The study, by scientists at the American Cancer Society, found that carrying around extra pounds is second only to smoking in terms of its overall effect on cancer mortality, raising the risk by as much as 50 percent to 60 percent for the heaviest people.
Eugenia E. Calle, an epidemiologist with the cancer group and lead author of the study, says being overweight increases the activity of certain hormones and proteins, such as estrogen and insulin, which can stimulate tumors. Not only does weight influence cancers of sex-neutral organs such as the esophagus, colon, liver and gallbladder, it also affects sex-specific sites such as the breasts, ovaries and cervix in women and the prostate gland in men.
The good news, Calle says, is that losing weight should reverse the effect. "There's every reason to assume that if you could lose weight and sustain the loss, then your cancer risk would decrease," says Calle, whose study appears in the April 24 issue of The New England Journal of Medicine.
More than 60 percent of American adults are overweight. Last year, more than 1.2 million Americans were diagnosed with cancer, according to the cancer society.
Most people know that smoking is the single greatest risk factor for many forms of cancer. But a recent survey from the cancer group found that fewer than 1 percent of Americans understand that being overweight can also raise the risk. "When you ask people about obesity they think of diabetes, heart disease and hypertension," Calle says.
The study, which began in 1982, included more than 900,000 men and women who were free of cancer at the start. Over the next 16 years, 57,145, or about 6 percent, died of cancer.
After controlling for smoking, the researchers found that overweight and obese people were more likely to die of cancer than others. The risk was greatest for those with a body mass index (BMI) -- a comparison of weight to height -- of at least 40, a figure considered dangerously obese. For this group, the chances of dying from cancer during the study period were 52 percent greater for men and 62 percent greater for women than if they had a normal body mass index. A person with a BMI of at least 25 is considered overweight, and one with a BMI of 30 or over is considered obese.
Not only is raw weight gain important in cancer, but the way those extra pounds are distributed in the body makes a difference, too, Calle says. People who carry their fat around the abdomen are at increased risk of cancer of the esophagus, the result of more gastric reflux that sends stomach acid back up the wrong way.
The researchers estimate that more than 90,000 deaths a year in this country could be avoided if people maintained a body mass index of no more than 25. Being overweight is also a major risk factor for cardiovascular disease, diabetes and several other serious health problems.
But Morgan Downey, executive director of the American Obesity Association, says thinning the country isn't going to be easy. "The strategy to get there is not apparent," says Downey, referring to a national BMI of 25 or less. "We need a lot more research to understand what works in obesity prevention and treatment."
Find your BMI with a calculator from the U.S. Centers for Disease Control and Prevention.