ABSTRACT
Vitamin A is the generic term for all derivatives of β-ionone (other than the carotenoids) that possess the biological activity of all-trans-retinol, while the term retinoids refers to both synthetic and natural forms of retinol regardless of biological activity (Figure 5.1) (Schumann et al., 1997). There are over 1000 synthetic retinoids, but only retinol, retinal and retinyl esters fulfill all vitamin A activities (Biesalski, 1997). Dietary vitamin A is generally obtained either from animal sources in the form of retinyl esters or from plant sources in the
form of the precursor β-carotene (Schumann et al., 1997). β-carotene belongs to a class of naturally occurring carotenoids (over 50 compounds) which, upon metabolic breakdown, exhibit vitamin A activity. However, there also are carotenoids that upon breakdown do not possess vitamin A activity and are termed non-provitamin A carotenoids (Figure 5.2). The ingestion of vitamin supplements can provide an additional source of vitamin A and the absorption of retinoids from these sources is more rapid than from dietary sources (Schumann et al., 1997). The amount of vitamin A ingested by humans or administered to laboratory animals generally is reported in either international units (IU) or retinol equivalents (RE). One international unit of vitamin A is defined as 0.3 µg of all-trans-retinol. Retinol equivalent is often used to obtain a single value that represents all dietary sources of carotenoids and vitamin A ingested. Using this nomenclature, 1 µg of all-trans-retinol is equivalent to 1 RE and 1 µg of all-trans-retinol is assumed to be biologically equivalent to 6 µg of β-carotene (IUPAC-IUB, 1982). Retinyl esters are considered to be as bioavailable as alltrans-retinol and therefore, after accounting for the difference in molecular weight, 1.83 µg of retinyl palmitate is equivalent to 1 µg RE.
