Retinol in the context of "By mouth"

Golden rice is a variety of rice (Oryza sativa) produced through genetic engineering to biosynthesize beta-carotene, a precursor of vitamin A, in the edible parts of the rice. It is intended to produce a fortified food to be grown and consumed in areas with a shortage of dietary vitamin A. Genetically modified golden rice can produce up to 23 times as much beta-carotene as the original golden rice.

Golden rice is generally considered to be safe, with the FDA, Health Canada, International Rice Research Institute and the Bill & Melinda Gates Foundation supporting its use. It has been met with significant opposition from some environmental and anti-globalisation activists, alleging risks regarding biodiversity and expressing concerns about unforeseen health effects and socioeconomic impacts. In 2016, 107 Nobel laureates wrote an open letter to Greenpeace and its supporters, asking them to abandon their campaign against genetically modified crops in general and golden rice in particular. In 2024, the Filipino Court of Appeals issued a cease and desist order for the growth of golden rice in the country, citing a lack of scientific certainty regarding its health and environmental impact.

Vitamin A is a fat-soluble vitamin that is an essential nutrient. The term "vitamin A" encompasses a group of chemically related organic compounds that includes retinol, retinyl esters, and several provitamin (precursor) carotenoids, most notably β-carotene (beta-carotene). Vitamin A has multiple functions: growth during embryo development, maintaining the immune system, and healthy vision. For aiding vision specifically, it combines with the protein opsin to form rhodopsin, the light-absorbing molecule necessary for both low-light (scotopic vision) and color vision.

Vitamin A occurs as two principal forms in foods: A) retinoids, found in animal-sourced foods, either as retinol or bound to a fatty acid to become a retinyl ester, and B) the carotenoids α-carotene (alpha-carotene), β-carotene, γ-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 retinol. Some carnivore species lack this enzyme. The other carotenoids do not have retinoid activity.

Retinoic acid (simplified nomenclature for all-trans-retinoic acid) is a metabolite of vitamin A₁ (all-trans-retinol) that is required for embryonic development, male fertility, regulation of bone growth and immune function. All-trans-retinoic acid is required for chordate animal development, which includes all higher animals from fish to humans. During early embryonic development, all-trans-retinoic acid generated in a specific region of the embryo helps determine position along the embryonic anterior/posterior axis by serving as an intercellular signaling molecule that guides development of the posterior portion of the embryo. It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages. In adult tissues, the activity of endogenous retinoic acid appears limited to immune function and male fertility.

γ-Carotene (gamma-carotene) is a carotenoid, and is a biosynthetic intermediate for cyclized carotenoid synthesis in plants. It is formed from cyclization of lycopene by lycopene cyclase epsilon. Along with several other carotenoids, γ-carotene is a vitamer of vitamin A in herbivores and omnivores. Carotenoids with a cyclized, beta-ionone ring can be converted to vitamin A, also known as retinol, by the enzyme beta-carotene 15,15'-dioxygenase; however, the bioconversion of γ-carotene to retinol has not been well-characterized. γ-Carotene has tentatively been identified as a biomarker for green and purple sulfur bacteria in a sample from the 1.640 ± 0.003-Gyr-old Barney Creek Formation in Northern Australia which comprises marine sediments. Tentative discovery of γ-carotene in marine sediments implies a past euxinic environment, where water columns were anoxic and sulfidic. This is significant for reconstructing past oceanic conditions, but so far γ-carotene has only been potentially identified in the one measured sample.