Dietary Antioxidants

The human endogenous antioxidant system is impressive but incomplete. Regular and adequate dietary intakes of (largely) plant-based antioxidants, most notably vitamin C, vitamin E, and folic acid, are needed. Fresh fruits and vegetables are rich in antioxidants (Figure 6), and epidemiological 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 antioxidant-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 allcause mortality.

To date, research on dietary antioxidant micronutrients has concentrated mainly on vitamin C and vitamin E. This is likely to be because humans have an undoubted requirement for these antioxidants, 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 carotenoids, 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 (vegetarian), and shellfish) and selenium (found especially in beef, cereals, nuts, and fish) are incorporated into the antioxidant enzymes SOD and glutathione peroxidase,
and the elements are themselves sometimes referred to as antioxidants. The levels of ascorbic acid, alfa-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, Beta-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 fattyacid and phytochemical mix.
6. The iron status of the body, environmental conditions, and lifestyle undoubtedly affect antioxidant demand.
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.

 Source  : Guide to Nutrition Suplement
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