Update on the Biological Characteristics of the Antioxidant Micronutrients: Vitamin C, Vitamin E, and the Carotenoids

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Abstract

Under normal circumstances, free radicals that are produced through biological processes and in response to exogenous stimuli are controlled by various enzymes and antioxidants in the body. Laboratory evidence suggests that oxidative stress, which occurs when free radical formation exceeds the ability to protect against them, may form the biological basis of several acute medical problems, such as tissue injury after trauma, and chronic conditions, such as atherosclerosis and cancer. A potential role for the antioxidant micronutrients (vitamin C, vitamin E, and the carotenoids) in modifying the risk for conditions that may result from oxidative stress has stimulated intense research efforts, increased interest in micronutrient supplements, and heightened consumer interest in these compounds. Much remains to be learned, however, about the bioavailability, tissue uptake, metabolism, and biological activities of these micronutrients. These biological characteristics will ultimately determine their clinical usefulness in modulating oxidative stress. Also, whether the antioxidant mechanism explains their relationship with risk for acute and chronic disease in epidemiologic studies remains to be determined. Increased knowledge in this area of nutrition science will have an impact on both clinical dietetics practice and public health nutrition guidelines. J Am Diet Assoc. 1996; 96:693–702.

Section snippets

The Antioxidant Hypothesis

Interest in the antioxidants stems from the growing understanding that a substantial number of free radicals are generated as an ordinary process of oxygen metabolism and that a wide range of conditions are associated with either an excess generation of free radicals or an inadequate antioxidant defense system (5). Exogenous sources, such as radiation, air pollution, and smoking, can also generate free radicals. Free radicals are reactive chemical species that contain one or more unpaired

Chemistry and Analysis

Vitamin C is the generic descriptor for all compounds exhibiting qualitatively the biological activity of ascorbic acid, which includes the oxidized form, dehydroascorbic acid (DHAA). Isoascorbic (erythorbic) acid, which is used widely as a food preservative, possesses antioxidant properties similar to L-ascorbic acid but lacks antiscorbutic activity (29). The newer high-performance liquid chromatography (HPLC) methods for quantifying vitamin C in biological specimens, foods, and

Chemistry and Analysis

Vitamin E is the generic term used to describe a group of at least eight compounds that exhibit the biological activity of a-tocopherol. The group comprises α-, β-, γ-, and δ-tocopherol and α-, β-, γ-, and δ-tocotrienol. All of these compounds occur as a variety of isomers; for example, eight stereoisomers of α-tocopherol result from the synthesis of this compound. The natural isomeric form is RRR-α-tocopheiol (also known as d-α-tocopherol), and the synthetic form is described as all racemic or

Chemistry and Analysis

Carotenoids are pigments that are found in plants and microorganisms but are not synthesized in animals. Nearly 600 of these compounds have been identified (see Fig. 2) (2), (78), and less than 10% of these compounds can be metabolized to retinol and function as vitamin A precursors in mammals. Human plasma contains only a fraction of the total carotenoids that have been identified in the food supply (79), The predominant plasma carotenoids are lutein, β-cryptoxanthin, lyco-pene, oc-carotene,

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