CAROTENOIDS

There are over 600 carotenoids in nature. Carotenoids generally contain a conjugated polyene structure which is efficient at absorbing light, and are the major yellow and red pigments in many fruits and vegetables. Beta-carotene (C40 H56) and alpha-carotene are responsible for the orange color of carrots, and lycopene for the red color of tomatoes; astaxanthin imparts a red or pink color to lobsters and salmon. The term "carotene" refers to carotenoids which contain only carbon and hydrogen (e.g. beta-carotene, alpha-carotene, lycopene), while the term "xanthophylls" refers to compounds which contain hydroxyl groups (lutein, zeaxanthin, beta-cryptoxanthin) or keto groups (canthaxanthin) or both (astaxanthin).

Major storage sites: Because the carotenoids are fat-soluble, they are found in fatty tissues in the body and are transported in blood by lipoproteins. The predominant carotenoids found in human tissues are beta-carotene, alpha-carotene, lycopene, lutein, zeaxanthin, and beta-cryptoxanthin; their relative abundance depends on dietary intake.

Functions: In animals some carotenoids (particularly beta-carotene) serve as dietary precursors to vitamin A, and many of them may function as fat-soluble antioxidants. Because they are present in much lower concentrations than is alpha-tocopherol, some questions have been raised about their physiological importance as antioxidants. However, increased consumption of foods rich in carotenoids is associated with decreased risk of some degenerative diseases, and there is some evidence also for their role in improving immune function. In plants they serve as antioxidants to protect the highly reactive photosystems and also act as accessory photopigments.

Deficiency: One study has found that carotenoid deficiency is associated with skin changes (including acne and dermatitis). These changes were detrimental but not life-threatening. This effect should be confirmed by additional studies before making dietary recommendations.

Diet recommendations: No formal diet recommendation for carotenoids has yet been established but some experts suggest intakes of 5 to 6 mg daily (about twice the average daily American intake). Individual dietary carotenoid consumption is quite variable.

Food sources: Carotenoids are biosynthesized only in plants and some bacteria, thus foods of plant origin are the primary dietary source for humans. Intestinal absorption can be poor, and depends on the presence of dietary fat. Mild cooking (steaming) increases bioavailability of carotenoids while overcooking can destroy some forms. The extent of conversion of provitamin A carotenoids to vitamin A seems to be variable but is less than 50%.

Toxicity: The carotenoids are remarkably devoid of toxicity, and serve as good nontoxic sources of vitamin A. Massive overconsumption of carotenoids can result in yellowing of the skin, especially of the hand and ears (xanthosis cutis), but has no adverse health effects. The color disappears within a week or so after reducing intake of carotene-rich foods.

Clinical uses: Canthaxanthin and beta-carotene have been used pharmacologically to treat erythropoietic porphyria, a disease characterized by extreme sensitivity of the skin to sunlight.

Recent research: Most recent interest has focused on antioxidant, anticancer, and immune-enhancing properties of carotenoids. Research is also continuing on food carotenoids as sources of dietary provitamin A.

For further information:

Britton, G. (1995) Structure and properties of carotenoids in relation to function. FASEB J. 9: 1551-1558

Krinsky, N.I. (1993) Actions of carotenoids in biological systems. Ann. Rev. Nutr. 13: 561-587.