Vitamin A was first discovered by a biochemist at the University of Wisconsin-Madison called Elmer McCollum, alongside colleague Marguerite Davis. The pair discovered a fat-soluble nutrient in cod liver oil and butterfat. Work performed by Lafayette Mendel and Thomas Osborne, also in 1913, at Yale tied in with the proposal that this nutrient existed. Later in 1947 this vitamin was for the first time successfully synthesized.

Fat-soluble and yellow, this antioxidant (reduces particular cancers) vitamin, retinol, is the animal form of vitamin A and is vital for healthy bones and good eyesight. Part of the chemical family of retinoids and ingested as a precursor form, hydrolysis of retinyl esters from animal sources like eggs and milk enables retinol to be formed. Plants, such as spinach and carrots, are sources of the retinol precursor carotenoids, which when ingested have their molecular structure cut down to produce retinaldehyde (retinal) and can then be chemically reversibly reduced to create retinoic acid.

All kinds of different isomers of retinol, retinal and retinoic acid exist in cis or trans molecular configurations, the structure determining bodily use.

Vitamin A is needed for the body's creation of the visual pigment, rhodopsin. Rhodopsin enables vision with low levels of light. This vitamin is also important so that epithelial cells function optimally. If someone is deficient in vitamin A cells that secrete mucus can get replaced by keratin producing cells resulting in xerosis. The production of gylcoproteins is facilitated by this A vitamin, in sufficient quantities. Liquefaction or corneal ulcers can occur from deficiencies. Fully functional epithelial tissues mean the barrier to infections is strong, vitamin A is also required in the upkeep of some types of immune system cells, e.g. myelocytes (e.g. macrophages, neutrophils) and lymphocytes (e.g. T-cells, B-cells). Iron metabolism has been witnessed from vitamin deficiency indicating this vitamins importance with haematopoiesis (red blood cell creation). Human growth hormone production has also been seen to require vitamin A.

Quantities of vitamin A tend to be measured in units known as retinal equivalents (RE), each RE equating to 0.001mg of retinal, 0.006mg of beta-carotene or 3.3 international units (IU) of vitamin A.

Vitamin E acts to protect vitamin A from structural modification in the intestine. Most of this vitamin is stored in your body, largely in your liver, released into your blood stream when required.

The recommended daily amount (RDA) of vitamin A for someone of 25 years is 900 micrograms/ day, or 3,000 IU.

The many retinoid forms of this A vitamin are used in medical and cosmetic industries for products aimed at cleaning or rejuvenating the skin. Retinyl palmitate, tretinoin, isotretinoin and retinoic acid all get used medicinally as a skin surface treatment for keratosis pilaris and acne. Severe or recurring acne is also sees the use of isotretinoin taken orally. Derivatives from vitamin A are also used in the cosmetics industry in products like skin creams to enhance short term collagen production in our skin, creating a more youthful appearance.

Rarely observed in developed countries, but frequent in developing nations, deficiency can lead to blindness due to the cornea of the eye becoming very dry, both the cornea and retina getting damaged. Alongside decreased bodily growth, fatalities can also result from weakened immune systems which lead to increased chances of catching diseases, and their severity becoming heightened.

Deficiency has been linked to iron deficiency in our diets. Too little iron results in poorer vitamin A uptake by our bodies. Large quantities of alcohol can also reduce vitamin A, alcoholic damage to the liver making it more susceptible to any excess/ toxic levels of this vitamin.

Toxic levels of vitamin A consumption for males aged 25 are thought to be around the 3000 micrograms per day, or 10,000 IU mark. Excessive quantities of the A vitamin can sometimes be fatal, through what is termed hypervitaminosis A. Osteoporosis can also be promoted, possible with far lower intakes. Carotenosis can result from excessive ingestion of carotenoids. Interestingly, past human trials have shown beta carotene to increase the chance of lung cancer in smokers whilst lowering that possibility in non-smokers. Birth defects are also more likely with too much of this vitamin during the early period of pregnancy.

Superb sources of vitamin A occur in a whole raft of foods. Minimums of 0.15mg of vitamin A or beta carotene for each 50-200g's can be found in kale, apricots, eggs, broccoli, mango, cantaloupe melon, pumpkin, sweet potatoes, collard greens, carrots, spinach and sweet peppers.

Moves, e.g. the Golden Rice Project, are being made at the moment to genetically modify rice, a staple diet in many developing countries, so that it contains larger amounts of beta carotene therefore addressing the large problems of vitamin A deficiency.