WASHINGTON -- Heart disease and diabetes get all the attention, but expanding waistlines increase the risk for at least nine types of cancer, too. And with the obesity epidemic showing no signs of waning, specialists say they need to better understand how fat cells fuels cancer growth so they might fight back.
What's already clear: Being overweight can make it harder to spot tumors early, catch recurrences, determine the best chemotherapy dose, even fit into radiation machines.
That in turn hurts chances of survival. One major study last year estimated that excess weight may account for 14 percent to 20 percent of all cancer deaths - 90,000 a year.
"Obesity makes taking care of cancer patients much more complicated," says Dr. Christopher Desch, a medical oncologist in Richmond, Va.
So why is cancer often the afterthought when listing obesity's multiple risks?
"The cancer picture is a little bit more subtle," says American Cancer Society epidemiologist Eugenia Calle, one of the nation's leading specialists on the link.
The risks of heart disease and diabetes from packing on pounds are much higher, and more immediate because cancer typically develops more slowly than those illnesses, she explains.
But with nearly two-thirds of U.S. adults now overweight plus an aging population - cancer is predominantly an older person's disease - oncologists want more attention to the link.
Fat is known to increase the risk of developing cancers of the colon, breast, uterus, kidney, esophagus, pancreas, gallbladder, liver and top of the stomach.
How big a role girth plays varies greatly, and the strongest connections are actually in less common cancers.
Weight is most strongly linked to cancer of the uterine lining, or endometrium. An overweight woman has twice the risk of developing that cancer as a lean one; once she becomes obese, the risk rises as much as 3.5- to 5-fold.
The obese have up to triple the risk of kidney cancer and a type of esophageal cancer as do the normal-weight.
The risk is somewhat smaller among two of the nation's most common cancers:
-Overweight or obese men are 50 percent to twice as likely as lean men to get colon cancer. For women, the extra risk is 20 to 50 percent. No one can yet explain the gender difference.
-Fat is linked to breast cancer in postmenopausal women only, increasing risk of the disease by 30 percent among the overweight and 50 percent among the obese.
For the other four cancers, the obesity risk falls somewhere in between.
The reason for the variation: Fat cells apparently play different roles that can spur different types of cancer growth.
"Fat cells are not just static storage depots," explains Calle. The worst, because it's most metabolically active, is visceral fat, the kind that builds up in the abdomen and surrounds organs there.
But exactly how fat cells work isn't well understood. They can spur surges of insulin and proteins that may in turn unleash out-of-control growth among certain cell types.
They also trigger inflammation, a known culprit in heart disease that's now increasingly suspect in cancer, too.
Another role: Fat cells make estrogen, which fuels breast cancer. After menopause, fat becomes a woman's leading source of estrogen. While anti-estrogen therapies are common, the fatter a woman is, the harder it is to lower hormone levels - one reason why the obese have at least a 50 percent greater chance of dying from breast cancer than lean patients, Calle says.
Then there are organ-by-organ reactions. For example, the obese are particularly prone to "gastroesophageal reflux," frequent heartburn where a backup of stomach acid onto the delicate esophagus eventually can lead to esophageal cancer.
Sorting out fat's roles could lead to new therapies, and while there's no clear evidence yet, it makes sense that losing weight would lower cancer risk, Calle says.
For the already diagnosed, the stereotype of cancer treatment causing dangerous weight loss isn't true for every cancer. Breast cancer chemotherapy, in fact, often causes weight gain, says Desch, speaking for the American Society of Clinical Oncology.
His advice: Exercise as much as possible, eat lots of fruits and vegetables and take a multivitamin during treatment - and try to achieve a healthy weight after battling the initial cancer.