Medicinal Plants Classification Biosynthesis and ... - Index of

Biological Effects of -Carotene
vegetables. Fifty carotenoids are typically present in the human diet and several are found in
human plasma, including β-carotene, �-carotene, lycopene, zeaxanthin and lutein.
Carotenoids are potent antioxidants and are known to affect many different cellular pathways
(Krinsky et al., 2002). Moreover, several observational and prospective epidemiological
studies have consistently shown an inverse relationship between dietary intakes or blood
levels of β-carotene, one of the most known carotenoids, and cardiovascular diseases
(Honarbakhsh and Schachter, 2008) or several types of cancer (Gerster, 1995).
On the other hand, there is some contradictory evidence from human intervention trials
using β-carotene supplements. An increase in the risk of lung cancer among smokers who
took β-carotene supplements was reported in the alpha-tocopherol, beta-carotene Cancer
Prevention Trial (ATBC) in Finland (The �-tocopherol, β-carotene Cancer Prevention Study
Group, 1994) and among smokers and asbestos workers in the β-carotene and retinol efficacy
trial (CARET) in USA (Hennekens et al., 1996), but not among healthy male physicians in
the Physicians' Health Study in USA (Omenn et al.,1996). Moreover, most of the clinical
studies for primary or secondary prevention of atherosclerosis failed to show a protective
effect or even showed adverse effects after administration of β-carotene (Siekmeier et al.,
2006). These findings aroused widespread scientific debate and raised the suspicion that
carotenoids may even have dangerous effects in human body under certain circumstances.
In 1981 it was hypothesized that a high dietary intake of β-carotene might reduce human
cancer rates. Since then, several observational epidemiologic studies have addressed this
topic. The results of both case-control and cohort studies show a remarkable consistency for
the association of increased lung cancer risk with low amounts of dietary β-carotene or low
plasma β-carotene concentrations. For stomach cancer, the evidence is also consistent,
although the number of studies is more modest. For breast and prostate cancer, the studies
indicate no consistent association of plasma or dietary β-carotene and reduced cancer risk.
For colorectal cancer, the effect will be moderate, if existent. However, overall results are
promising and several plausible cancer preventive mechanisms have been reported for βcarotene
(van Poppel et al., 1995).
Prospective and retrospective studies suggest that carotenoids may reduce the risk of
certain other cancers; however, too few studies have looked at these sites to examine the
consistency of the evidence. Although clinical trials of the efficacy of β -carotene in cancer
prevention are underway, it is still necessary and prudent to continue well-designed
prospective and retrospective studies of the carotenoid hypothesis. However, the importance
of other carotenoids, other constituents of vegetables and fruits, and other nutrients whose
levels in the blood are partially correlated with those of β-carotene has not been adequately
explored.
Although initial studies suggested that persons with lower levels of serum retinol have
higher future rates of lung cancer, this idea was not confirmed in subsequent investigations.
Prediagnostic levels of β-carotene in blood, however, have been inversely related with risk of
lung cancer. Available data thus strongly support the hypothesis that dietary carotenoids
reduce the risk of lung cancer, but the data are also compatible with the possibility that some
other factor in these foods is responsible for the lower risk. Even if ultimately shown to be
causal, the relation between diet and lung cancer is modest compared with the deleterious
effect of cigarette smoking.
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