Three approaches may be used when populations have low vitamin A levels:[11] Retinol is also used to reduce the risk of complications in measles patients.

The first documented death possibly caused by vitamin A poisoning was that of Xavier Mertz, a Swiss scientist, who died in January 1913 on an Antarctic expedition that had lost its food supplies and fell to eating its sled dogs.

A review of all randomized controlled trials in the scientific literature by the Cochrane Collaboration published in JAMA in 2007 found that supplementation with beta carotene or vitamin A increased mortality by 5% and 16%, respectively.

[22] This effect has been attributed to the role of retinol and retinoic acid in increasing circulating cholesterol and triglycerides as well as promoting cancer incidence.

[24] Similarly, dosing newborn infants with 50,000 IU (15 mg) of vitamin A within two days of birth can significantly reduce neonatal mortality.

Retinoic acid is not required for patterning of the retina as originally proposed, but retinoic acid synthesized in the retina is secreted into the surrounding mesenchyme where it is required to prevent overgrowth of perioptic mesenchyme which can cause microphthalmia, defects in the cornea and eyelid, and rotation of the optic cup.

It is thought to initiate differentiation into several different cell lineages through activation of the Retinoic acid receptor.

Night blindness, the inability to see well in dim light, is associated with a deficiency of vitamin A, a class of compounds that includes retinol and retinal.

In the early stages of vitamin A deficiency, the more light-sensitive and abundant rods, which have rhodopsin, have impaired sensitivity, and the cone cells are less affected.

[35] Vitamin A appears to modulate the innate immune response and maintains homeostasis of epithelial tissues and mucosa through its metabolite, retinoic acid (RA).

[35] Another way that vitamin A helps maintain a healthy skin and hair follicle microbiome, especially on the face, is by reduction of sebum secretion, which is a nutrient source for bacteria.

[35] Retinol has been the subject of clinical studies related to its ability to reduce the appearance of fine lines on the face and neck.

[4][36] Vitamin A may be needed for normal red blood cell formation;[37][38] deficiency causes abnormalities in iron metabolism.

These are obtained from fruits and vegetables containing yellow, orange and dark green pigments, known as carotenoids, the most well-known being β-carotene.

[45] When required by a particular part of the body, the liver releases some vitamin A, which is carried by the blood and delivered to the target cells and tissues.

[46] The Dietary Reference Intake (DRI) Recommended Daily Amount (RDA) for vitamin A for a 25-year-old male is 900 micrograms/day, or 3000 IU.

[47] During the absorption process in the intestines, retinol is incorporated into chylomicrons as the ester form, and it is these particles that mediate transport to the liver.

Liver cells store vitamin A as the ester, and when retinol is needed in other tissues, it is de-esterifed and released into the blood as the alcohol.

Each of the following contains at least 0.15 mg of retinoids per 1.75–7 oz (50–198 g): Many different geometric isomers of retinol, retinal and retinoic acid are possible as a result of either a trans or cis configuration of four of the five double bonds found in the polyene chain.

The cis isomers are less stable and can readily convert to the all-trans configuration (as seen in the structure of all-trans-retinol shown at the top of this page).

The process of vision relies on the light-induced isomerisation of the chromophore from 11-cis to all-trans resulting in a change of the conformation and activation of the photoreceptor molecule.

[52] Retinol is made industrially via total synthesis using either a method developed by BASF[53][54] or a Grignard reaction utilized by Hoffman-La Roche.

[56] The world market for synthetic retinol is primarily for animal feed, leaving approximately 13% for a combination of food, prescription medication, and dietary supplement use.

[56] Pure retinol is extremely sensitive to oxidization and is prepared and transported at low temperatures and oxygen-free atmospheres.

In 2001, the European Commission imposed total fines of 855.22 Euros on these and five other companies for their participation in eight distinct market-sharing and price-fixing cartels that dated back to 1989.

[56] In 1912, Frederick Gowland Hopkins demonstrated that unknown accessory factors found in milk, other than carbohydrates, proteins, and fats were necessary for growth in rats.

[57] One year later, Elmer McCollum, a biochemist at the University of Wisconsin–Madison, and colleague Marguerite Davis identified a fat-soluble nutrient in butterfat and cod liver oil.

Their work confirmed that of Thomas Burr Osborne and Lafayette Mendel, at Yale, also in 1913, which suggested a fat-soluble nutrient in butterfat.

[57] Retinoic acid and retinol were first synthesized in 1946 and 1947 by two Dutch chemists, David Adriaan van Dorp and Jozef Ferdinand Arens.

Sommer classified historical accounts related to vitamin A and/or manifestations of deficiency as follows: "ancient" accounts; 18th- to 19th-century clinical descriptions (and their purported etiologic associations); early 20th-century laboratory animal experiments, and clinical and epidemiologic observations that identified the existence of this unique nutrient and manifestations of its deficiency.