The World Cancer Research Fund (WCRF) said the link between body fat and cancer is closer than generally realised. It found convincing evidence of a link to six types of cancer, five more than in its last report, 10 years ago.
Among the new types are colorectal (bowel) and post-menopausal breast cancer.
Professor Michael Marmot, chair of the panel of 21 eminent scientists who compiled the report, 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."
The report, which selected 7,000 studies from a worldwide pool of 500,000 written since records began in the 1960s, includes five key findings.
They are that processed meats, such as ham and bacon, increase the risk of colorectal cancer, and should be eaten sparingly.
Another is the link between red meat and colorectal cancer, for which the evidence is stronger than ever. People should not eat more than 500g of cooked red meat a week - or between 700g and 750g for "blue" or uncooked meat.
A further finding was the strongest evidence yet that alcohol is a cause of cancer. If people must drink, the report said, they should limit their intake to two units a day for a man or one for a woman. A unit is a half pint of beer or a small glass of wine.
The report recommended mothers breast-feed exclusively for the first six months after birth followed by complementary breast-feeding, after evidence showed breast-feeding protects the mother against breast cancer.
It did not recommend dietary supplements as prevention.
Scientists believe there are several reasons for the link between body fat and cancer. One is the relationship between excess fat and the hormonal balance in the body. Research has shown that fat cells release hormones such as oestrogen, which increases the risk of breast cancer, while fat around the waist encourages the body to produce growth hormones, which can increase levels of risk.
Evidence of a link is most convincing for cancer of the oesophagus, pancreas, colorectum, endometrium (uterus lining), kidney and post-menopausal breast cancer.
The WCRF report can be found at: http://www.dietandcancerreport.org/
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