Vitamin A refers to any compound possessing the biological activity of retinol (1). The term ‘retinoids’ includes both the naturally occurring forms of vitamin A as well as the many synthetic analogues of retinol with or without biological activity (2).All-trans retinol, the parent retinoid compound, is a primary alcohol. In most animal tissues, the predominant retinoid is retinyl palmitate but other fatty acid esters, such as retinyl oleate and retinyl stearate, are also found. Most of these compounds also occur in the all-trans configuration. Furthermore, the 11-cis aldehyde form, 11-cis retinal, is present in the retina of the eye, and several acid forms such as all-trans retinoic acid, 13-cis retinoic acid, and 9-cis retinoic acid can be present in many tissues (3, 4).Vitamin A exists in the plant world only in the form of precursor compounds such as β-carotene. β-carotene is one of 50 to 60 members of a large class of naturally occurring compounds called carotenoids that have vitamin A activity. In all cases, a requirement for vitamin A activity is that at least one intact molecule of retinol or retinoic acid can be obtained from the carotenoid.Recommendations on vitamin A include both vitamin A activity as retinol and some provitamin A carotenoids. The term ‘retinol equivalents’(RE) is used to convert all sources of preformed retinol and provitamin Acarotenoids in the diet into a single unit. The conversion factors for therelevant carotenoids are based on human studies that showed that theabsorption of a single dose of 45 mg to 39 mg β-carotene ranges from9% to 22% (5). In addition, a number of factors such as protein-energymalnutrition, zinc deficiency, dietary fat, alcohol consumption, infections,and the degree of food processing and food matrix can affect the bioavailabilityand bioconversion of retinol and carotenoids (3–5). Based on theseand similar studies, the US Institute of Medicine, IoM (5) introduced theconcept ‘retinol activity equivalents’ (RAE). 1 RAE is equal to:• 1 μg of dietary or supplemental preformed vitamin A (i.e. retinol)• 2 μg of supplemental β-carotene• 12 μg of dietary β-carotene• 24 μg of other dietary provitamin A carotenoids (e.g. α-carotene andβ-cryptoxanthin)The same factors are used in the NNR, but the term ‘retinol equivalents’(RE) is maintained.