Body fat and obesity are far more closely linked to cancer than is generally realised, a landmark study has found.
Researchers say there is "convincing" evidence that excess body fat can cause six different types of common cancers, including those affecting the breast, bowel and pancreas.
The World Cancer Research Fund (WCRF) report is based on an in-depth analysis of 7,000 cancer studies from around the world dating back to the 1960s. The study also concluded that processed meat, including ham and bacon, was such a risk factor for bowel cancer that people should avoid it completely.
Simply adding fruit and vegetables to a diet, meanwhile, does not appear to offer the degree of protection from cancer as was previously thought, the study found. It includes recommendations from a panel of 21 world-renowned scientists.
The study is the second major investigation of the causes of cancer to be conducted by the WCRF.
Since the first report was published in 1997, the number of types of cancer for which there is "convincing" evidence of body fat being a causal factor has risen from one to six.
Previously, "high body mass" was found to be a likely cause of endometrial cancer - cancer of the womb lining. The new report says there is powerful evidence that excess body fat is also a trigger for oesophagus, pancreatic, bowel, post-menopausal breast, and kidney cancers.
A specific strong link is said to exist between fat around the abdomen and bowel cancer. There is also a probable connection between body fat and gall bladder cancer, and abdominal fat and pancreatic, post-menopausal breast, and endometrial cancer, the evidence suggests.
Professor Sir Michael Marmot, professor of Public Health at University College London, chaired the expert panel. He said: "We are recommending that people aim to be as lean as possible within the healthy range, and that they avoid weight gain throughout adulthood."
Professor Martin Wiseman, project director of the new report, said: "If people follow our recommendations, they can be confident they are following the best advice possible based on all the scientific research done up to this point."
The study found strong evidence that red meat and processed meats were a cause of bowel cancer. The panel recommended that people consume less than 500 grams, or 18 ounces, of cooked red meat per week, and avoid processed meat altogether if possible.
Processed meat was defined as meat preserved by smoking, curing, salting or the addition of preservatives. Examples included ham, bacon, pastrami, salami, and frankfurters.