The structure and spectral characteristics of nearly 750 carotenoids present in our natural surroundings have been identified. About 50 of these carotenoids are present in the human diet but mostly in small amounts. The major carotenoids in human blood are lutein, β-cryptoxanthin, lycopene, and α-and β-carotene. Their structures are shown in Figure 10.1, and isocratic, reverse-phase chromatography has been shown to separate these carotenoids by many researchers. The carotenoids have at least two major functions in the human body: to provide a source of vitamin A and to act as antioxidants. Of the principal carotenoids in serum, only α-carotene, β-carotene, α-cryptoxanthin, and β-cryptoxanthin are provitamin A carotenoids, that is, they contain at least one β-ionone ring linked to a hydrocarbon chain with a conjugated double bond structure, which can be cleaved by the oxidation of the 15,15′ double bond to form β-apo-15-carotenal. β-apo-15-carotenal is subsequently reduced to retinol. Most carotenoids are not vitamin A precursors. Although many carotenoids possess a β-ionone ring, they contain a variety of substituents that commonly include oxygen. These oxygen-containing carotenoids are known as xanthophylls. A few of these, such as β-cryptoxanthin (Figure 10.1), are vitamin A precursors, but most are not. All carotenoids have antioxidant capacity associated with their conjugated carbon structure. Typically, the longer the polyene chain, the greater the ability of a carotenoid to deactivate reactive oxygen species, such as oxy radicals and singlet oxygen.