LONDON (Reuters) - Many of the world's six million cancer deaths each year could be prevented if people stopped smoking and lost weight, a leading British scientist said on Wednesday.

Tobacco is the biggest single cause of cancer and accounts for more than half of all cancer deaths in smokers. In non-smokers, 10% of cancer deaths in America and 7% in Europe are linked to being overweight. ``In terms of public health, the most important aspect is smoking. If you give up smoking, even late in life, it will have a big effect on mortality,'' Professor Julian Peto told a news conference. The epidemiologist at Britain's Institute of Cancer Research and the London School of Hygiene and Tropical Medicine conducted an overview of the major causes of cancer discovered during the past 50 years. His research is reported in the science journal Nature. Because tobacco is such a major risk factor, Peto divided people into smokers and non-smokers to determine what causes the major types of the disease. ``About 60% of all cancer deaths in smokers are caused by smoking, and smoking causes as many deaths from other diseases as it does from cancer. It is absurd for smokers in the West to worry about anything except stopping smoking,'' he said. LUNG CANCER Peto estimates up to three-quarters of cancers in smokers can be avoided if people quit, and the sooner the better. Smoking is the leading cause of lung cancer and also contributes to stomach, liver, cervical, pancreatic, bladder and kidney cancers. The cancer picture is not as clear in non-smokers, but Peto said he was surprised that being overweight had such an impact. Scientists aren't sure of the mechanism through which excessive weight increases the risk of the disease or if it can be reduced if people lose weight. ``There is now a consensus that cancer is commoner in those who are overweight. The evidence on weight is strongest for post-menopausal breast cancer and cancer of the endometrium (lining of the womb), gall bladder and kidney,'' Peto said in the Nature paper. Infections and a weakened immune system are other known risk factors for cancer, along with excessive alcohol, sunlight and pollution. Helicobacter pylori, a bacteria linked to ulcers and stomach cancer, and the human papilloma virus found in most cervical cancers are among infections that contribute to about 15% of cancers. Doctors also suspect childhood leukaemia is triggered by an infection, but scientists have not pinpointed the cause. ``Viruses are particularly important in the developing world. Hepatitis B virus, which can be prevented by vaccination, causes almost as many cancers as smoking does in China,'' said Peto. Genetic susceptibility is also a risk factor in many types of cancer, but Peto added that most of the genes involved remain to be discovered.

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