Normally, the body regulates the growth of new cells but occasionally abnormal cells are produced. These abnormal cells do not function properly and if they are not destroyed by the body's surveillance system, they may develop (mutate) and also rapidly increase in number, causing cancer.

The abnormal cells may also spread to other parts of the body and multiply there. Cancer can occur in different parts of the body. In the UK, the most common cancers in men are lung cancer, prostate cancer and colon cancer. The most common cancers in women are breast cancer, lung cancer and colon cancer.

Diet has a greater influence on some types of cancer than others. The strongest links are with some cancers of the gastrointestinal tract e.g. of the mouth, throat, stomach and large bowel (colon). Dietary factors may protect against or reduce the risk of cancer.

For example, fruit and vegetables, consumed regularly, are thought to help reduce risk, whereas a low fibre intake or a high alcohol intake increase risk. Diet is only usually one of several factors involved.

There is moderate evidence that higher vegetable consumption will reduce the risk of colon cancer and that higher fruit and vegetable intake will reduce the risk of stomach cancer.

There is also moderate evidence that higher consumption of fruit can reduce lung cancer risk although the major risk factor is smoking. The more of these foods consumed, the greater the reduction in risk. At least five portions of a variety of fruit and vegetables per day is recommended.

One of the mechanisms proposed to explain the effect of fruit and vegetables is via the antioxidants they contain, such as vitamin C, carotenoids and other plant phytochemicals.

However, although dietary intakes and blood levels of antioxidant nutrients such as vitamin E and beta-carotene have generally been associated with a lower risk of cancer (e.g. lung cancer), supplementation studies have not generally supported a protective effect.

In fact, supplementation with large doses of beta-carotene may increase the risk of lung cancer in high risk subjects (e.g. smokers). Maintaining body weight within the normal range may reduce the risk of some types of cancer. Being obese (very overweight) and having high intakes of energy may increase the risk of some cancers. For example in postmenopausal women, breast cancer risk is associated with being overweight.

Fat provides a large amount of energy so a high fat diet may make weight gain more likely. People who drink large amounts of alcohol have an increased risk of certain types of cancer, particularly liver cancer and cancers in the mouth and oesophagus. If such people also smoke, this makes the risk of cancer even greater.

Carcinogens are substances which can start the process of cancer. Tobacco smoke contains carcinogens which cause lung cancer. Foods may also contain carcinogens. However, the risk from carcinogens in foods is low because if they are present at all, it is usually in very small amounts.

Carcinogens in foods may be substances that occur naturally, they may be due to contamination, or they be formed during cooking or processing (e.g. in smoked foods or foods that have been blackened during cooking e.g. barbecuing).

Aflatoxins are examples of carcinogens present in poorly stored foods. They are linked to mouldy produce, especially peanuts that are contaminated by the growth of moulds.