Nov. 5, 2009 -- As many as 100,000 cases of cancer could be prevented in the U.S. each year if Americans get rid of their excess body fat.
That's according to estimates released by the American Institute for Cancer Research. The estimates suggest that heart disease, diabetes, and joint problems aren't the only illnesses in which rampant obesity is causing havoc.
A 2007 report from the American Institute for Cancer Research and the World Cancer Research Foundation reviewed hundreds of studies and found what researchers called "convincing evidence" that obesity was tied to several cancers. Those included cancer of the esophagus, pancreas, and kidneys. It also included colorectal cancer and endometrial cancer (a form of uterine cancer).
Researchers also said it was "probable" that excess abdominal fat was a cause of breast cancer in postmenopausal women.
Experts took estimates of obesity's influence on cancer and applied them to a breakdown of the approximately 1.6 million U.S. cancer cases per year.
The researchers estimate that excess body fat is the cause of 33,000 breast cancer cases each year, nearly one-sixth the total cases in postmenopausal women. Obesity could be to blame for nearly 21,000 cases of endometrial cancer and more than 13,000 cases of colorectal cancer per year.
Researchers stressed that the figures are only estimates, and that individual cancer cases can have many, inter-connected causes.
"We believe these estimates are as good as it is possible to achieve, given the available data," says Tim Byers, MD, PhD, interim director of the University of Colorado Cancer Center and a co-author of the report.
Cancer is more often blamed on influences like smoking and other toxic exposures than it is blamed on obesity. Smoking does cause many more malignancies than excess body fat.
But Larry Kolonel, MD, PhD, deputy director of the Cancer Research Center of Hawaii, says there are strong reasons to believe that excess fat can give rise to cancer. Fat cells produce estrogen, which are now known to be a factor in breast cancer and endometrial cancer. Fatty tissue also affects the way the body metabolizes insulin, which can alter how sugar is processed and how it ultimately gets to cells.
Fatty tissue, also known as adipose tissue, produces hormones on its own that could play a role in promoting cancer cells, Kolonel says. It also has been shown to produce chronic, low-grade inflammation in the body. That inflammation can spark immune responses that may also be linked.
"It is not implausible that adipose tissue can be a risk factor or a causal factor for cancers," he says.
The estimates suggest maintaining a normal weight could prevent half of all endometrial cancers, a third of all esophageal cancers, and a quarter of all kidney cancers.
"We can have a very substantial influence," Kolonel says.