The earliest clues to be discovered that led to the identification of Vitamin A and its deficiency date back as far as 1819, when a physiologist called Magendie found that malnourished dogs tended to develop corneal ulcers and their risk of death was increased.
In 1912, an English biochemist called Frederick Gowland Hopkins found unknown factors present in milk that were not fats, proteins or carbohydrates, but were required to aid growth in rats. Hopkins was later awarded the Nobel Prize (in 1929) for this discovery. In 1917, Elmer McCollum from the University of Wisconsin–Madison along with Lafayette Mendel and Thomas Burr Osborne from Yale University discovered one of these substances while researching the role of dietary fats. In 1918, these “accessory factors” were described as fat soluble and in 1920, they were referred to as vitamin A.
As carotenoids such as beta-carotene are converted to vitamin A in the body, researchers have attempted to establish how much of the carotenoids in the diet are equal to a certain amount of retinol. The reasoning behind this was that foods could then be compared to assess their different benefits. However, the accepted values of equivalences have changed over the years. For many years, a system was used where one international unit (UI) was classed as being equivalent to 0.3 μg of retinol, 0.6 μg of beta-carotene or 1.2 μg of other provitamin A carotenoids. In subsequent years, the retinol equivalent (RE) unit was introduced. Prior to 2001, a system was used where 1 RE was equivalent to 1 μg retinol, 2 μg β-carotene dissolved in oil, 6 μg β-carotene in normal food, or 12 μg of α-carotene, γ-carotene or β-cryptoxanthin in food.
However, in 2001, the US Institute of Medicine recommended the use of a new unit, referred to as the retinol activity equivalent (RAE). This was advised because studies demonstrated that the amount of provitamin-A carotenoids absorbed was only half of what it was believed to be previously. According to the new classification, one μg RAE is equal to 1 μg retinol, 2 μg of β-carotene in oil, 12 μg of beta-carotene found in the diet, or 24 μg of the provitamin-A carotenoids α-carotene, γ-carotene and β-cryptoxanthin.