The antioxidant vitamins C and E

oxygen-dependent prolyl hydroxylase enzymes in vitro (11,13), its significance to this
“oxygen sensing” pathway in vivo remains to be determined.
Antioxidant Activity
An antioxidant has been defined as “a substance that, when present at low concentrations
compared with those of an oxidizable substrate, significantly delays or prevents
oxidation of that substrate” (14). Several properties make vitamin C an ideal
antioxidant in biological systems. First, the low one-electron reduction potentials
of ascorbate and the ascorbyl radical enable these compounds to react with and
reduce virtually all physiologically relevant reactive oxygen species (ROS) and
reactive nitrogen species (RNS), including superoxide, hydroperoxyl radicals,
aqueous peroxyl radicals, singlet oxygen, ozone, nitrogen dioxide, nitroxide radicals,
and hypochlorous acid (15). Hydroxyl radicals also react rapidly, although not
preferentially, with vitamin C; hydroxyl radicals are so reactive that they combine
indiscriminately with any substrate at a diffusion-limited rate. Vitamin C also acts
as a co-antioxidant by regenerating α-tocopherol from the α-tocopheroxyl radical.
This may be an important function because in vitro experiments have found that αtocopherol
can act as a prooxidant in the absence of co-antioxidants such as vitamin
C (16). Another property that makes vitamin C an ideal antioxidant is the low
reactivity of the ascorbyl radical formed when ascorbate scavenges ROS or RNS.
The ascorbyl radical is neither strongly oxidizing nor strongly reducing and it
reacts poorly with oxygen. Thus, when a reactive radical interacts with ascorbate, a
much less reactive radical is formed. The ascorbyl radical scavenges another radical
or rapidly dismutates to form ascorbate and DHA. Alternatively, the ascorbyl
radical and DHA can be reduced enzymatically or recycled back to ascorbate (see
above).
Dietary Iron Absorption
The ability of ascorbate to maintain metals ions in a reduced state is critical to the
function of the mono- and dioxygenases discussed above (7,8). Concomitant consumption
of vitamin C from food or supplements enhances nonheme iron absorption
from a single meal in a dose-dependent manner (17), probably because the reduction
of iron by ascorbate makes it less likely to form insoluble complexes with phytate and
other ligands (1). Iron deficiency is the most common nutrient deficiency in the world
and is associated with a number of adverse health effects (18). Consequently, the
potential for increased vitamin C intake to improve iron nutritional status by increasing
the bioavailability of dietary nonheme iron has received considerable attention.
Despite consistent findings of enhanced iron absorption from a single meal in the
presence of vitamin C, several intervention studies were not able to demonstrate that
increasing vitamin C intake improved iron nutritional status (19,20). More recent
research indicates that the enhancing effect of vitamin C on iron absorption from a
complete diet, rather than a single meal, may be offset partially by dietary inhibitors of
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