Relative value of carotenoids as precursors of vitamin A
The carotenoid pigments are very conspicuous in nature, owing to their yellow to red hues, and occur without exception in photosynthetic tissue. They are responsible for the colour in some species of yeast, bacteria and fungi as well as many vegetables and fruits. Although animals are incapable of de novo synthesis, they are able to deposit the carotenoid pigment as absorbed or with some alteration of the basic structure. Thus, the red to yellow colour in the flesh, skin, shell or exoskeleton of salmon, lobster, crab, prawn, carp, flamingo, etc. is directly or indirectly diet-related. The intestinal cleavage of carotenoids to form vitamin A active retinoids represents the main contribution of the carotenoids to nutrition. The colour associated with foods such as vegetable, fruits, butter, egg yolks, salmon, etc. represents an aesthetics contribution made by these pigments.
An inspection of other papers in this symposium demonstrated that the retinoids have a wider application to health than has been traditionally thought. Likewise, recent findings support to suggestion for a wider, non-vitamin A role of the carotenes.
The structures of some common carotenoids are shown in Fig. 1 β-Carotene, while not the most abundant carotene is, nevertheless, very widespread in nature. Both halves of β-carotene are related to retinol, thus the compound possesses maximal provitamin A activity. γ-Carotene, with one ring, α-carotene with a 4’, 5’ double bond in the ring, and β-cryptoxanthin (xanthophyll) and β-carotene-5,6-epoxide, with substituted rings, contribute 50% of the activity of β-carotene.