Is vitamin D supplementation responsible for the allergy pandemic?

Purpose of review A link between vitamin D supplementation and allergy was already suspected soon after it became possible to chemically synthesise vitamin D-2 by means of ultraviolet radiation. During the past decade, the assumed allergenic effect was confirmed by clinical and epidemiological studies although the most recent discussion has centred more on vitamin D insufficiency. The purpose of this review is to summarise studies published during the past year while attempting to reconcile some apparent inconsistencies. Recent findings Two new concepts are presented here - epigenetic programming of the fetal vitamin D system by low maternal vitamin D supply (Barker's paradox) and ubiquitous vitamin D exposure of the newborn (Rose's paradox). Taken together a misdirected epigenetic programming offers an explanation why also vitamin D insufficiency in pregnancy may be associated with increased allergy rates in the offspring. At least eight studies examined the association of early 25-hydroxy-vitamin D levels and atopic diseases in 2011, whereas no new study addressed the question of vitamin D supplementation in the newborn period. One study tested the whole range of 25-hydroxy-vitamin D levels in cord blood describing a U-shaped association with 2.4-fold odds ratio of low and 4-fold odds ratio of high levels to develop allergen-specific immunoglobulin E. Summary Randomised clinical trials with vitamin D supplements are therefore highly required. Several key points are presented for designing vitamin D trials.