ABSTRACT
Introduction Fleshy growths of the colon and rectal endothelium (polyps) are usually found in the distal part of the colon and rectum. Often, these polyps develop into benign colo-rectal adenomas. The growths can be removed surgically, but unfortunately there is a high recurrence rate (around 20%/year). Moreover, the hyperplastic tissue can lose its normal cell characteristics and progress to a colonic or rectal cancer. Indeed, most colo-rectal carcinomas arise from adenomas, although only 1-10% of adenomas progress to cancers. The risk of progression to a malignant tumour is increased if the adenoma is large, if the histology is villous rather than tubular, and if the lesion contains many abnormal cells. A meta-analysis based on 18 studies suggested that in individuals of average risk, the prevalence of colo-rectal adenomas was 30.2%, but that of colo-rectal cancer was only 0.3%.1 However, critics of this analysis have suggested that the apparently very high prevalence of colo-rectal adenomas may reflect in part the fact that data have often been collected on elderly people. The effectiveness of ongoing surveillance of adenomas is limited by the relatively high costs of detection and low recurrence rates.2 The screening costs associated with the detection of an advanced adenocarcinoma and a stage 1 colo-rectal cancer have been estimated at US$27,962 and US$922,762, respectively.3 Given these high costs and the adverse prognosis following carcinogenic change, there is considerable interest in prevention by various lifestyle measures. This chapter looks specifically at the value of physical activity in preventing the development of colo-rectal adenomas. There is now widespread agreement that an increase of habitual physical activity is helpful in reducing the likelihood of developing various types of benign and malignant tumours. Evidence on this point seems particularly strong for adenomas and carcinomas of the colon, where risks seem to be at least 20-40% lower among those individuals who are taking regular and adequate volumes of physical activity. Nevertheless, detailed analysis of the benefits of an active lifestyle has been hampered by a long disease latency. This has made it necessary to explore the physical activity patterns and overall lifestyle of large populations for periods of 20 years and more. Occupational comparisons have
been helpful in that the physical demands of work have remained consistent for many people over long periods. On the other hand, mechanization and automation have progressively reduced energy expenditures in many previously “heavy” occupational categories, and it is now increasingly difficult for epidemiologists to find people who are employed in jobs that still require hard physical work.
