrated. The safety of loading doses of vitamin D should be demonstrated before these regimens become recommended as routine clinical practice. The current dilemma of defining vitamin D insufficiency and identifying safe and efficacious repletion regimens needs to be resolved.