Sitting around could be waiting for cancer to strike say researchers

By Dr. Ananya Mandal, MDNov 6 2011

A new analysis reveals that more than 90,000 new cancer cases a year in the United States may be due to physical inactivity and prolonged periods of sitting. The researchers presented their findings at the annual conference of the American Institute for Cancer Research (AICR) in Washington, D.C., citing about 49,000 cases of breast cancer and 43,000 of colon cancer from over 200 studies worldwide.

“This gives us some idea of the cancers we could prevent by getting people to be more active,” said epidemiologist Christine Friedenreich of Alberta Health Services in Calgary, Canada. Calculations are based on U.S. physical activity data and cancer incidence statistics. “This is a conservative estimate…The more physical activity you do, the lower your risk of these cancers,” she said.

Friedenreich reviewed more than 200 cancer studies worldwide and found convincing evidence that regular physical activity reduces the risk of breast cancer, colon cancer and endometrial cancer by 25% to 30%. There's some evidence that regular exercise also reduces the risk of lung, prostate and ovarian cancer, she said. Emerging research indicates that prolonged sitting also increases the risk of some types of cancer, such as colon, endometrial and ovarian cancers, Friedenreich said.

Alpa Patel, an American Cancer Society epidemiologist who looked at the data, said the numbers “seem like very reasonable estimates.” Experts have known for years that physical activity decreases the risk of chronic diseases including cancer, heart disease and diabetes, Friedenreich said, but the new data give estimates on the number of cases that might be prevented if people were more physically active. Patel and others also have investigated the health dangers of sitting too long without moving around, which is called “sitting disease.”

In a study of 123,000 people, Patel found that the more time people spent sitting, the higher their risk of dying early. “Even among individuals who were regularly active, the risk of dying prematurely was higher among those who spent more time sitting,” she said. Even if you are doing half an hour of aerobic activity a day, you need to make sure you don't sit the rest of the day, Patel said. “You have to get up and take breaks from sitting.”

James Levine, a professor of medicine at Mayo Clinic in Rochester, Minn., says many people sit an average of seven to 9 and half hours a day. “If you've sat for an hour, you've probably sat too long,” he said. “A brisk daily walk of at least 30 minutes could lower a person's risk over time for breast cancer and colon cancer,” says Alice Bender, a registered dietitian with AICR.

Friedenreich said looking into why exercise reduces cancer risk she found some clues. In a study of 320 post-menopausal women, she has found that physical activity appears to decrease the risk of cancer by increasing insulin sensitivity and reducing body fat, inflammation, metabolic hormones and sex steroids hormones.