Too much body fat raises the risk of about a dozen different cancers, according to a major new study that adds to growing evidence linking excess weight to cancer at most sites in the body.
British scientists who pooled data from 141 studies on 20 cancer types found a higher body mass index is associated with:
- An increased risk of thyroid, kidney and colon cancers, cancer of the esophagus, multiple myeloma (a cancer of the blood cells), leukemia and non-Hodgkin lymphoma in both sexes;
- Rectal cancer and malignant melanoma (a deadly skin cancer) in men and;
- And gallbladder, pancreas, endometrial and postmenopausal breast cancers in women.
The study is published in this week's edition of the journal The Lancet.
Today, most Canadians are overweight or obese. According to the 2004 Canadian Community Health Survey, 23 per cent of Canadians aged 18 or older -- about 5.5 million adults -- had a body mass
index (BMI) of 30 or more, putting them in the obese category. Another 8.6 million, or 36 per cent, were overweight, with a BMI greater than 25. (BMI is a measure of body fat based on height and weight).
Researchers warn fat is becoming the new tobacco: Many of the associations between a bigger body mass and cancer risk were for cancers that aren't related to smoking.
"Conceivably, as cigarette smoking (which is the largest cause of cancers in developed countries) decreases, excess body weight could become the dominant lifestyle factor that contributes to cancer occurrence in such countries," the authors conclude.
And, unlike smoking, it's not known whether it's too late to lower cancer risks once the weight is already on.
"This study is all about preventing weight gain in the first place," says lead author Dr. Andrew Renehan, senior lecturer in cancer studies and surgery at the University of Manchester.
"When you ask populations what do they think are the risk factors for cancers, they all know smoking, they all know about family risk, but not many mention obesity. There's an awareness that needs to be echoed over and over again."
A key report last fall from the World Cancer Research Fund concluded body fat is associated with an increased risk of cancers of the esophagus, pancreas, colorectum, post-menopausal breast, endometrium and kidney.
Renehan's group went further. They looked at less common cancers as well, and at differences between sexes and populations to quantify the cancer risks.
The team estimated cancer risk with every five-unit increase in BMI. Overall, the increased cancer risk corresponds to a weight gain of about 37 pounds in men, and 28.6 pounds in women who have an average BMI of 23, or about "the middle of normal weight," Renehan says.
The study reinforces that "the leaner we are, the better off we are in terms of health status," says Dr. David Lau, professor of medicine at the University of Calgary and president of Obesity Canada. He said even people who aren't overweight might benefit "if you can slim off a few pounds. It goes beyond just overweight and obesity."
Just how excess weight increases cancer risk isn't fully understood. One theory is that excess body fat causes changes in circulating concentrations of hormones that can cause cells to proliferate, leading to runaway cancer formation.
It's also possible that fat cells, especially those inside the belly, produce a basket of messengers that may induce cells to become cancer cells.
"The bottom line is that fat cells produce toxins that can wreak havoc, and not just in sugar and lipid metabolism, which can in turn lead to high risk for heart disease and diabetes. We now know this may also be mechanistically linked to the formation of a number of cancers," Lau says.
The team analyzed 141 articles that involved 282,137 cancer cases. In men, a higher BMI raised the risk of esophageal cancer by 52 per cent, thyroid cancer by 33 per cent and colon and kidney cancers each by 24 per cent.
In women, increased BMI raised the risk of endometrial and gallbladder cancer each by 59 per cent, cancer of the esophagus by 51 per cent and kidney cancers by 34 per cent.
The association for colon cancer was stronger in men than in women (24 per cent versus nine per cent). The magnitude of associations between increased BMI and cancer were similar across different populations, though the link between increased body fat and breast cancers was particularly strong in Asia-Pacific populations.
It's unknown whether excess weight in adolescence or early adulthood increases the cancer risk, or whether where the fat is located makes a difference. The team used BMI as a measure of body fat, but some say abdominal or belly fat might be a better predictor of cancer risk, which is the case for cardiovascular disease.
According to an accompanying article, obesity accounts for about 30,000 deaths in the U.K. each year, "and 10 times that" in the U.S., where obesity is now thought to have overtaken smoking as the main preventable cause of sickness and premature death.
It will take "multiple public health approaches to reverse the obesity epidemic, the Swedish authors write, including restricting junk food ads, taxing sugary drinks and other high-calorie, high-fat foods, lowering the cost of healthy foods and promoting physical activity in schools and workplaces.
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