Dietary Antioxidants

The human endogenous antioxidant system is impressive but incomplete. Regular and adequate dietary intakes of (largely) plant-based antioxi-dants, most notably vitamin C, vitamin E, and folic acid, are needed. Fresh fruits and vegetables are rich in antioxidants (Figure 6), and epidemio-logical evidence of protection by diets rich in fruits and vegetables is strong. To decrease the risk of cancer of various sites, five or more servings per day of fruits and vegetables are recommended. However, it is not known whether it is one, some, or all antioxidant(s) that are the key protective agents in these foods. Furthermore, it may be that antioxidants are simple co-travellers with other, as yet unidentified, components of antioxi-dant-rich foods. Perhaps antioxidants are not 'magic bullets' but rather 'magic markers' of protective elements. Nonetheless, the US recommended daily intakes (RDis) for vitamin C and vitamin E were increased in 2000 in recognition of the strong evidence that regular high intakes of these antioxidant vitamins are associated with a decreased risk of chronic disease and with lower all-cause mortality.

To date, research on dietary antioxidant micro-nutrients has concentrated mainly on vitamin C and vitamin E. This is likely to be because humans have an undoubted requirement for these antiox-idants, which we cannot synthesize and must obtain in regular adequate amounts from food. However, there are a plethora of other dietary antioxidants. Some or all of the thousands of car-otenoids, flavonoids, and phenolics found in plant-based foods, herbs, and beverages, such as teas and wines, may also be important for human health, although there are currently no RDis for these. Furthermore, while there are recommended intakes for vitamin C, vitamin E, and folic acid, these vary among countries, and there is currently no agreement as regards the 'optimal' intake for health. in addition, there is growing evidence that other dietary constituents with antioxidant properties, such as quercetin and catechins (found in teas, wines, apples, and onions), lycopene, lutein, and zeaxanthin (found in tomatoes, spinach, and herbs) contribute to human health. Zinc (found especially in lamb, leafy and root vegetables, and shellfish) and selenium (found especially in beef, cereals, nuts, and fish) are incorporated into the antioxidant enzymes SOD and glutathione perox-idase, and the elements are themselves sometimes referred to as antioxidants.

The levels of ascorbic acid, a-tocopherol, folic acid, carotenoids, and flavonoids within the body are maintained by dietary intake. While the role and importance of dietary antioxidants are currently unclear, antioxidant defense can be modulated by increasing or decreasing the intake of foods containing these antioxidants. There are a number of reasons for recommending dietary changes in preference to supplementation for achieving increased antioxidant status, as follows.

1. it is not clear which antioxidants confer protection.

2. The hierarchy of protection may vary depending on body conditions.

3. A cooperative mix of antioxidants is likely to be more effective than an increased intake of one antioxidant.

4. Antioxidants, including vitamin A, ^-carotene, vitamin C, selenium, and copper, can be harmful in large doses or under certain circumstances.

5. Antioxidant status is likely to be affected by the overall composition of the diet, e.g., the fatty-acid and phytochemical mix.

6. The iron status of the body, environmental conditions, and lifestyle undoubtedly affect antioxi-dant demand.

Repair r i

DNA repair enzymes (Mg, Zn, folate, vitamin B12, Amino-acid and riboflavin, niacin, and MSR) sugar products

Mitochondrial dysfunction

Mercapturic acids

GR (riboflavin)

Preventive and scavenging (also vitamin A)

DNA and protein damage

Haptoglobin Hemopexin Transferrin Caeruloplasmin (Cu) Metallothionein (Zn)

Dienes, epoxides, carbonyls

Dienes, epoxides, carbonyls

DNA and protein damage

Haptoglobin Hemopexin Transferrin Caeruloplasmin (Cu) Metallothionein (Zn)

'Activated' carotenoids

Mercapturic acids

GR (riboflavin)

Hexose monophosphate shunt (Mg)

GSSG

PUFA alcohols

'Activated' carotenoids

Energy

Energy

PUFA alcohols y

Lutein

Astaxanthin

Other carotenoids

Phytic acid

Flavonoids

Isoflavonoids

Other phenols

Organosulfur compounds

S-AA

Fe Cu

Constituents of herbs and spices , transition-metal-catalyzed oxidant damage to biomolecules; ?, biological relevance.

Hexose monophosphate shunt (Mg)

Vitamin A

Vitamin E

Riboflavin Nicotinic acid Folate Vitamin B12

GSSG

Chain-breaking

(Alos urate, SH-proteins, billirubin)

Ubiquinol a-lipoic acid Oestrogens

Figure 5 The integrated antioxidant defense system comprises both endogenous and dietary-derived antioxidants. GR, glutathione reductase (EC1.6.4.2); GSH, reduced glutathione; GSH-Px, glutathione peroxidase (EC1.11.1.9); GSSG, oxidized glutathione; GST, glutathione-S-transferase (EC 2.5.1.18); MSR, methionine sulfoxide reductase (EC1.8.4.5); NADPH and, NADP, are, respectively, the reduced and oxidized forms of the co-factor nicotinamide adenine dinucleotide phosphate; PUFA, polyunsaturated fatty acid; S-AA, sulfur amino-acids; SH, sulfydryl; SOD, superoxide dismutase (EC1.15.1.1).

Potato Carrot Tomato Broccoli Mange-tout Choy Sum Pineapple Banana Grape Apple Kiwi Orange Strawberry

0 5 000 10000 15000 20000

Antioxidant capacity (as the FRAP value); ^ol 100 g-1 fresh weight wet

Figure 6 Antioxidant capacity varies among different fruits and vegetables. FRAP, Ferric Reducing/Anti-oxidant Power.

Antioxidant defense, therefore, is likely to be optimized through a balanced intake of a variety of antioxidants from natural sources rather than by pharmacological doses of one or a few antioxidants.

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