NUTRIENT FUNCTIONS AND THE INDICATORS USED TO ESTIMATE REQUIREMENTS FOR VITAMIN C, VITAMIN E, SELENIUM, AND THE CAROTENOIDS

Vitamin C (ascorbic acid) functions physiologically as a water-soluble antioxidant by virtue of its high reducing power. To provide antioxidant protection, the Recommended Dietary Allowance (RDA) for adults for vitamin C is set at 75 mg/day for females and 90 mg/day for males. This intake should maintain near maximal neutrophil ascorbate concentrations with little urinary excretion. Because smokers suffer increased oxidative stress and metabolic turnover of vitamin C, their recommended intake is increased by 35 mg/day.

Vitamin E is thought to function primarily as a chain-breaking antioxidant that prevents the propagation of lipid peroxidation. To estimate the requirement, data were examined on induced vitamin E deficiency in humans and the intake that correlated with in vitro hydrogen peroxide-induced hemolysis and plasma α-tocopherol concentrations. In addition, vitamin E acts as an in vivo antioxidant, maintaining normal physiological function in humans. The RDA for both men and women is 15 mg/day of α-tocopherol. Other naturally occurring forms of vitamin E (β-, γ-, δ-tocopherol and the tocotrienols) do not meet the vitamin E requirement because they are not converted to α-tocopherol in humans and are recognized poorly by the α-tocopherol transfer protein. In establishing recommended intakes α-tocopherol is defined as RRR-α-tocopherol, the only form of α-tocopherol that occurs naturally in food, and the 2R-stereoisomeric forms of α-tocopherol (RRR-, RSR-, RRS-, and RSS-α-tocopherol) that occur in fortified foods and supplements.

Selenium functions through selenoproteins, several of which are oxidant defense enzymes. The method used to estimate the requirement for selenium relates to the intake needed to maximize the activity of the plasma selenoprotein glutathione peroxidase, an oxidant defense enzyme. The RDA for both men and women is 55 µg/day. It is not clear if the diseases associated with selenium deficiencies, Keshan disease or Kashin-Beck disease, are due to oxidative stress. The selenium in several selenoproteins has a biochemical role in oxidant defense, and as such plays a role as a dietary antioxidant.

β-Carotene and other provitamin A carotenoids function as a source of vitamin A and, due to this provitamin A activity, can prevent vitamin A deficiency. Because specific functions beyond this role have not yet been sufficiently identified, no Dietary Reference Intakes (DRIs)



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