Beyond its significance as provitamin A, specific functions of p-carotenes proper have been discovered during the past decades. Therefore, p-carotenes can be considered essential for humans. When p-carotenes are absorbed they can either be split into retinal or absorbed as is. In the latter case they have "anti-oxidant" functions in the cell. In all human cells—particularly in skin— oxygen radicals and other related free radicals form continuously, under the influence of light (A). These free radicals are characterized by extremely high reactivity: they have an extremely short half-life (10-11 sec), after which they react with other substances, forming new radicals. This triggers chain reactions, which may damage essential cell components. The damage may cause acutely reduced cell function due to membrane damage, inactivation of enzymes, destruction of receptors, or lowered rates of protein biosynthesis. DNA damage may reduce the cell's lifespan. The resulting defect may also be passed on during cell division and thereby potentially affect entire tissues.
Physiological mechanisms interrupting such chain reactions are, therefore, extremely important. Mainly in skin, exposure to UV light causes the formation of active singlet oxygen (jO2), which returns to its normal state by reacting with a p-carotene (1). The resulting p-carotene radical regenerates by releasing heat energy. This inactiva-tion of radicals is called "quenching." UV light can also foster direct formation of radicals, resulting in lipid peroxidation of membrane fatty acids (RH) (2). Thus, the chain reactions triggered result in rapid, further peroxidation of membrane lipids. It can be inhibited by direct chemical reaction with p-caro-tenes, which, however, are used up in the process. Radical formation is enhanced by many factors: besides UV light, mainly by O2-dependent reactions in mitochondria and microsomes, arachidonic acid metabolism, leukocyte and macrophage activity, many enzymatic reactions, certain sulfur compounds, reduced iron complexes, and exogenous toxins. This indicates that certain risk groups, such as smokers, might have an increased need for anti-oxidants including p-carotenes. In spite of this, heavy smokers should avoid high doses (>10 mg) from supplements.
Most plants contain p-carotenes in varying concentrations since they are the most important carotenoids (B). Depending on fruit and vegetable intake, p-carotenes play a more or less important role as a source of vitamin A: in the U.S., half of the required vitamin A comes from p-carotenes, in England just a third. To date, no official requirements have been set for p-carotenes.
For an optimal antioxidant effect as well as for cancer prevention ~15-50 mg/d of carotenoids are recommended. High intakes of carotenoid-rich vegetables have been shown to correlate with the lowest cancer morbidity.
- A. The Skin-Protective Effect of ß-Carotenes -
Oxidation of a substrate
Carbohydrates Nucleic acid
Peroxidation e. g., oxidation of SH-enzymes
Cross-linking hydroxylation of bases
—► Consequences Membrane damage Inactivation
Inhibition of protein and nucleotide biosynthesis
- B. Occurrence and Daily Requirement-
The daily requirement of 2 -4 mg p-carotene is contained in:
800 - 1000 g broccoli
300 - 600 g tomatoes. Brussels sprouts
kale, spinach 50 - 100 g carrots — 50 - 100 g red bell peppers-
150 - 300 g escarole-
800 - 1000 g broccoli
400 - 800 g nectarines, sour cherries
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