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 nmo 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 vegetables, fruits, butter, egg yolks, salmon, etc. represents an aesthetic contribution made by these pigments.