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Mechanisms and Biomarkers (WG 4) page 17
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diet induced a significant increase of lipid oxidised by-products in the plasma suggesting that
carotenoids are necessary to prevent lipid peroxidation; moreover, a supplementation with ß-
carotene normalise plasma malodialdehyde level. (Dixon et al., 1998). Other studies showed
that ß-carotene supplementation may decrease plasma and tissue vitamin E concentration. The
most well known of adverse effects of ß-carotene is shown in the ATBC intervention study
where supplementation with that carotenoid in Finnish male smokers actually increased,
rather than reducing, lung cancer incidence (ATBC, 1994). In the same study, it was also
shown that in this male smokers population, ß-carotene supplementation also increased
coronary heart disease mortality by 11%. One explanation for such an observation is that
antioxidant activity of ß-carotene may shift into prooxidant one, depending on oxygen
tension, concentration and also with other antioxidants (Palozza, 1998). Whether one or more
of these conditions exist in smokers has to be established. On the other hand, other large scale
studies including the CARET (Omenn et al., 1996) and Physicians’ Health Study (Hennekens
et al., 1996) did not show any beneficial effect of ß-carotene on atherosclerosis or its
sequelae. It should be emphasised that in contrast with cholesterol-lowering trials, no
individuals were selected on grounds of low antioxidant intakes or high cardiovascular
disease susceptibility. Another carotenoid of interest is lutein mainly found in spinach and
which accumulates in the lens. High intake of this carotenoid has recently been associated
with decreased risk of cataract formation (Brown et al., 1999; Chasan-Taber et al., 1999). The
possible biological mechanisms may be the antioxidant activity and light absorption, but no
experiments specifically showed the actual mechanisms by which lutein may confer its
protective effect on lens opacity.
Other phytomicronutrient antioxidants
Many metabolites found in plants are supposed to act as antioxidant. These include phenols,
polyphenols, flavonoids and possibly lignans, all of which are characterised by the presence
of one or more hydroxyls conferring either metal chelating or radical scavenging properties
(Arora et al., 1998).
Phenolics - Many phenolic compounds exit in plants either in free or conjugated forms
(ferulic acid, thymol, ...). Investigations of their antioxidant properties clearly show their
ability to inhibit lipid peroxidation of liposomes by scavenging peroxyl radicals or to protect
deoxyribose against hydroxyl radical attack. However, the beneficial effects of consumption
of food-plants high in phenols has not been clearly established and it is difficult to reach high
circulating levels of these phenolic compounds by means of diet. However, they may present