Even people of a healthy weight should slim if they want to reduce the risk of developing cancer, a landmark study concluded today.
Everyone should aim to be as slim as possible without being underweight, the World Cancer Research Fund study said.
Its analysis of 40 years of international research into the links between lifestyle and cancer also advised that people should not eat processed meat such as ham and bacon, should gain no weight after 21 years of age, and should avoid drinking alcohol.
The researchers found "convincing" evidence that excess body fat could cause six different types of common cancers, including those of the breast, bowel and pancreas.
They said that, in order to enjoy maximum protection against cancer, everyone should be at the lower end of the healthy weight range.
"We are recommending that people aim to be as lean as possible within the healthy range, and that they avoid weight gain throughout adulthood," Sir Michael Marmot, who chaired the expert panel, said.
"This might sound difficult, but this is what the science is telling us more clearly than ever before. The fact is that putting on weight can increase your cancer risk, even if you are still within the healthy range."
He said the best advice for cancer protection was to avoid weight gain, adding that "if you are already overweight … you should aim to lose weight".
People with a Body Mass Index (BMI) of 18.5 to 24.9 are considered to be within a healthy weight range. Those with a BMI of 25 to 29.9 are considered overweight, while anyone with a 30+ rating is classified as clinically obese.
However, the study said the risk of cancer rose as people got nearer to a BMI of 25 and everyone should try to be as close to the lower end of the weight range as possible.
The researchers said body fat was a key factor in the development of cancer, claiming that its links to cancer were much stronger than was generally realised.
The report - based on an analysis of 7,000 cancer studies published since the 60s - includes recommendations from a panel of 21 world-renowned scientists.
It also linked diet to the risk of developing certain cancers, particularly those of the bowel, warning people to avoid eating processed meat.
The document found a strong link between eating red meat and colorectal cancer, and recommend that people consume no more than 500g of cooked red meat a week.
All alcoholic drinks should be avoided to protect against cancer, but the study acknowledged that modest consumption was said to have a protective benefit for heart disease. Alcohol was particularly linked to mouth, oesophagus and breast cancers.
The most recent data showed that, in 2004, nearly a quarter of men and women in England were obese, as well as 10% of girls and 8% of boys aged under 20.
A government report, published earlier this month, warned that some 60% of UK men, 50% of women and a quarter of all children could be clinically obese by 2050 if the trends continued.
"After smoking, obesity is the biggest risk factor for cancer, and the evidence showing the link continues to strengthen," Dr Lesley Walker, Cancer Research UK's director of cancer information, said.
"Small changes can have a big effect on our cancer risk and everyone needs to take action, individuals as well as government. We have been disappointed with the government's response to the problem of obesity."
She said people needed to "make the healthy choice the easy choice in all aspects of our lives" and called for action to "defuse the obesity timebomb while we still can".
