Antioxidants recommended dietary intakes

Based primarily on immunological studies in model systems, dietary recommendations, though tentative, should include moderation of caloric and lipid intake. The recommended dietary intakes of trace elements and antioxidants, for selected populations such as pregnant women, should be reexamined with reference... 

Vitamin C occurs as L-ascorbic acid and dihydroascorbic acid in fruits, vegetables and potatoes, as well as in processed foods to which it has been added as an antioxidant. The only wholly undisputed function of vitamin C is the prevention of scurvy. Although this is the physiological rationale for the currently recommended intake levels, there is growing evidence that vitamin C may provide additional protective effects against other diseases including cancer, and the recommended dietary allowance (RDA) may be increased in the near future. Scurvy develops in adults whose habitual intake of vitamin C falls below 1 mg/d, and under experimental conditions 10 mg/d is sufficient to prevent or alleviate symptoms (Bartley et al., 1953). The RDA is 60 mg per day in the USA, but plasma levels of ascorbate do not achieve saturation until daily intakes reach around 100 mg (Bates et al., 1979). Most of the ascorbate in human diets is derived from natural sources, and consumers who eat five portions, or about 400-500 g, of fruits and vegetables per day could obtain as much as 200 mg of ascorbate. 

Numerous studies have demonstrated an inverse correlation between cancer rates and dietary intake of various antioxidants (foods rich in vitamins C and E, beta-carotene, lycopene, etc.) (Cl3, G5, E3, P2, H10, Y5). For example, a recent review (G8) showed a consistently lower risk of developing a variety of cancers in those with a higher consumption of tomatoes and tomato-based products (rich in lycopene, a potent carotenoid antioxidant), adding further support for the current recommendations to increase the consumption of fruits and vegetables. In addition... 

The polyphenols such as anthocyanidins might have a protective effect on atherosclerosis. This concept suggests that the Recommended Optimal Intake (ROI) is a more important factor for the antioxidation than the Recommended Dietary Allowance (RDA). Now, considering the absence of the appropriate methodology to identify anthocyanidins in the plasma, the analysis of the plasma antioxidant capacity might be a suitable index to define the optimal nutritional intake [95]. 

The major functions of selenium can be attributed to its antioxidative properties and its role in the regulation of thyroid hormone metabolism. Selenium is important as a nutritional factor, and the US Food and Nutrition Board s Committee on Dietary Allowances has, perhaps somewhat arbitrarily, proposed a recommended daily intake of 50-200 micrograms/day. 

The optimal dietary requirements of vitamin E for humans are not yet known, especially with the emergence of new paradigms regarding adequate levels of dietary micronutrients (Chalem, 1999). Recommendations in the United States and Canada have been reevaluated, and a new concept of Dietary Reference Intake (DRI, 2000) was issued for vitamin E and other antioxidants. The DRI recommendation should prevent specific deficiency disorders, support health in general ways and minimize the risk of toxicity, which carries more tasks than the previous recommendations (DRI, 2000). Accordingly, the recommendations for intakes were set to higher levels than previously. Estimated Average Requirements (EAR) for adults, both men and women, were set to 12 mg a-tocopherol/day, RDA to 15 mg/day and Tolerable Upper Intake Level (UL) to 1000 mg/day. Moreover, the EAR and RDA are based only on the 2R-stereoisomeric forms of a-tocopherol, because the other vitamers... 

The dietary intake of polyunsaturated fats and minerals is clearly not a simple problenL Dietary recommendations for polyunsaturated fats and especially the highly oxidizable n-3 polyunsaturated lipids must consider their susceptibility to oxidation as a significant risk factor. When the interrelationships between the metabolic effects of different polyunsaturated fatty acids are better understood, an appropriate target may be to achieve a balanced eicosanoid metabolism. It would seem imprudent to make dietary reconunendations to the public before the mechanisms of polyunsaturated hpid nutrition, in vivo activity of antioxidants, and in vivo lipid peroxidation are better understood. 

Low levels of vitamin Bg may cause hyperhomocysteinaemia since vitamin Bg acts as a cofactor for tra .s-sulfation of cysteine (Siri et al. 1998). There is evidence that an elevated homocysteine level is a risk factor for heart disease and stroke. Endo et al. (2006) reported that vitamin Bg deficiency induced the oxidant stress which accelerates atherosclerosis. They also highlighted that the antioxidant activity of vitamin Bg may suppress the homo-cysteine-induced atherosclerosis. Vitamin Bg levels may be important in the prevention of coronary heart disease intake of vitamin Bg above the current recommended dietary allowance has been shown to be instrumental in the primary prevention of coronary heart disease among women (Rimm et al. 1998). 

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. 

National Academy of Sciences (USA) [11] has recommended a daily intake of at least five servings of fruit and vegetables, particularly citric fruits, carotene-rich fruits and vegetables, and cmciferous vegetables to reduce the risk of both cancer and heart disease scientific research has demonstrated that the presence of dietary fibers and polyphenols in fruits and vegetables reduces the risk of cancer because of their antioxidant and anti-inflammatory properties. 

Vitamin E is the most potent fat-soluble antioxidant in human plasma. Although vitamin E was first discovered in 1922, its metabolic function remains an enigma. There are eight different molecular forms with vitamin E antioxidant activity, yet the body preferentially retains a-tocopherol. This preference for a-tocopherol has led the Eood and Nutrition Board in its 2000 Dietary Reference Intakes (DRIs) for vitamin E to recommend that only a-tocopherol, not the other forms, meets human requirements for vitamin E. Moreover, only a-tocopherol is recognized by the hepatic a-tocopherol transfer protein (a-lT P). This protein regulates plasma a-tocopherol concentrations and genetic abnormalities in the protein (or its absence) leads to vitamin E deficiency in humans. 

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