Total antioxidant capacity: appraisal of a concept.

In this contribution, I discuss the applicability of total antioxidant capacity (TAC) data obtained from plasma to human health issues and the use of TAC data for dietary items in epidemiological applications. Against the background of knowledge that major antioxidant defense is enzymatic, the use of the term "total" is not appropriate. Because dietary phytochemicals undergo uptake and metabolism, extrapolation to health effects requires direct molecular information, not a global parameter that uses an arbitrarily selected prooxidant source. Suitable alternatives are given in measuring functional biomarkers (surrogate endpoints). Although using TAC may be helpful in comparing different food items, the extrapolation to their contribution of antioxidant defense in vivo and, further, to health issues, should be discouraged, with the possible exception of the gastrointestinal tract. This is of particular importance because dietary phytochemicals and other small molecules have nonantioxidant activities. Direct assay of urate, ascorbate, and tocopherol, the major small-molecule contributors to TAC, is recommended.