Food types and bowel cancer

Fibre is found mostly in fruit, vegetables and cereals, including flour and bread. A lot of research has looked at fibre and colorectal cancer.

Recent research from the World Cancer Research Fund, including EPIC (the European study into the effect of diet on cancer risk), has suggested that fibre protects against bowel cancer. More than 8 separate studies showed that people who ate the most fibre had the lowest bowel cancer risk. And those who ate the least had the highest bowel cancer risk. The foods studied were all natural foods, such as fruit, vegetables and cereals, and not fibre supplements or foods with artificially added fibre. 

In November 2011 a large meta analysis looked at fibre and bowel cancer risk. It included 25 studies and the findings supported the EPIC study. In people who had a high intake of dietary fibre, in particular cereal fibre and whole grains, there was a lower risk of colorectal cancer. 

So, fibre is recommended as part of a healthy diet. It can help to prevent other chronic diseases such as heart disease and diabetes. It is not clear whether the protective effect against bowel cancer is due to fibre itself or to other protective properties of fruit, vegetables and cereals. Another possible explanation is that people who eat a high fibre diet also tend to have less food in their diets that increases the risk, such as red and processed meat.

There are a couple of theories to explain the protective effect of fibre

• You need fibre to help waste products travel through the bowel
• Some types of fibre help to carry bile acids that could potentially cause cancer through the bowel more quickly

People who do not eat enough fibre tend to be constipated. So any cancer causing agents are in contact with the bowel lining for longer and increase bowel cancer risk.

Eating more fruit and vegetables may lower the risk of bowel cancer, according to results from the EPIC study. But the evidence is not conclusive. Fruit and vegetables may be protective because they contain vitamins and minerals. Researchers think that antioxidant vitamins and minerals help prevent cell damage that may lead to cells becoming cancerous. There is quite strong evidence that a diet rich in folate is linked to a lower risk of bowel cancer. Folate is a B vitamin found in green and leafy vegetables.

Fruit and vegetables may also be protective because of their fibre content. Diets higher in fruit and vegetables tend to be lower in meat and fat. We do not really know whether it is the fibre or the antioxidant vitamins that make the most difference.

If you take vitamin supplements instead of eating more fruit and vegetables, you miss out on the fibre and on other substances in plant foods that may help prevent cancers. These are called flavonoids, phyto oestrogens and tannins. We don't know whether these help to prevent bowel cancer.

Protein foods are meats, fish, eggs, dairy foods, beans, lentils, soya and tofu. There is reasonable evidence that eating a lot of red meat and processed meats increases bowel cancer risk. Red meats include lamb, pork, veal and beef. Processed meats include sausages, salami, ham, bacon, paté and corned beef. 

Poultry meats, such as chicken and turkey, probably don't increase your risk of bowel cancer.

Recent results from a large European study (EPIC) showed that people who eat fish at least every other day on average, lower their risk of bowel cancer by about a third compared to people eating fish about once a fortnight. Other studies show that the more fish people eat, the lower their risk of bowel cancer. But a review by the World Cancer Research Fund (WCRF) found the results of these studies to be inconsistent. Fish might protect against bowel cancer because some types contain fats called long chain polyunsaturated fatty acids.

Cooking methods may increase the cancer risk of meat and fish. Foods cooked at very high temperatures can form chemicals called polycyclic hydrocarbons and aromatic amines. These chemicals are thought to increase cancer risk, but studies have not linked them specifically to bowel cancer.

In some studies, high fat diets have been linked to bowel cancer. But many researchers think this may be tied up with meat intake. Other researchers think that it is not fat that causes the problem, but an unhealthy lifestyle in general. The studies have not generally looked at different types of fats. But if you are trying to eat a healthier diet, it makes sense to use mainly monounsaturated and some polyunsaturated vegetable fats and cut down on saturated animal fat.

There is a strong link between obesity and cancer of the large bowel (colon cancer), especially in men. Body mass index (BMI) is a better measure of obesity than just your weight as it links weight and height. The link takes you to the glossary, which tells you how you can work out your BMI.

Compared to men of a healthy bodyweight, overweight men (with a body mass index of 25 or higher) have a 25% increase colon cancer risk. Obese men (with a BMI of 30 or higher) have a 50% increased risk of colon cancer. There is a smaller risk increase for colon cancer in overweight and obese women.

Calcium may protect against bowel cancer. The World Cancer Research Fund reviewed calcium and bowel cancer risk in 2007. They found that people with the highest amounts of calcium in their diets (over 1,300mg a day) had a lower risk of bowel cancer than people with the lowest amount (less than 500mg a day). But you only have to increase your calcium intake by a small amount to begin to lower your bowel cancer risk. Most of the calcium in our diets comes from milk and other dairy products. To give you an idea of how much calcium is in everyday foods,

• A pint of milk has about 700mg
• A regular pot of plain, full fat yoghurt has about 300mg
• 100g (4oz) of cheddar cheese has about 700mg of calcium but also contains fats that can be harmful

Calcium may help because it prevents polyps from developing. Polyps are growths in the bowel that may develop into cancer over a long period of time. A research review in 2008 found that a daily intake of about 1g of calcium may help to prevent bowel polyps. You can read this review on dietary calcium in the Cochrane Library. It was written for researchers and specialists so it's not in plain English.

Researchers have brought together the results of several studies looking at alcohol and bowel cancer risk. The combined results show that you have a 15% increase in bowel cancer risk if you drink more than 12.5 units of alcohol a week on average. The risk increases further if you drink more than this.

Several large reviews of research into smoking and bowel cancer seem to show that smoking also increases bowel cancer risk.