Carotenoids

Carotenoids are a class of natural pigments found in plants, algae, and some bacteria. They are lipophilic tetraterpenoids built from eight isoprene units (C40). Carotenoids are divided into carotenes, which are purely hydrocarbons, and xanthophylls, which contain oxygen. They contribute red, orange, and yellow colors to many fruits, vegetables, and flowers.

In plants, carotenoids serve roles in photosynthesis as accessory pigments that broaden light absorption and help harvest energy. They also protect the photosynthetic apparatus from photooxidative damage by quenching excited singlet oxygen and dissipating excess energy. Biosynthesis occurs via the isoprenoid pathway, with steps forming phytoene, desaturation to lycopene, and cyclization to beta- or alpha-carotene; subsequent oxygenation yields various xanthophylls.

Nutritionally, some carotenoids are provitamin A compounds, notably beta-carotene, alpha-carotene, and beta-cryptoxanthin, which can be converted to retinal in the body. Not all carotenoids have vitamin A activity. Dietary sources include carrots, sweet potatoes, leafy greens, corn, peppers, and tropical fruits. Carotenoids also act as antioxidants, and dietary intake has been studied for associations with disease risk, though results vary.

Absorption is enhanced by dietary fat, and carotenoids are carried in micelles and ultimately in lipoproteins after intestinal uptake. They are stored mainly in the liver and adipose tissue. Carotenoids can be sensitive to heat, light, and oxygen, undergoing isomerization that can affect activity. Analytical methods such as high-performance liquid chromatography are commonly used to profile individual carotenoids.