A third of cancers are caused by diet and lack of exercise and could be prevented, according to a report which urges people to stay slim and abstain from too much fast food, red meat and preserved meat such as ham and bacon, and alcohol.
The report from the World Cancer Research Fund, which had input from more than 200 scientists and took five years to produce, is the most authoritative overview of the role that food, drink, obesity and exercise play in causing cancer. It concludes that changes in our lifestyle could play almost as big a role as stopping smoking in preserving us from disease and that being fat is a big risk for cancer.
Top of the 10 recommendations for a healthier life in the report is that people should keep their weight down throughout their life.
"The most striking thing to emerge from the report is the importance of overweight and obesity," said Sir Michael Marmot, professor of epidemiology and public health at University College London and chair of the panel which reviewed 7,000 studies on causes of cancer.
Obesity is normally measured by body mass index (BMI) -- which is a person's weight in kilograms divided by height in meters squared. A BMI above 30 is considered obese, while 25-29 is overweight. The WCRF recommends a BMI at the lower end of the healthy range, which is 18.5 to 25.
Excess body fat is not only a trigger for endometrial cancer, but also oesophagus, pancreatic, bowel, post-menopausal breast, and kidney cancers, according to the report. And fat around the abdomen is also linked to bowel cancer.
"What we're saying is that young adults should try not to put on weight throughout their adult life. They should stay as lean as possible," said Sir Michael, who said he had himself been "a bit shocked" by this conclusion.
The report has similarly robust recommendations on alcohol, exercise and meat consumption. Everyone should have at least half an hour of exercise a day -- but the panel says it should be vigorous, not moderate exercise. If the exercise is moderate, it should last for an hour a day.
The third recommendation is to avoid energy-dense foods, in which, said Sir Michael, they included fast foods which were high in fat and sugar. Sugary drinks were a particular problem, he said, and even fruit juices should not be drunk to excess because of their sugar content. Tea and coffee posed no risk, he added.
Eating mostly plant-based foods, such as fruit and vegetables, reduced cancer risk, but too much red meat raised it. The panel recommended no more than 500g a week. Processed meat -- such as the ham and bacon with added preservatives sold in supermarkets -- should be avoided altogether, it said, because they raised the cancer risk by around 10 percent.
Alcohol was a difficult area, Sir Michael said, because it was the only item they looked at which had a beneficial effect in other diseases -- small amounts reduce the risk of heart disease. But it raises the risk of six -- and possibly seven -- different cancers, including mouth, throat and breast cancers.
The panel recommends women should not have more than one drink a day and men not more than two.
Sir Michael said the importance of the report was that it looked at the totality of the evidence.
"We could be giving grandma's wisdom," he said. "This is a very positive message. What we're saying is that perhaps a third of cancers are diet-related. Cancer is largely preventable. It is a very positive message."