Vitamin A occurs as retinyl esters in foods of animal origin and in the form of provitamin A carotenoids in plant foods. Some carotenoids found in colourful fruits and vegetables are called provitamin A; they are metabolized in the body to vitamin A.
Among the carotenoids, β-carotene, a retinol dimer, has the most significant provitamin A activity in human nutrition, since its concentration in food and feed ingredients, particularly of leaf origin, greatly exceeds that of the other vitamin A active compounds. Other provitamin A carotenoids, such as α-carotene and β-cryptoxanthin, are half as active as β-carotene.
The β-carotene molecule contains two β-ionic rings. Theoretically the cleavage of that chain at –C15 = C15′– position provides two retinol molecules.
Carotenoids play a prominent role in protecting bodily cells and thereby act as powerful antioxidants. It has been observed that carotenoid pigments in all photosynthetic organisms, bacteria, algae, and higher plants, play an important role in protecting these organisms against the seriously damaging effects of photooxidation by their own endogenous photo-sensitizer, chlorophyll.
Due to its high bioactivity, β-carotene is also widely used in medicine. Among the numerous functions of β-carotene in the human body, the important one is related to provitamin A supply, further affecting embryonic development, correct growth, and sight. It is considered as an inhibitor of some genes; moreover, it exhibits anticancer and antioxidant properties.
Vitamin A and β-carotene
