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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 colo-rectal (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.
Key findings
Processed meats, such as ham and bacon, increase the risk of colo-rectal cancer, and should be eaten sparingly.
Another important finding is the link between red meat and colo-rectal 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: evidence showed breast-feeding protects the mother against breast cancer.
Supplements not recommended
The report did not recommend dietary supplements as prevention.
"This report is a real milestone in the fight against cancer, because its recommendations represent the most definitive advice on preventing cancer that has ever been available anywhere in the world," said Professor Martin Wiseman, project director of the report.
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, colon-rectum, endometrium (uterus lining), kidney and post-menopausal breast.
10 recommendations
The report makes 10 recommendations including 30 minutes of moderate activity a day, rising to 60 minutes; drinking water rather than sugary drinks; eating fruit, vegetables and fibre and limiting salt consumption. - (Reuters Health)
The WCRF report can be found at: http://www.dietandcancerreport.org/
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