Does a widening waistline put you at risk for cancer? Apparently so. Based on a new study, nearly 6 per cent of cancers are attributable at least in part to obesity and diabetes.

Researchers compared incidence data for 12 cancers in 175 countries in 2012 with body mass index and diabetes prevalence figures from 2002, on the assumption that it takes at least 10 years for cancer to develop. They found that in 2012, diabetes and a body-mass index (BMI) above 25 were independent risk factors for 792,600 new cases of cancer, about 5.6 per cent of the more than 14 million cases reported to a worldwide cancer registry.

Among the cancers associated with diabetes and high BMI were tumours of the colon, gall bladder, liver and pancreas. Obesity and diabetes were not the only causes of these cancers, but the conditions played a role.

"We know a lot about what causes obesity and diabetes, but what it is about being obese or diabetic that causes cancer is less clear," said lead author Jonathan Pearson-Stuttard, a clinical fellow at Imperial College London. "It may be that exposure to high insulin levels or insulin resistance may also be a cause of cancer."

The study, in The Lancet Diabetes and Endocrinology, found considerable differences in the proportions of cancer cases attributable to BMI on the one hand and to diabetes on the other.

For example, high BMI was associated with about twice as many cases of colorectal cancer as diabetes, and nearly three times as many cases of breast and endometrial cancers. Diabetes was not associated with kidney cancer at all, but high BMI was linked to about one-fifth of kidney cancer cases.

High BMI and diabetes combined accounted for 38.4 per cent of endometrial cancers but only 8.9 per cent of breast cancers. In men, the two conditions combined made up 23.3 per cent of liver cancers but just 8.6 per cent of colorectal cancers.

There were variations by region as well. About 16 per cent of cancer cases were attributable to high BMI in high-income Western countries. But only about 5 per cent of cancer cases in the high-income Asia-Pacific region were linked to BMI.

Diabetes and obesity are increasing worldwide, and the researchers project that cancers, particularly those of the gall bladder, liver and endometrium, will rise as well.

Unless diabetes and obesity are much better controlled, the growth in cancers will be significant.

"By 2035, because of increases in obesity and diabetes, there could be a 20 per cent increase in men's cancer and a 30 per cent increase for women," Dr Pearson-Stuttard said.

NYTIMES