THURSDAY, Nov. 3, 2011 (HealthDay News) -- The hours Americans spend sitting may be increasing their risk for cancer, just as the time they spend exercising can reduce the risk, according to new research.
"For colon and breast cancer we can now say there is convincing evidence that being physically active reduces your risk of developing those two major cancers," said Christine Friedenreich, a senior research epidemiologist at the Alberta Health Services-Cancer Care in Calgary, Canada.
"It's not enough to just meet the physical activity guidelines like doing 150 minutes a week of exercise," she added. "If you are spending the rest of your time sitting, like in front of a computer, that may be a problem."
For colon cancer, regular exercise can reduce the risk up to 35 percent, Friedenreich said. "There's a dose response -- that means more physical activity lowers the risk more," Friedenreich said. The same is true for breast cancer, where exercise can reduce the risk up to 25 percent.
One expert said the new findings do support a connection between sedentary lifestyles and cancer.
Dr. Freya Schnabel, director of breast surgery at NYU Langone Medical Center in New York City, said that "there is beginning to be evidence that the way obesity increases the risk of cancer is through an increase in inflammation in the body, and by doing exercise you can lower the makers of inflammation, which might lower the risk of cancer." But she stressed that more research is needed to clarify these links.
Friedenreich agreed. Although the connection between lack of exercise and the increased risk for heart disease is well-established, the association between a "couch potato" lifestyle and cancer risk is a relatively new finding and one that needs further investigation, she said.
Friedenreich, who has studied the connection between exercise and cancer for years, is slated to present the findings Thursday at the American Institute for Cancer Research (AICR) annual conference in Washington, D.C.
In her most recent work, Friedenreich and her colleagues have found an association between exercise and the reduction of markers of inflammation, such as one called C-reactive protein, which might explain how exercise reduces the risk of breast cancer.
At the moment there is no conclusive evidence that increases in these markers of inflammation actually cause cancer or increase the risk of developing cancer, Friedenreich noted. It's probably a much more complicated process, she said.
"It's a bunch of different mechanisms that are going to have an impact," she said. "We think there are probably pathways through body fat, through hormones, through insulin resistance. Inflammation is one way cancer is affected; I wouldn't say it's the primary one, it's just one of the pathways."
The study also appears in the October issue of the journal Cancer Prevention Research.
It's not just lack of exercise that increases these markers of inflammation -- just sitting around or leading a sedentary life style may have the same effect. Friedenreich's presentation also describes findings from a study first published in 2009 in the British Journal of Sports Medicine. It suggested that simply sitting around watching TV can cut years off your life.
To counteract the risk of leading a sedentary life, Friedenreich suggests regular exercise. But she also suggests that taking breaks from sitting, especially at work, will help lower inflammation and also the risk of cancer.
"Too much sitting, sedentary behavior, actually increase the risk of cancer," she said. "Some of the mechanisms seem to be the same as for lack of physical activity. If you can break up your sitting time, even by little bits, that can help reduce your risk of cancer," Friedenreich said.
Experts estimate that people sit about 15.5 hours a day, including at meals, traveling to and from work or school, working on the computer and watching television. Office workers may spend 75 percent of their time sitting.
"It is clear to me it's never too late" to start exercising to reduce your risk of cancer, Friedenreich said. "Anything you can do to maintain a healthy body weight, especially later in life, will reduce the risk of colon and breast cancer."
Of course people who lead sedentary lives are also at risk for other unhealthy habits that can lead to obesity, a known risk factor for some cancers. In addition, sedentary people might not eat the most healthful foods, which also increase the risk for some cancers, experts say.
Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, commented that "we know that exercise is a good tool to maintain normal body weight."
But, it isn't known whether reducing inflammation markers in the body really reduces the risk for cancer, she said.
"The things that are good for you in general appear to be good for you in terms of reducing your risk for some cancer," Bernik said.
Sit Less, Move More at Work
The AICR presentation includes tips for getting active at work:
For more information on exercise and cancer, visit the U.S. National Cancer Institute.