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Tobacco has long been the number one cause of preventable cancer, but experts say obesity may soon take its spot.

“There are varying estimates [when this will happen],” Dr. Nigel Brockton, the vice president of research at the American Institute for Cancer Research , told Global News.

“The American Cancer Society said five to 10 years, and I’ve heard some people say by 2030, but it is going to roll around pretty quickly.”

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Brockton said increasing rates of obesity in North America paired with decreasing tobacco use is behind this shift.

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Canada’s obesity rate has doubled since the 1970s, and as of 2017, 64 per cent of Canadians were overweight or obese. Nearly 40 per cent of adults in the U.S. are obese.

Obesity is a progressive chronic disease, similar to diabetes or high blood pressure, which is characterized by abnormal or excessive fat accumulation that may impair health, Obesity Canada says.

The disease is linked to at least 12 types of cancer, Brockton said, including stomach, colorectal, pancreatic, esophageal, endometrial, ovarian and postmenopausal breast cancer among others.

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While tobacco is still the number one cause of preventable disease and death in Canada, the Canadian Cancer Society says, fewer people are heavy smokers today than a decade ago.

The government also aims to reduce tobacco use to less than five per cent by 2035.

“The kind of societal and structural problems that created the obesity epidemic, as it’s often referred to, haven’t really changed, and arguably the reservoir of childhood obesity is increasing,” Brockton said.

“It’s very concerning.”

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Why is obesity linked to cancer?

Doctors are not entirely sure why obesity is so closely tied to cancer, and more research is needed, Brockton said. Still, doctors are confident there is a relationship.

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According to the American Cancer Society, excess body weight is currently thought to be responsible for about eight per cent of all cancers in the U.S., and around seven per cent of all cancer deaths.

“If you have excess body weight then you are at a higher risk [for cancer],” Brockton said. “The way that that actually impacts the cells to transform them from a normal cell to a malignant cell is less certain. There may be multiple ways that it happens.”

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Brockton said inflammation may be a factor, as inflammatory conditions like Crohn’s disease and colitis are associated with an increased risk of cancer.

“Obesity is actually a chronic inflammatory state,” he said. “So that’s certainly one way in which obesity or excess depository body fat can increase cancer risk.”

Another potential reason why obesity increases the risk of cancer is tied to insulin levels, Brockton said.

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“As people gain body weight, they often develop Type 2 diabetes… and there’s this insulin resistance, which means higher levels of insulin circulating,” he said. Research suggests there may be a link between insulin resistance, diabetes and cancer.

“There are many potential mechanisms,” Brockton added.

Raising public awareness and preventing obesity

While obesity and being overweight has long been linked to heart disease and diabetes, there’s not enough public awareness around the relationship between obesity and cancer, Brockton says.

“I think awareness is increasing…but it does come as a bit of a surprise to people that obesity increases your risk of cancer — and many, many types of cancer,” he said.

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Brockton said that tackling obesity can help reduce the risk of cancer. Plus, maintaining a healthy body weight may even help patients diagnosed with cancer.

“We know that if you’re at a healthy body weight at diagnosis, overall, you have better outcomes for most cancers,” he explained.

Dr. Yoni Freedhoff, medical director of the Canadian Bariatric Institute and an assistant professor of family medicine at the University of Ottawa, previously told Global News that in order to deal with rising obesity rates, it’s important to look at our “food environment.”

Today, unlike decades ago, people are pushed towards unhealthy food choices at every turn. Not only is unhealthy food everywhere, but it’s also become a part of every celebration or activity.

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“If we had a time machine that deposited us all back in the ’40s, living the lives that existed back then with the portions that existed back then and the societal norms that existed back then, I bet you we would all lose a fair bit of weight,” he said.

Freedhoff said small changes can have an effect. This could include taxes on sugary beverages, limits on food advertising to children, changes to food labelling and the return of home economics classes.

— With files from Leslie Young

Laura.Hensley@globalnews.ca