a b c Wu L, Guo X, Wang W, Medeiros DM, Clarke SL, Lucas EA, etal. (November 2016). "Molecular aspects of β, β-carotene-9', 10'-oxygenase 2 in carotenoid metabolism and diseases". Experimental Biology and Medicine. 241 (17): 1879–1887. doi: 10.1177/1535370216657900. PMC 5068469. PMID 27390265. a b "Overview on Dietary Reference Values for the EU population as derived by the EFSA Panel on Dietetic Products, Nutrition and Allergies" (PDF). 2017. Archived (PDF) from the original on 28 August 2017. Dietary retinol is absorbed from the digestive tract via passive diffusion. 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. [5] Storage of retinol is in lipid droplets in the liver. 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. [3] Only when the liver stores are nearly depleted will signs and symptoms of deficiency show. [3] Retinol is reversibly converted to retinal, then irreversibly to retinoic acid, which activates hundreds of genes. [8] Saeed A, Dullaart RP, Schreuder TC, Blokzijl H, Faber KN (December 2017). "Disturbed Vitamin A Metabolism in Non-Alcoholic Fatty Liver Dise Animal models have shown that at the enterocyte cell wall, β-carotene is taken up by the membrane transporter protein scavenger receptor class B, type 1 (SCARB1). Absorbed β-carotene is converted to retinal and then retinol. The first step of the conversion process consists of one molecule of β-carotene cleaved by the enzyme

The USDA has conducted extensive studies on the percentage losses of various nutrients from food types and cooking methods. [60] Some vitamins may become more "bio-available" – that is, usable by the body – when foods are cooked. [61] The table below shows whether various vitamins are susceptible to loss from heat—such as heat from boiling, steaming, frying, etc. The effect of cutting vegetables can be seen from exposure to air and light. Water-soluble vitamins such as B and C dissolve into the water when a vegetable is boiled, and are then lost when the water is discarded. [62] Vitamin U.S. Food and Drug Administration. CFR – Code of Federal Regulations Title 21. Retrieved 16 February 2014. Newer research has shown that the absorption of provitamin-A carotenoids is only half as much as previously thought. As a result, in 2001 the US Institute of Medicine recommended a new unit, the retinol activity equivalent (RAE). Each μg RAE corresponds to 1 μg retinol, 2 μg of β-carotene in oil, 12 μg of "dietary" beta-carotene, or 24 μg of the three other dietary provitamin-A carotenoids. [4] Substance and its chemical environment (per 1 μg) AI US and EFSA Adequate Intake; AIs established when there is not sufficient information to set EARs and RDAs. Some countries require or recommend fortification of foods. As of January 2022, 37 countries, mostly in Sub-Saharan Africa, require food fortification of cooking oil, rice, wheat flour or maize (corn) flour with vitamin A, usually as retinyl palmitate or retinyl acetate. Examples include Pakistan, oil, 11.7mg/kg and Nigeria, oil, 6mg/kg; wheat and maize flour, 2mg/kg. [59] An additional 12 countries, mostly in southeast Asia, have a voluntary fortification program. For example, the government of India recommends 7.95mg/kg in oil and 0.626mg/kg for wheat flour and rice. However, compliance in countries with voluntary fortification is lower than countries with mandatory fortification. [59] No countries in Europe or North America fortify foods with vitamin A. [59] FoodRetinoic acid diffuses into the cell nucleus where it regulates more than 500 genes by binding directly to gene targets via retinoic acid receptors. [5]

Maton, Anthea; Hopkins, Jean; McLaughlin, Charles William; Johnson, Susan; Warner, Maryanna Quon; LaHart, David; Wright, Jill D. (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. ISBN 978-0-13-981176-0. OCLC 32308337. Rodahl K, Moore T (July 1943). "The vitamin A content and toxicity of bear and seal liver". The Biochemical Journal. 37 (2): 166–168. doi: 10.1042/bj0370166. PMC 1257872. PMID 16747610. Vitamins have diverse biochemical functions. Vitamin A acts as a regulator of cell and tissue growth and differentiation. Vitamin D provides a hormone-like function, regulating mineral metabolism for bones and other organs. The B complex vitamins function as enzyme cofactors (coenzymes) or the precursors for them. Vitamins C and E function as antioxidants. [7] Both deficient and excess intake of a vitamin can potentially cause clinically significant illness, although excess intake of water-soluble vitamins is less likely to do so.The capacity to store retinol in the liver means that well-nourished humans can go months on a vitamin A deficient diet without manifesting signs and symptoms of deficiency. Two liver cell types are responsible for storage and release: hepatocytes and hepatic stellate cells (HSCs). Hepatocytes take up the lipid-rich chylomicrons, bind retinol to retinol-binding protein 4 (RBP4), and transfer the retinol-RBP4 to HSCs for storage in lipid droplets as retinyl esters. Mobilization reverses the process: retinyl ester hydrolase releases free retinol which is transferred to hepatocytes, bound to RBP4, and put into blood circulation. Other than either after a meal or when consumption of large amounts exceeds liver storage capacity, more than 95% of retinol in circulation is bound to RBP4. [14] Carnivores [ edit ] Welch AD (1983). "Folic acid: discovery and the exciting first decade". Perspect. Biol. Med. 27 (1): 64–75. doi: 10.1353/pbm.1983.0006. PMID 6359053. S2CID 31993927.