ABSTRACT
An antioxidant may be broadly defined as “any substance that when present at low
concentrations compared to those of an oxidizable substrate, significantly delays
or prevents oxidation” (1). Carotenoids are often regarded as dietary antioxidants,
but how effective are they? Although more than 600 naturally occurring
carotenoids have been identified (2), only 40 of these are ingested in the human
diet, but fewer than 20, including both polar xanthophylls and apolar cyclic and
acyclic carotenes from dietary sources, have been found in plasma and tissues
(3,4). Trace levels of a number of carotenoid metabolites and potential oxidation
products of dietary carotenoids (4), including, for example, 2,6-cyclolyopene-1,5-
diols (5), anhydrolutein (6), and acycloretinol (7), have also been detected.
Although a number of these oxidation products may be produced by interaction of
carotenoids with reactive oxygen species (ROS) under controlled conditions (8,9;
see below), their functional significance (if any) in biological tissues is not yet
understood (see Ref. 10).
