Different populations worldwide
experience different levels of colorectal cancer, and these levels change
with time.
Populations living in one
community whose lifestyles differ from those of others in the same community
also experience
different levels of colorectal
cancer. Groups of migrants quickly lose the risk associated with their
original home community and acquire the patterns of the new community,
often starting within one generation of arrival. Ethnic and racial differences
in colorectal cancer, as well as studies on migrants, suggest that environmental
factors play a major part in the aetiology of the disease. In Israel male
Jews born in Europe or the United States are at higher risk of colon cancer
than those born in Africa or Asia. Risk in the offspring of Japanese populations
who have migrated to the United States has changed—incidence now approaches
or surpasses
that in white people in the
same population and is three or four times higher than among the Japanese
in Japan. For reasons such as these, colorectal cancer is widely believed
to be an environmental disease, with “environmental”
defined broadly to include
a wide range of ill defined cultural, social, and lifestyle practices.
As much as 70-80% of colorectal cancers may owe their appearance to such
factors; this clearly identifies colorectal cancer as one of the major
neoplasms in which causes may be rapidly identified, and a large portion
of the disease is theoretically avoidable.
The move from theoretically
avoidable causes to implementation of preventive strategies depends on
the
identification of risk factors,
exposures that have been associated with an increased (or decreased) risk
of colorectal
cancer, and the smaller
subset of risk determinants, whose alteration would lead directly to a
reduction in risk. From
analytical epidemiology
some clear ideas have now emerged about measures for reducing the risk
of colorectal cancer.
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