The European Union and various countries have set recommendations for dietary intake, and upper limits for safe intake. Neither of these measures indicates the status of liver reserves. Breast milk retinol can indicate a deficiency in nursing mothers. Plasma retinol is used as a biomarker to confirm vitamin A deficiency. Reversible night blindness is an early indicator of low vitamin A status. Vitamin A deficiency is estimated to affect approximately one-third of children under the age of five around the world, resulting in hundreds of thousands of cases of blindness and deaths from childhood diseases because of immune system failure. Deficiency can occur at any age but is most common in pre-school-age children and pregnant women, the latter due to a need to transfer retinol to the fetus. Vitamin A deficiency is common in developing countries, especially in Sub-Saharan Africa and Southeast Asia. Retinol is reversibly converted to retinal, then irreversibly to retinoic acid, which activates hundreds of genes. Only when the liver stores are nearly depleted will signs and symptoms of deficiency show. A high capacity for long-term storage of retinol means that well-nourished humans can go months on a vitamin A- and β-carotene-deficient diet, while maintaining blood levels in the normal range. Storage of retinol is in lipid droplets in the liver. Unlike retinol, β-carotene is taken up by enterocytes by the membrane transporter protein scavenger receptor B1 (SCARB1), which is upregulated in times of vitamin A deficiency. ĭietary retinol is absorbed from the digestive tract via passive diffusion. The other carotenoids have no vitamin activity. Vitamin A occurs as two principal forms in foods: A) retinol, found in animal-sourced foods, either as retinol or bound to a fatty acid to become a retinyl ester, and B) the carotenoids alpha-carotene, β-carotene, gamma-carotene, and the xanthophyll beta-cryptoxanthin (all of which contain β-ionone rings) that function as provitamin A in herbivore and omnivore animals which possess the enzymes that cleave and convert provitamin carotenoids to retinal and then to retinol. Vitamin A has multiple functions: it is essential for embryo development and growth, for maintenance of the immune system, and for vision, where it combines with the protein opsin to form rhodopsin – the light-absorbing molecule necessary for both low-light ( scotopic vision) and color vision. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably beta-carotene ). Vitamin A is a fat-soluble vitamin and an essential nutrient for humans.