The more weight you carry on your body, the greater your odds of developing cancer, British researchers report.
This is true not only of fairly common cancers such as colon and breast, but also of lesser known varieties, including gallbladder. Moreover, the degree of risk differs between men and women and among different ethnic groups, report the authors of a comprehensive new paper appearing in this week's issue of The Lancet.
"This is a profoundly important issue. Obviously, the obesity epidemic is a huge problem itself, and the relationship to cancer is only one of the many adverse health effects of being overweight and obese," said Dr. Michael Thun, head of epidemiological research at the American Cancer Society. "The evidence has been accumulating now for over 10 years. . . This study tries to provide a quantitative measure of how much the relative risk goes up with each increment, basically jumping from one BMI [body-mass index] category to another."
Although extra fat has already been identified by research as a risk factor for several different types of cancer, Thun said, "the problem of obesity is so large and so difficult to solve that there's a very sound reason for ongoing studies of things that have become increasingly well-known, just because it helps the momentum in stimulating approaches that will actually help people maintain a healthy weight."
Last year, a report issued by the American Institute of Cancer Research and the U.K.-based World Cancer Research Fund concluded that body fat is associated with an increased risk for several different types of cancer including esophageal adenocarcinoma, as well as cancers of the pancreas, colon and rectum, breast (postmenopausal), endometrium and kidney.
Although that report was one of the most comprehensive to date, it did leave some questions unanswered. For instance, are there associations between less common cancers and body weight, and do the associations differ between the sexes and people of different ethnic backgrounds?
The issue is a pressing one, with about two-thirds of adult men and women in the United States overweight or obese. That number is only expected to increase as people continue to eat more and exercise less.
This study, from scientists at the University of Manchester, analyzed 141 articles involving 282,137 cancer cases and 20 different types of malignancies to determine the cancer risk associated with a 5 kilogram-per-meter-squared increase in BMI, roughly the increase that would bump a person from middle-normal weight into overweight.
In men, such an increase in BMI raised the risk of esophageal adenocarcinoma by 52 percent, thyroid cancer by 33 percent, and colon and kidney cancer by 24 percent each.
In women, the same increase in BMI increased the risk of endometrial and gallbladder cancer by 59 percent each, esophageal adenocarcinoma by 51 percent, and kidney cancer by 34 percent.
In men, there were weaker associations between increased BMI and rectal cancer and melanoma. In women, there were weaker associations between increased BMI and postmenopausal breast, pancreatic, thyroid and colon cancers.
In both genders, there were associations between increased BMI and leukemia, multiple myeloma and non-Hodgkin's lymphoma.
For colon cancer, the associations were stronger in men than in women (24 percent vs. 9 percent).
There were stronger associations in Asia-Pacific populations between greater BMI and both premenopausal and postmenopausal breast cancers.
Although the main message is still to maintain a healthy weight, this research might indicate earlier screening for certain cancers, said Dr. Greg Cooper, interim chief of the gastroenterology division at Ireland Cancer Center of University Hospitals and Case Comprehensive Cancer Center in Cleveland. "If someone is obese, then lower the threshold for screening," he said. "One of the cancers they identified is esophageal adenocarcinoma, which is not as common as colon cancer, but it is increasing in incidence. It is thought to be related to reflux, so as a gastroenterologist, if I have a patient who has reflux and is obese, I might lower the threshold for doing an endoscopy. For other cancers like colon cancer, those guidelines are pretty well-established, and this probably wouldn't change practice."
Experts aren't sure why extra fat can lead to malignancies, but changes in the circulating levels of various hormones (insulin, insulin-like growth factors and sex steroids) might explain the link.
Here's more bad news as the world heads for a smoke-free future: An accompanying commentary from Swedish researchers notes that as people quit smoking (the biggest cause of cancer in developed countries), weight gain may become the main lifestyle factor contributing to new cancers.