Half-million cancers worldwide linked to obesity

Excess body weight caused about 481,000 new cancer cases in 2012, according to a new study by the International Agency for Research on Cancer, an arm of the World Health Organization. 

That works out to about 3.6 per cent of all cancers worldwide, the majority of which occur in North America and Europe, according to the study published in the medical journal Lancet Oncology on Wednesday.

The study estimates that a quarter of all obesity-related cancers in 2012 are directly linked to rising average body mass index (BMI), especially in developed parts of the world where BMI has been increasing since the 1980s.

BMI is a measure of body fat based on a person's height and weight, and has become the standard in determining whether someone is at a healthy body weight.

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Scientists hypothesize that excess body fat may trigger cancer by changing insulin, glucose and hormone levels, and increasing chronic inflammation.

Women have greater cause for concern than men. 

The study indicates that obesity-related cancer causes 5.4 per cent of new cancers in women worldwide in 2012.

"For women, we know there's more cancer sites that are related to obesity, such as post-menopausal breast cancer but also breast cancer and cancer of the womb," said Dr. Melina Arnold, the study's lead researcher, in an interview with CBC News.

"So, those are very common cancers that are only affecting women. That's why we found this higher burden."

For men, excess weight is responsible for about 1.9 per cent of new cancer cases globally. 

6,000 cases in Canada

The geography varies widely.

Apart from North America, researchers found that the number of obesity-related cancer cases is high in Barbados and Puerto Rico. In Europe, they're particularly prevalent in Czech Republic. 

Obesity-related cancers showed up least in Sub-Saharan Africa, where they accounted for only about 1.5 per cent of the region's total cases.

Dr. Darren Brenner, who works with Alberta Health Services and is not linked to the Lancet Oncology-published study, said 3.6 per cent doesn't sound like much, but it is significant.

"In Canada, that works out to be about 6,000 cancer cases per year that are attributable to excess body weight. So when we think about it in those absolute terms, I think it's very very important that people take these results seriously," said Brenner, who's also studied the link between obesity and cancer.

The study noted that "about 0.9 [per cent] of all cancers diagnosed in 2012 could therefore be regarded as realistically avoidable by prevention of high BMI."

Obesity rates are stabilizing in North America, but researchers said the health benefits won't be obvious for a while.

"There's, of course, the lag time between being obese and developing cancer so we'll still expect the cancer burden related to obesity to increase in the next 10, 20 years," said Arnold. 

And what could we do to stave off the possibility of developing these cancers?

"A general recommendation of the World Health Organization is to assure a healthy diet, a balanced diet, and also to engage in regular physical activity ... to maintain a healthy body weight," said Arnold.