vitamin a traffic sign on blue sky

vitamin retinol retinal retinoic acid carotenoids betacarotene liver eyesight vision immune system skin health antioxidant cell growth reproduction bone deficiency night blindness xerophthalmia keratinization retinoids dietary sources supplements daily intake recommended allowance rda toxicity pregnancy breastfeeding children adults elderly absorption metabolism transport binding protein retinyl esters gene expression pigment epithelium photoreceptors rhodopsin opsin cones rods visual cycle function regulation epithelial integrity mucous membranes respiratory tract gastrointestinal hormone