After the discovery of
vitamin A in 1913, the yellow pigments of fruits and vegetables were soon implicated as compounds with similar nutritional effects.
beta-Carotene was shown to be converted into
vitamin A by Moore in 1929, and the chemical structures of both
vitamin A and
beta-carotene were determined two years later. Thus, the sole function of
beta-carotene in human health was considered to be its conversion into
vitamin A. On the basis of observational epidemiologic studies, conducted in the mid-1970s, however,
carotenoids were implicated as
protective agents, first against
lung cancer and then against a variety of other
chronic diseases. Intervention trials employing
beta-carotene, however, either have shown no preventive effect or indeed, in two cases, have enhanced the incidence of
lung cancer in middle-aged male smokers and
asbestos workers. The possible protective action of
carotenoids can be attributed to their properties as
singlet oxygen quenchers and as
antioxidants, whereas their
cancer-enhancing actions in lung can be ascribed to the prooxidant action of
carotenoid free radicals in damaged cells. Apart from
chronic diseases,
beta-carotene has shown significant therapeutic value in individuals suffering from
photosensitivity disorders and provides temporary relief to persons afflicted with
leukoplakia. A part from a medical context, the colored
carotenoids found in many living organisms and in many foods delight both the eye and the palate. Thus, human health and the enjoyment of life are greatly benefited by the presence of these interesting pigments in nature, whether or not they ultimately prove to have more specific protective effects against
chronic diseases.