Obesity is the single most important cause of cancer among non-smokers, concludes a major review of research carried out over the last 50 years. But the nature of the relationship remains unclear and the benefits of dieting are unknown.

In 17 May Nature Julian Peto from the Institute of Cancer Research, Surrey points out that tobacco remains the biggest health risk by far, causing cancer in 60% of smokers. Other causes include alcohol, sunlight and air pollution, each responsible for about 1% of cancers. Microorganisms such as human papilloma virus and Helicobacter pylori are also identified as important causes of cancer. But around 10% of cancers in non-smoking Americans and 7% in Europeans can be directly attributed to obesity.

Peto observed that being overweight is strongly associated with cancers of the breast, endometrium, gall bladder and kidney, aside from its link with heart disease. He added that he found...