More evidence is needed before general supplementation

Holick is right to highlight the high prevalence of vitamin D insufficiency in many groups of people living at higher latitudes.¹ However, the evidence base to support general supplementation, including aggressive fortification, does not currently exist.

An impressive array of observational data suggests that low vitamin D concentrations are associated with a wide variety of diseases, including diabetes, cardiovascular disease, and cancer. This does not necessarily mean that intervention with vitamin D supplementation will improve health outcomes—witness the problems of hormone replacement therapy despite encouraging observational data, not to mention the lack of effect and possible harms surrounding vitamins A, E, and β carotene supplementation.²

Although traditional vitamin D toxicity may be unlikely even with relatively large doses of vitamin D, we cannot be sure that long term vitamin D supplementation does not in fact cause some harms—and the case for benefits is far from proved. The recent women’s health initiative study of low dose calcium and vitamin D supplementation showed no change in risk of colorectal cancer, diabetes, stroke, or cardiovascular disease, but the regimen did produce an increase in the number of renal stones.³ Recent observational data from the Framingham study suggests that the risk of cardiovascular events may be lowest at 25- hydroxy vitamin D levels of around 60 nmol/l, possibly increasing slowly at levels above this.⁴

The only way to resolve these uncertainties is to conduct large scale randomised controlled trials comparing different doses of vitamin D with placebo. Only then will we be able to weigh up the balance of benefit and risk, decide which groups of people would gain net benefit, and thus make safe and sensible recommendations on vitamin D supplementation.