How does cancer begin?
Cancer occurs when the cells in a certain area of your body divide and multiply too rapidly. This produces a lump of tissue known as a tumour.
Most cases of bowel cancer first develop inside clumps of cells on the inner lining of the bowel. These clumps are known as polyps. However, if you develop polyps, it does not necessarily mean that you will get bowel cancer.
Exactly what causes cancer to develop inside the bowel is still unknown. However, research has shown that several factors may make you more likely to develop it. You cannot do anything about some of these factors, but others you can change.
Family history
There is evidence that bowel cancer can run in families. Around 20% of people who develop bowel cancer have a close relative (mother, father, brother or sister) or a second-degree relative (grandparent, uncle or aunt) who have also had bowel cancer.
It is estimated that if you have one close relative with a history of bowel cancer, your risk of getting bowel cancer is doubled. If you have two close relatives with a history of bowel cancer, your risk increases four-fold.
Diet
A large body of evidence suggests that a diet high in red and processed meat can increase your risk of developing bowel cancer. For this reason, the Department of Health advises people who eat more than 90 grams (cooked weight) of red and processed meat a day to cut down to 70 grams. For more information, see Red meat and bowel cancer risk
There is also good evidence that a diet high in fibre and low in saturated fat could help reduce your bowel cancer risk. Cancer experts think this is because this type of diet encourages regular bowel movements.
Smoking
People who smoke cigarettes are 25% more likely to develop bowel cancer, other types of cancer and heart disease than people who do not smoke.
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Alcohol
A major study, called the EPIC study, showed that alcohol was associated with bowel cancer risk. Even small amounts of alcohol can put you at higher risk of getting bowel cancer. The EPIC study found that for every two units of alcohol a person drinks each day, their risk of bowel cancer goes up by 8%.
Obesity
Obesity is linked to an increased risk of bowel cancer. Obese men are 50% more likely to develop bowel cancer than people with a healthy weight. Morbidly obese men, who have a body mass index (BMI) of over 40, are twice as likely to develop bowel cancer.
Obese women have a very small increased risk of developing the condition, and morbidly obese women are 50% more likely to develop bowel cancer than women with a healthy weight.
Inactivity
People who are physically inactive have a higher risk of developing bowel cancer. You can help reduce your risk of bowel and other cancers by being physically active every day. Your risk could be cut by up to one-fifth if you do an hour of vigorous exercise every day or two hours of moderate exercise (such as vacuum cleaning or brisk walking).
Digestive disorders
Some conditions may put you at a higher risk of developing bowel cancer. People with Crohn’s disease are 2-3 times more likely to develop bowel cancer. The risk of developing bowel cancer is much higher in people with ulcerative colitis, and as many as 1 in 20 of these people will go on to develop it.
Genetic conditions
There are two rare inherited conditions that can cause bowel cancer. They are:
- familial adenomatous polyposis (FAP)
- hereditary non-polyposis colorectal cancer (HNPCC), also known as Lynch syndrome
FAP affects 1 in 10,000 people. The condition triggers the growth of non-cancerous polyps inside the bowel. Although the polyps are non-cancerous, there is a very high risk that, over time, at least one will turn cancerous. Almost all people with FAP will have bowel cancer by the time they are 50 years of age.
People with FAP have such a high risk of getting bowel cancer, they are often advised by their doctor to have their large bowel removed by surgery before they reach the age of 25. Families who are affected can find support and advice from the FAP registry at St Mark’s Hospital, London.
HNPCC is a type of bowel cancer caused by a mutated gene. An estimated 2-5% of all cases of bowel cancer are due to HNPCC. Around 90% of men and 70% of women with the HNPCC mutation will develop bowel cancer by the time they are 70 years of age.
As with FAP, removing the bowel as a precautionary measure is usually recommended in people with HNPCC.
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