In Europe and the US, the average life time risk of developing colorectal cancer is one in 20, but the risk can go up or down as a result of many factors:

The presence of non malignant colonic polyps (adenomas). As many as 40 per cent of people over 60 are estimated to have polyps, but the vast majority of these are benign adenomas which never develop into cancer. However, 2 per cent of polyps will become cancerous. The current treatment is to surgically remove all polyps as soon as they are detected. This is believed to substantially reduce the likelihood of later tumour development, but studies show only a 15 per cent reduction in mortality after surgery following the occult blood test, which measures blood in the faeces. To date, there have been no clinical trials to evaluate other screening methods or follow up surgery (Schweiz Med Wochenschr, 1998; 128: 999-1011).An immediate family history of bowel cancer. This increases the risk to one in five. The risk is even higher if a close relative has contracted the disease under age 50 (J Cell Biochem, 1996; 25: 131-5). Genetic testing is becoming available, but this will only detect, at most, 20 per cent of those at risk (Digestion, 1998; 59: 481-92).

A personal history of breast, endometrial or ovarian cancer (New Eng J Med, 1991; 325: 37-41).

A personal history of adult onset diabetes mellitus (type 2) (J Natl Cancer Inst, 1999; 91: 542-7), and the use of the diabetes drug troglitazone (Nature Medicine, Sept 1998).

Being more than 40 per cent overweight, if a man (Am J Clin Nutr, 1996; 63: 442S-4S), although higher body mass indices are associated with increased risk in both sexes (Int J Obes Relat Metab Disord, 1998; 22: 178-84).

Having chronic inflammatory disease of the colon, such as ulcerative colitis or Crohn's disease.

Long term frequent constipation (Epidemiology, 1998; 9: 385-91).

Heavy consumption of cigarettes (more than 30 per day), beer (every other day) and red meat (twice a day) (Int J Cancer, 1998; 77: 549-53). Moderate to high alcohol intake (more than two drinks a day) increases the incidence of colon polyps and doubles the risk of colon cancer in men (J Natl Cancer Inst, 1995; 87: 265-73).

Being over the age of 50.