Experts have linked eight more cancers to being overweight or obese, nearly tripling the list from five to 13.
The study, by World Health Organisation scientists, suggests the number of people who get the disease because of an unhealthy lifestyle is thousands higher than previously thought.
Until now, doctors believed only five cancers were linked to excess weight – breast, womb, bowel, kidney and oesophageal.
But a review of evidence, analysed by the WHO International Agency for Research on Cancer, adds eight to the list – ovarian, stomach, liver, pancreatic, gall bladder, thyroid, the bone marrow cancer myeloma, and a type of brain tumour called meningioma.
Experts warned that obesity crisis may mean cancer rates increase at a higher rate than estimated.
Twenty years ago scientists began to realise that poor diets and sedentary lifestyles may be linked to some types of cancer.
They found that having more fat in the body changes the balance of hormones such as oestrogen, testosterone and insulin – which can each drive tumour growth.
In 2002 the IARC produced a list of five cancers which were more likely to occur if someone was overweight.
The same group of scientists – led by experts at Washington University School of Medicine in St Louis – released the extended list of 13. Cancer Research UK has estimated that 18,000 cases of cancers in Britain every year are caused by excess weight, making obesity the biggest preventable cause of the disease after smoking.
But the latest findings mean that number will be far higher.
Study leader Dr Graham Colditz, of Washington University, said: ‘The burden of cancer due to being overweight or obese is more extensive than what has been assumed. ‘Many of the newly identified cancers linked to excess weight haven’t been on people’s radar screens as having a weight component.’
In the past, doctors stressed that cancer was down to genetics, and a diagnosis was simply ‘bad luck’. But in recent years, experts have emphasised that the risk could be reduced with a healthy lifestyle. They estimate that roughly 40 per cent of cancer cases may be avoidable – compared with 85 to 90 per cent of heart disease cases. Dr Colditz, whose findings were published in the New England Journal of Medicine, said: ‘Lifestyle factors such as eating a healthy diet, maintaining a healthy weight and exercising, in addition to not smoking, can have a significant impact on reducing cancer risk.
‘Public health efforts to combat cancer should focus on these things that people have some control over. This is another wake-up call. It’s time to take our health and our diets seriously.’ The findings were based on more than 1,000 studies of weight and cancer risk.
Combining the results, scientists calculated that obesity – having a body mass index of more than 30 – raises the risk of pancreatic cancer by 50 per cent. For bowel cancer, the risk rises 30 per cent, and for ovarian cancer by 10 per cent.
Obese people are 80 per cent more likely to get liver cancer than those of a healthy weight, and nearly 400 per cent more likely to get oesophageal cancer if their BMI is 40 or above.
They found that for most forms of the disease, the risk rises as a person gets heavier, proving there is a direct link. For breast cancer, for example, the risk goes up by an additional 10 percentage points for every five BMI points above 30.
Cancer Research UK’s Dr Jyotsna Vohra said: ‘This report adds to the overwhelming evidence that carrying too much weight increases the risk of cancer … Yet last week the Government failed to take obesity seriously … Unless the Government acts, future generations will face ill-health, will develop cancers that could have been avoided and a financially overwhelmed NHS will be unable to cope.’