Being even slightly overweight can increase the risk of a range of common cancers including breast, bowel and pancreatic cancer, a landmark study has found.

The largest review of links between diet and cancer, incorporating more than 7,000 studies, concludes that there is convincing evidence that excess body fat can cause at least six different types of the disease. The researchers give warning that everyone should be at the lower end of the healthy weight range.

Their recommendations include avoiding processed meats such as ham, bacon, salami or any other meat preserved by smoking, curing or salting; only consuming small amounts of red meat; moderate consumption of alcohol; and avoiding junk food and sweet drinks.

Professor Sir Michael Marmot, who chaired the expert panel assembled by the World Cancer Research Fund (WCRF) to review evidence on the dietary causes of cancer, said he had been shocked to find that weight was so important.

A report by the fund published ten years ago linked only one cancer to being overweight. Professor Marmot said the evidence now showed that at least six – cancers of the oesophagus, pancreas, bowel, breast postmenopause, kidney and endometrium (womb lining) – were linked and that the risks were increased by even quite modest weight gains.

The finding is particularly alarming, given the expanding girth of the British population. An official report last month gave warning that by 2050, 60 per cent of men, 50 per cent of women and a quarter of all children could be clinically obese.

A healthy weight is defined as having a body mass index (BMI) below 25; BMI is calculated by dividing an individual’s body weight in kilograms by the square of the height in metres.

Sir Michael said: “A BMI of 25 is fine, but it would be a bit finer if it was lower. The healthiest thing is to be as low as possible within the normal range.” The report suggests moderation in the consumption of red meat, suggesting a limit of 500g (18oz) per person per week. A total avoidance of processed meats is recommended because of convincing evidence that eating meat increases the risk of colon cancer.

The WCRF report emphasises the benefits of exercise, for its direct effects on some cancers, and because it helps to prevent becoming overweight or obese. It made ten recommendations which do not, save in one case, conflict with advice given for the avoidance of other common causes of death, such as heart disease.

The exception is alcohol, which Professor Marmot said had been shown to cut the risk of heart disease. For cancer prevention the optimum level is zero, but for heart disease it is two units a day for men and one for women, he said. The panel agreed that the levels set for minimum heart risk should be accepted.

The report is based on an analysis of cancer studies from around the world dating back to the 1960s. The initial trawl produced half a million studies, which was pared down to the best 7,000. The results were analysed by nine teams and then presented to a panel of twenty-one leading scientists for their recommendations. They looked at cancers at 17 different sites in the body and at a wide range of factors, mostly dietary, that can affect risk of developing the disease.

Professor Marmot 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. This might sound difficult but this is what the science is telling us more clearly than ever. The fact is that putting on weight can increase your cancer risk, even if you are within the healthy range.”

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