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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