The most important antioxidant in cell membranes is a-tocopherol, the major member of the vitamin E family. This molecule acts as a 'chain-breaking antioxidant', intercepting lipid peroxyl radicals and so terminating lipid-peroxidation chain reactions. Vitamin E is found in many dietary fats and oils, especially those containing PUFA (Table 9.2). Thus, the dietary intake of vitamin E is related to the intake of PUFA. Intakes among adults in the UK vary between 3.5 and 19.5 (median 9.3) mg a-tocopherol equivalents day-1 for men and
Table 9.2. Dietary sources of antioxidant vitamins.
Vitamin C Citrus fruits, blackcurrants, kiwi fruit, strawberries
Red peppers, broccoli, Brussels sprouts
Vitamin E Whole grains, vegetable oils, wheat germ, eggs Carotenoids p-carotene Carrots, broccoli, watercress, spinach, apricots
Lycopene Tomatoes and processed tomato products (sauce and paste)
Lutein Peas, spinach, broccoli and dark green leafy vegetables p-cryptoxanthin Mandarins, satsumas, apricots, orange peppers between 2.5 and 15.2 (median 6.7) mg day-1 a-tocopherol equivalents for women (Department of Health, 1991). Another group of lipid-soluble compounds that can act as antioxidants are the carotenoids, such as p-carotene, lycopene and lutein, found in highly pigmented fruits and vegetables (Mangels et al., 1993). The major water-soluble free radical scavenger is ascorbic acid (vitamin C), which also plays a role in 'sparing' vitamin E, by regenerating a-tocopherol from the oxidized tocopheroxyl radical (Bendich et al., 1986). The estimated average requirement for vitamin C in adults in the UK is 25 mg day-1 (Department of Health, 1991). More recently, attention has also focused on the antioxidant properties of plant polyphenols, found in tea and red wines (Rice-Evans, 1995), but considerably more information on the absorption, metabolism and excretion of these compounds in humans is required before their relative contribution to preventing oxidative damage can be assessed.
A balanced diet, containing at least five or six varied portions of fruits and vegetables per day, should provide an adequate supply of antioxidants for healthy individuals. Concerns regarding the taking of supplements centre around the possibilities that certain compounds might have a toxic effect if taken in doses significantly higher than can be obtained from a healthy diet and that a reliance on supplements will lead to a reduced consumption of fresh fruit and vegetables, which probably contain a multitude of compounds whose health benefits we have yet to appreciate. However, in elderly individuals, whose diet might be restricted (e.g. by loss of appetite, dental conditions) and where absorption of nutrients is impaired, there might be a case for supplementation with certain nutrients. The case is probably strongest for vitamin E, because it is impossible to obtain high intakes of this nutrient without consuming a high-fat diet. Table 9.2 identifies dietary sources of anti-oxidant vitamins.
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