Does a carrot a day keep the allergy away?

Vitamin A is an important micronutrient, from plants diet taken up as carotenoids, from animal food sources as retinol. Its active metabolite retinoic acid (RA) binds to nuclear hormone receptors, thereby regulating gene transcription programs in various cells. Adequate nutritional intake of vitamin A is essential for pre- and postnatal development, eyesight and reproduction, and it contributes to the maintenance and regulation of the immune system. Recent molecular studies indicate that lipocalins play an important role in the bioavailability of RA and its immune modulation against Th2 responses. There is emerging evidence that supply with vitamin A determines the susceptibility to allergic diseases: significantly reduced serum vitamin A levels are commonly observed in allergic patients compared to healthy controls. In line, findings from nutritional and clinical trials suggest that sufficient vitamin A supplementation in pregnancy prevents the development of allergic diseases in the offspring, and helps in controlling symptoms in adult asthmatics. Overall, retinoids have a key role in regulating immune homeostasis on mucosal surfaces because they are able to interfere with inflammatory signalling pathways. In this mini-review we will concentrate on the current knowledge about the influence of dietary and supplementary vitamin A on allergic diseases in humans from infancy to adulthood.