Age. This is the dominant factor. Risk climbs steadily from the mid-forties and most cases are diagnosed in people over 60. That is why screening programmes are built around age rather than lifestyle.
Family history. A first-degree relative — parent, sibling or child — diagnosed with bowel cancer roughly doubles your risk, and more so if they were diagnosed young or if several relatives are affected. Inherited conditions such as Lynch syndrome and familial adenomatous polyposis carry much higher risks again, and need specialist surveillance rather than screening tests.
Inflammatory bowel disease. Long-standing Crohn's disease or ulcerative colitis increases risk, particularly where a lot of the colon is involved and the disease has been present for many years. People in this group should be on a colonoscopy surveillance programme.
A previous bowel cancer or previous polyps. Both raise the chance of another, which is why follow-up intervals exist.
Processed meat, and red meat in quantity. Processed meat — bacon, ham, salami, sausages cured or preserved — is classified by the International Agency for Research on Cancer as carcinogenic to humans, with bowel cancer the cancer in question. Red meat is classified as probably carcinogenic. This does not mean a bacon sandwich is dangerous; it means the risk rises with how much you eat, and UK advice is to keep red and processed meat to no more than about 70g a day.
Alcohol. Risk rises with intake, with no threshold below which it disappears entirely. Cutting down helps; the relationship is dose-dependent.
Smoking. Associated with both polyps and cancer. Stopping helps at any age.
Body weight. Carrying excess weight, particularly around the middle, is consistently associated with higher risk.
Inactivity. Regular physical activity is associated with lower risk, and the effect appears independent of weight.
Fibre. Diets higher in fibre from wholegrains, pulses, fruit and vegetables are associated with lower risk. This is one of the more consistent dietary findings.
Two honest caveats. First, most of this evidence is observational: it shows association across populations, not cause and effect in an individual. Second, the effect sizes for lifestyle factors are modest next to age and family history. Someone who eats well, exercises and has never smoked can still develop bowel cancer, and does.
That is the argument for screening rather than against lifestyle change. You do the sensible things because they help across the board, and you screen because the sensible things do not make you immune.
ColoAlert looks for hidden blood and tumour DNA in one stool sample. Free UK delivery, results in about 10 days, and a GP reads anything abnormal.
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