|
 study
by scientists at the Univer- sity of Texas Medical Branch at
Galveston (UTMB) and the University of Bologna in Italy casts
further doubt on the value of taking beta-carotene supplements
as a means to prevent cancer.
 According
to research being published in the April 29 issue of the journal
Nature, beta-carotene supplements increase the activity
of certain proteins which, during the normal processing of certain
toxic compounds, turn those compounds into potentially harmful,
cancer-causing ones.
If those proteins
are hyper-activated, "you're getting more cancer-causing
bang for your buck every time you're exposed to a carcinogen
such as when you smoke a cigarette," says Marvin Legator,
a UTMB professor of environmental toxicology and of preventive
medicine and community health, corresponding author of the new
study.
The new results
which were obtained by studying rats point to at least one potential
mechanism behind the seemingly paradoxical health effects of
supplementing one's diet with beta-carotene. Previous epidemiological
and animal studies suggested that taking beta-carotene supplements
might prevent cancer in humans. That inference was supported
by the fact that beta-carotene is an antioxidant a chemical that
can mop up harmful oxygen molecules called free radicals which,
if allowed to run rampant in cells, can damage genetic material
in ways that may lead to cancer.
But studies
of humans who consented to take vitamin supplements unexpectedly
showed that taking beta-carotene either alone or in combination
with vitamin A or vitamin E actually increased lung-cancer incidence
and mortality. Those studies, which were published in 1994 and
1996, looked at heavy smokers and asbestos workers.
The new rat
study is among the first to suggest a mechanism by which beta-carotene
might lead to these adverse effects. Researchers measured levels
of several enzymes belonging to a family of proteins known as
cytochrome P450s. These enzymes metabolize a number of carcinogenic
compounds (such as polycyclic aromatic hydrocarbons, dioxins
and nitrosamines) contained in cigarette smoke and in the environment.
The activity of those enzymes was boosted in the lungs of rats
that were fed beta-carotene supplements but not in rats that
weren't.
Researchers
don't know whether beta-carotene has a similar effect in humans.
But if it does, these results are decidedly relevant to public
health, they say. That's because most carcinogens acquire their
cancer-causing capabilities after being acted on by enzymes such
as the cytochrome P450s, which act similarly in humans and in
rats. Only after this interaction are carcinogens able to induce
the genetic damage that leads to cancer.
"Most
compounds that are considered carcinogenic are not carcinogenic
by themselves," Legator explains.
Sherif Abdel-Rahman,
UTMB assistant professor of environmental toxicology and preventive
medicine and community health, a coauthor of the study, notes:
"In humans and particularly in smokers high levels of these
enzymes would predispose an individual to greater cancer risk."
Abdel-Rahman adds that the cancer-boosting effect of beta-carotene
might be more pronounced in people who inherit forms of cytochrome
P450 that are naturally highly active.
Professor
Moreno Paolini of the University of Bologna, senior author of
the study, says, "We think that our findings are relevant
to public health policy and that they should be considered before
widespread supplementation with these micronutrients is recommended."
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