3.4. Vitamin A Supplementation against Viral Infections
Vitamin A supplementation is correlated with a reducing in the infection-related morbidity and mortality associated with vitamin A deficiency (Table 2) [33,34,35,36,37,38,39,40,41,42,43].
A meta-analysis of 47 studies that included 1,233,856 children found that vitamin A supplementation is associated with a reduction of all-cause mortality of 12% [33]. This association could be explained by the low-to-moderate evidence that vitamin A supplementation in children can reduce the incidence of diarrhea and measles [34,35]. In contrast with previous several trials [33], in a large cluster-randomized trial (DEVTA trial) that included more than 1 million pre-school children in North India, a region with high frequency of vitamin A deficiency, supplementation with high-dose vitamin A (200,000 UI every 6 months] did not achieve a significant mortality reduction. However, in the same article, a meta-analysis that included DEVTA plus eight previous randomised trials of supplementation yielded a weighted average mortality reduction of 11% [32]. Moreover, although it has been reported that there is a significant association between low serum concentration of retinol and acute lower respiratory tract infections [36], several studies in children, particularly regarding the role of vitamin A for treatment of respiratory syncytial virus infection [37,38,39], have shown that vitamin A supplementation is not effective in reducing the incidence of lower respiratory tract infections [40,41,42,43]. However, considering the evidence of the role of vitamin A in supporting an effective immune system, and of the effect of vitamin A on child mortality, supplementation should be offered to children in population at risk of vitamin A deficiency, which could also include patients with disorders associated with fat malabsorption.