Carotenoid

Carotenoids are a class of naturally occurring, fat-soluble pigments with long chains of conjugated double bonds. They are widely distributed in plants, algae, bacteria, and fungi and play essential roles in photosynthesis and photoprotection. Their vibrant red, orange, and yellow hues lend color to many fruits and vegetables and to animal tissues through diet.

Carotenoids are divided into two major groups: carotenes, which are purely hydrocarbons such as beta-carotene, alpha-carotene, and lycopene; and xanthophylls, which contain oxygen such as lutein, zeaxanthin, violaxanthin, and beta-cryptoxanthin. Some carotenoids, notably beta-carotene, alpha-carotene, and beta-cryptoxanthin, are provitamin A carotenoids because they can be converted to retinol (vitamin A) in the body.

In plants, carotenoids participate in light harvesting and protect chloroplasts by quenching reactive oxygen species and dissipating excess energy. They are synthesized through the plastid-localized isoprenoid pathway, starting from geranylgeranyl pyrophosphate.

In humans and other animals, carotenoids are obtained from the diet. Absorption occurs with dietary fat, and carotenoids are incorporated into micelles and transported by chylomicrons. Provitamin A carotenoids can be enzymatically converted to retinal and then to retinol, contributing to vitamin A status; non-provital carotenoids act mainly as antioxidants and may influence vision and skin health. Plasma carotenoids serve as biomarkers of fruit and vegetable intake in research.

Dietary sources include carrots, leafy greens, corn, squash, tomatoes, and many colorful fruits. Carotenoids are also used as natural colorants in foods and cosmetics; stability varies with light, heat, and oxygen. High-dose beta-carotene supplements have shown adverse effects in certain populations, so intake should follow dietary guidelines.