Obesity link to cancer

Some cancers are linked to viruses

One in 10 fatal cancers affecting non-smokers is caused by the sufferer being obese say researchers.

A major review of statistical evidence has clarified the principal causes of cancer in the UK - and produced some surprising results.

Historically, tobacco smoking is such an overwhelming cause that it overshadows virtually everything else, causing 60% of fatal cancers which affect smokers.

However, in non-smokers, it causes virtually no cancers at all, allowing researchers to highlight other reasons why the disease is emerging.

The research project, led by Professor Julian Peto, and paid for by the Cancer Research Fund, also reinforces the importance of infections as a cause of cancer.

Infections and obesity led to higher cancer rates

Worldwide, 15% of cancer deaths can be attributed to some sort of infection - although the figure is lower in the US and Europe.

The most important of these are hepatitis, which can lead to liver cancer, human papillomavirus, which is strongly linked to cervical cancer, and Helicobacter pylori, the bacterium linked to stomach cancers.

It is now thought that as many as 50% of younger women have had the papillomavirus infection at some point or other during their lives.

Weight and infections

Professor Peto said: "In developed countries such as Britain and America obesity and carcinogenic infections are the major avoidable causes of cancer deaths in non-smokers."

Some of the cancer deaths linked to obesity are breast and ovarian cancers - it is thought that the excess oestrogen produced by obese patients increases their chance of developing hormone-sensitive versions of these cancers.

Professor Peto said: "Apart from these, we are still not exactly sure why obese people are more prone to cancer."

He stressed the importance of preventing people becoming obese in the first place - as there was no real evidence that an obese person who slimmed down actually reduced their risk of dying from cancer - although this does reduce the chance of developing heart disease or diabetes.

Professor Karol Sikora told the BBC that the importance of diet in cancer should not be underestimated.

He said: "People at risk have a very high fat content in their diets, low fibre and eat fewer fruits and vegetables."

Occupation-related cancers still figure strongly in statistical tables, he said, mainly due to the toll of asbestos disease.

While exposure to the lethal asbestos fibres happened in most cases almost half a decade ago, deaths caused by mesothelioma, the cancer involved, are now emerging at the rate of 1,500 a year.

Statistical evidence from the US examined by Professor Peto also suggested that no clear case could be made for PSA screening for prostate cancer - which may be offered to UK men.

Death rates stable

Despite detecting many more cases of prostate cancer, death rates from the disease in the US have not fallen significantly over the past decade, when testing has been widespread.

Many prostate cancers are extremely slow-growing, and may not shorten the lifespan of the man involved.

And many men who have surgery to remove cancerous prostate glands risk serious damage to nerves which control urination and sexual function.

Professor Peto said: "All these poor US men are going around being impotent and incontinent - the statistics are very far from showing that prostate screening is desirable."

The statistical survey is published in the journal Nature.