Most in U.S. unaware of cancer-obesity link

WASHINGTON (CNN) -- The problems of obesity and cancer are major concerns for many Americans, but the majority appear unaware that being overweight increases their risk for the disease.

An American Institute for Cancer Research study sought to see how much Americans knew about a link between obesity and the risk for cancer. The institute and World Cancer Research Fund presented the data Thursday at a scientific conference on diet and cancer in Washington.

The results of a June poll of 1,205 adults show that most Americans know their chances of getting heart disease and diabetes rise when they are overweight, but just 25 percent of those polled knew the risk of cancer also increases.

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"Gaining half a pound per year or five pounds per decade" can contribute to cancer risk, said Dr. George Bray, professor of medicine at Louisiana State University Medical Center, who presented some of the data at the conference.

Sixty-one percent of Americans are either overweight or obese, according to the most recent statistics from the Centers for Disease Control and Prevention.

And more than half a million people will die from cancer in the United States this year, the American Cancer Society says. The World Health Organization estimates one-quarter to one-third of cancer cases worldwide are related to being overweight and physically inactive.

The risk for post-menopausal breast cancer, colon cancer, prostate cancer, esophageal cancer, endometrial cancer and kidney cancer increases among nonsmoking overweight and obese adults, according to the report.

The amount of weight gain that can lead to higher cancer risk builds up over time, researchers have found.

"Gaining weight over a long period of time -- that causes health problems," Bray said.

At highest risk were people who gained 22 pounds or more beyond about age 20, Bray said. People should limit weight gain during adulthood to no more than 11 pounds, he said.

Excess body fat appears to be the culprit. Bray calls fat cells "hormone pumps," which produce higher levels of insulin and excess estrogen, both contributing to accelerated cell division and replication.

Recent research suggests that the more cells duplicate, the higher the chance that something can go wrong in the cell and a cancer cell is born. Then these additional hormones lead to the rapid reproduction of the cancer cells.

Also, fat cells can keep carcinogens -- cancer-causing agents -- trapped in the body, which can lead to the development of cancer.

The American Institute for Cancer Research report recommends that people should "strive to be physically active," including moderate exercise such as brisk walking for 30 minutes a day and vigorous activity such as jogging for one hour a week.

The American Cancer Society defines "moderate" exercise as walking a mile in 15 minutes, said Colleen Doyle, the society's director of nutrition and physical activity.

Research indicates that "science increasingly shows that vigorous exercise, as opposed to moderate exercise, is more beneficial for breast cancer and colon cancer," Doyle said.