NO - Besoin d'assistance

Mechanisms and Biomarkers (WG 4) page 4
__________________________________________________________________________________________
Introduction
A basic concept on which all nutritional scientists can agree is that the increased consumption
of diets rich and varied in fruit and vegetables will improve the health of almost any human
population. However, what is by no means clear, despite extensive research in recent years, is
the means by which the protective components in these diets mediate their effects against
oxidative stress. The plant kingdom was first to evolve a sophisticated defence system to
successfully colonise the hostile terrestrial environment, and what omnivorous and
herbivorous animals (including humans) have done is to effectively hijack these systems for
their own survival. However, it is interesting to note that flavonoids and carotenoids, which
have been shown to have excellent anti-oxidant properties in vitro are only poorly bioavailable
compared with vitamins E and C. Moreover, there appears to be no specific uptake
and transport system for flavonoids and carotenoids which are absorbed passively in the
human GI tract. This is not to say that they may play a crucial role in the GI tract by titrating
out reactive oxygen, nitrogen and chlorine species (ROS, RNS and RCS- hereafter referred to
globally as RS) and other genotoxins and chelating redox active iron and copper. It has been
calculated that daily ROS damage to the colon could be as high as a 10,000 rad equivalent
daily radiation dose (Babbs, 1990) in the absence of adequate anti-oxidant protection. Plants
are a rich source of both oil soluble and water soluble anti-oxidant pigments and vitamins
which play a crucial role both in intermediary metabolism and cellular protective mechanisms
in animals. The greatest intellectual challenge in this field of research is to dis-entangle the
immensely complex milieu of conflicting and interacting variables that form what constitutes
a superficially healthy diet. Superimpose on this human genetic variability and susceptibility
to environmental hazards and the problems become truly daunting. Early attempts to spend
our way out of these problems through expensive large scale intervention studies have been of
dubious value. With hindsight it is easy to see how a basic understanding of biochemistry and
physiology led to a misinterpretation of the earlier epidemiological data. Subjects consuming
fruit, vegetable and grain-rich diets tend to have higher plasma levels of vitamins C and E,
carotenoids and some flavonoids, and on average are at lower risk of developing cancer and
heart disease (Gey, 1995). Such data should not, however, be used to infer that the above
components are the protective agents, or that they are mediating their beneficial effects via
antioxidant mechanisms. In fact, any agent in such protective diets would appear to correlate
with decreased disease incidence. The associative evidence comes from a wide range of case-