Dietary Reference Intakes: Proposed Definition and Plan for Review of Dietary Antioxidants and Related Compounds (1998)

Dietary Antioxidants and Related Compounds also identifies the nutrients and food components that it will evaluate in detail, assuming adequate scientific data are available, to establish Dietary Reference Intakes (DRIs) for these selected substances.

DRIs are reference values that are quantitative estimates of nutrient intakes to be used for planning and assessing diets for healthy people. They include Recommended Dietary Allowances (RDAs) as goals for intake by individuals, but also include three other reference values. These include the Estimated Average Requirement (EAR), Adequate Intake (AI), and the Tolerable Upper Level (UL).

In its second report, the panel will provide a comprehensive set of DRIs for those nutrients and food components selected, in the current report, for detailed examination. This selection is not based on the panel's definition of a dietary antioxidant. Beta-carotene, vitamin C, and vitamin E are included as requested by the federal steering committee for Dietary Reference Intakes. Similarly, other dietary compounds will be included in the review that may be related in function, but may not necessarily act as antioxidants. Therefore, the nutritional recommendations that will be presented in the second report for some of these dietary components may not be determined by or related to their possible action as antioxidants.

In order to elicit other perspectives about the attributes of dietary antioxidants, the Panel on Dietary Antioxidants and Related Compounds held an open meeting and workshop. A broad range of definitions from experts in the field was presented (see the Appendix for Acknowledgments), however, there was no firm agreement among the presenters. The situation is complicated as some compounds, at least in vitro, can be antioxidants under one condition and pro-oxidants under others. Adding to the complexity is the fact that antioxidants contained in foods are not interchangeable and may differ from one another both in their sites and mechanisms of action. For example, vitamin E is a most effective chain-breaking antioxidant, whereas beta-carotene is a most effective singlet oxygen quencher. In contrast to these fat-soluble antioxidants that function in cell membranes, lipoproteins, and lipid droplets, water-soluble vitamin C directly quenches a variety of reactive oxygen and nitrogen species in the aqueous portion of cells and extracellular fluids. Selenium, not present in high enough quantities to be a direct antioxidant, participates as an essential constituent of glutathione peroxidases and thioredoxin reductase in the breakdown of peroxides, thus preventing the potentially damaging generation of reactive oxygen species.