ANTIOXIDANT VITAMINS AND CHRONIC DISEASE: WHO NEEDS SUPPLEMENTS?
B. Frei
Linus Pauling Institute, Oregon State University, Corvallis, OR 97331
The antioxidant vitamins E and C, and certain carotenoids, flavonoids
and other phytochemicals may exert beneficial effects in age-related
diseases such as cancer, cardiovascular diseases (CVD), eye diseases
and other chronic diseases that are caused, in part, by oxidative or
nitrative stress. This abstract will focus on vitamin C as an example
of an antioxidant vitamin. Vitamin C (ascorbate) has several
well-defined biological functions, e.g., it is required for collagen,
catecholamine and carnitine biosynthesis. Ascorbate also is a potent
antioxidant capable of scavenging numerous reactive oxygen and nitrogen
species, and regenerating other small molecule antioxidants. For
example, ascorbate is the most effective water-soluble antioxidant in
human plasma exposed to different types of oxidative stress; it
effectively protects low-density lipoproteins from oxidative
modification, a process implicated in atherosclerosis and CVD; and
vitamin C supplementation in humans dose-dependently increases the
resistance of plasma to ex vivo lipid peroxidation and, in some
studies, lowered in vivo levels of F2-isoprostanes, a biomarker of
lipid peroxidation. Vitamin C may also lower CVD risk by preventing
endothelial dysfunction and increasing synthesis of endothelium-derived
nitric oxide. Over thirty double-blind, placebo-controlled clinical
studies have shown that vitamin C treatment restores normal endothelial
function and vasodilation in patients with CVD or coronary risk
factors. Related to these findings, vitamin C treatment also lowers
blood pressure in moderately hypertensive patients.
In light of these findings and the proven safety of vitamin C
supplements other than occasional gastrointestinal upset or mild
diarrhea plasma and tissue saturation of vitamin C appears desirable,
as it would maximize the antioxidant benefits derived from the vitamin.
Pharmacokinetic data indicate that in young, healthy, non-overweight
males and females, 200 mg/d of vitamin C is sufficient to saturate
cells and tissues, and 400 mg/d to saturate plasma. These intake levels
can be achieved by dietary means, especially by eating five to nine
servings of fruits and vegetables. However, detailed pharmacokinetic
data for vitamin C are not available for elderly persons, patients
suffering from infections or chronic diseases such as CVD, diabetes or
cancer, obese people or smokers. It is known that smokers have a higher
requirement for vitamin C, as reflected by their RDA, which is 35 mg/d
higher than for non-smokers. In addition, in the above clinical studies
of vitamin C and endothelial function or hypertension, daily doses of
500 mg or intravenous infusion of large amounts of vitamin C were used,
but no detailed dose-response studies were performed. Given these
uncertainties, and the facts that vitamin C supplements are safe and
most people do not consume the recommended five to nine daily servings
of fruits and vegetables, vitamin C supplementation, and
supplementation with a multivitamin/multimineral in general, is
recommended.
Key words:
antioxidants, supplements, vitamin C, cardiovascular diseases
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