Submitted to: Encyclopedia of Human Nutrition
Publication Type: Book / Chapter
Publication Acceptance Date: February 28, 2005
Publication Date: August 1, 2005
Citation: Ishida, B.K., Bartley, G.E. 2005. Carotenoids: Chemistry, Sources, and Physiology. Encyclopedia of Human Nutrition. 2nd Edition. Vol 1. Chapter C. p. 330-338. Interpretive Summary: Carotenoids are made by plants, bacteria and fungi. They are compounds that give fruits and vegetables like tomatoes, carrots, and papayas their red, orange, and yellow colors. Lycopene is the red pigment of ripe tomato fruit, and beta-carotene makes carrots orange. Carotenoids are soluble in fat, sensitive to high temperatures, and easily degraded in the presence of acid, oxygen, and light. They are powerful antioxidants that can protect tissues against damage from free radicals produced by different reactions in tissues and by light. Humans cannot synthesize carotenoids and must get them mostly from eating fruits and vegetables. Green leafy vegetables also contain carotenoids, but their colors are masked by the green color of chlorophyll. Carotenoid content varies with growing conditions and maturity and ripeness. It can decrease with storage and processing. They are absorbed in the intestine along with fat (in micelles) and secreted back into the blood. In blood, LDL-cholesterol carries three-quarters of the lycopene and b-carotene, and lutein and zeaxanthin are carried equally by HDL- and LDL-cholesterol. Carotenoids are stored in fat, liver, lung, breast cervix, skin, and the eye, where they are believed to protect against damage from free radicals leading to various cancers, cardiovascular diseases, and skin damage from light. Lutein and zeaxanthin are high in the middle of the retina of the eye, where they can shield photoreceptors from light and protect against age-related macular degeneration. In addition, some carotenoids form Vitamin A and are important in vision. They are also needed for reproduction, and regulating growth and development.
Technical Abstract: Carotenoids are synthesized in plants, bacteria, and fungi. The first carotenoid in the pathway phytoene is desaturated, producing eleven conjugated double bonds in lycopene, the red pigment of ripe tomato fruit. Carotenoids are hydrophobic, sensitive to high temperatures, and easily degraded in the presence of acid, oxygen, and light. Their conjugated double bond system absorbs energy from light and free radicals generated from oxidative reactions, making them powerful antioxidants that can protect tissues against damage. Humans cannot synthesize carotenoids and must obtain them mostly from red, orange, and yellow fruits and vegetables and green leafy vegetables with carotenoids masked by chlorophyll. Carotenoid content varies with growing conditions and maturity and can decrease with storage and processing. Intestinal mucosal cells absorb carotenoids in lipid micelles, passing into lymph and blood. They are taken up by the liver and secreted back into the blood combined with VLDL. In plasma, 75% of the lycopene and b-carotene combine with LDL; lutein and zeaxanthin are equally distributed between HDL and LDL. Carotenoids in adipose tissue, liver, lung, breast cervix, skin, and the eye are believed to protect against oxidative damage from free radicals that lead to various cancers, cardiovascular diseases, and skin damage from light. The macula lutea has a xanthophyll-binding membrane protein. Its central area is high in zeaxanthin and lutein, which can shield photoreceptors from blue light and protect against age-related macular degeneration. In addition, some carotenoids are precursors to Vitamin A and are important in vision. Derivatives also are essential for reproduction, growth regulation, and cell differentiation.