Bowel cancer – risk factors

Everyone is at risk of developing bowel cancer, and the risk of developing bowel cancer increases with age.

Polyps are small growths on the inner lining of the bowel or rectum that are quite common. About 1 in 4 people will have had a polyp by the age of 50, and 1 in 2 people will have had them by age 70.

People with a history of polyps in the bowel, especially those who have had multiple polyps, have a higher risk of developing bowel cancer.

People who have previously recovered from bowel cancer may develop the disease again in other parts of the bowel. If the person was younger than 50 years of age when the bowel cancer was originally diagnosed the risk of developing a subsequent cancer becomes even greater.

There are two types of chronic inflammatory bowel disease: ulcerative colitis and Crohn’s disease.

These are conditions in which the bowel becomes inflamed due to injury, infection or irritation. If this inflammation occurs for long periods of time it causes damage to the lining of the bowel.

This damage increases the chances of bowel cancer developing. The risk of developing bowel cancer depends on how badly inflamed the bowel is and how long the inflammation occurs for.

A person’s risk of bowel cancer may be increased if one or more of their close relatives have been diagnosed with bowel cancer. However, only 15 per cent to 20 per cent of people with bowel cancer have a history of the disease within their family.

Two inherited conditions that can increase bowel cancer risk are:

• Familial Adenomtous Polyposis (FAP)
• Hereditary Non-Polyposis Colorectal Cancer (HNPCC).

Both are very uncommon conditions but can significantly increase the risk of bowel cancer for those who are affected.

FAP accounts for less than 1 per cent of all bowel cancers. FAP is caused by a change in a particular region of our DNA, called the APC gene. DNA contains all the information our cells need to make an identical copy of themselves.

The change in the APC gene causes multiple polyps to grow in the bowel and these polyps can turn into cancer if left untreated. The polyps commonly start growing during the late teens or early twenties, but may occur at any age. FAP is passed down in families. If a parent has the altered APC gene there is a 50 per cent chance a child may also have it.

HNPCC accounts for about 1 to 4 per cent of bowel cancers. HNPCC, also known as Lynch syndrome, is also caused by an altered gene. HNPCC is characterised by no or very few polyps, which makes the condition hard to diagnose. People with HNPCC often have bowel cancers that develop when they are in their 30s or 40s.

Type 2 diabetes, or non-insulin-dependent diabetes, is a condition in which the body can no longer respond to the hormone insulin. This results in high levels of insulin in the blood, known as hyperinsulinemia.

High levels of insulin lead to increased levels of a protein called insulin-like growth factor-1 (IGF-1) in the blood. IGF-1 is a protein that is believed to help cancer cells grow.

People with type 2 diabetes often have the disease long before it is diagnosed and treated. As a result, their bodies are exposed to high levels of IGF-1 for a long time. People with type 2 diabetes also put on weight (body fat). These 2 factors therefore increase the chances of people with type 2 diabetes developing bowel cancer.