For the great majority of people the cause of bowel cancer is not known.

There is evidence that diet can play a part in the development of bowel tumours. A high intake of animal fat seems to increase the risk of developing a colonic or rectal cancer. It has also been suggested that a high alcohol intake, especially of beer, may be linked to an increased risk. On the other hand, a diet that is rich in vegetables, and in particular rich in fibre, seems to reduce the chances of developing one of these growths.

Having a close relative with bowel cancer is also a risk factor (although the precise reason why is not known) especially if they developed their tumour at a relatively early age. Someone who has a close relative who developed a colorectal cancer before the age of 45, or who has two or more close relatives with bowel cancer is about two to three times more likely to develop the condition than somebody who has no family history. Anyone who is in this situation should talk to their family doctor to discuss whether they might need genetic counselling to assess their possible risk and whether they need any form of screening to ensure they do not have bowel cancer.

As well as this family tendency there are several rare inherited conditions which strongly predispose to bowel cancer developing. The two most common of these are

• familial adenomatous polyposis (FAP) and
• hereditary non-polyposis colorectal cancer (HNPCC).

With FAP, there are large numbers of benign (non- cancerous) polyps in the large bowel and there is a high risk that one or more can develop into cancer. If someone has the gene that causes FAP than there is a 50% risk of passing the gene on to their children.

With HNPCC (also known as Lynch syndrome) there is usually a history of bowel cancer in the family at an early age and there may also be a history of other cancers such as stomach, ovary and womb (endometrium). Usually families who have one of these conditions will already be known to their doctors who will arrange any necessary screening or monitoring tests for early detection of cancer development.

Two conditions that cause chronic inflammation of the bowel can also occasionally lead to bowel cancer. These are ulcerative colitis and Crohn's disease. With ulcerative colitis the risk is greatest for people who have had the condition for more than ten years, particularly if it started during their childhood. Crohn's disease usually affects the small intestine but can involve the colon and when this happens over a long period of time there is a small chance of bowel cancer developing.