WASHINGTON — Our culture of sitting may be responsible for 173,000 cases of cancer each year, according to new estimates.

Physical inactivity is linked to as many as 49,000 cases of breast cancer and 43,000 cases of colon cancer a year in the United States, said Christine Friedenreich, an epidemiologist at Alberta Health Services-Cancer Care in Canada.

Breast and colon cancer appear to be the cancers most influenced by physical activity, according to the research we have to date, Friedenreich said, in presenting her findings here today (Nov. 3) at the American Institute for Cancer Research (AICR) conference.

But her findings also suggested that an estimated 37,200 cases of lung cancer, 30,600 cases of prostate cancer, 12,000 cases of endometrial cancer and 1,800 cases of ovarian cancer could be prevented if people were more physically active.

The work adds to a growing body of research indicating that prolonged sitting has lethal consequences, regardless of how active people are the rest of the day.

"It seems highly likely that the longer you sit, the higher your risk [of cancer]," said Neville Owen, head of behavioral epidemiology at the Baker IDI Heart and Diabetes Institute in Australia, who also presented findings at the meeting. Owen's study showed that U.S. adults, on average, sit 15.5 hours a day.

The amount of time we spend standing up and walking "makes up such a tiny sliver of a person's waking hours," Owen said.

However, there's good news. It seems that long, uninterrupted periods of sitting are what is most unhealthy, and that by frequently breaking up long bouts of sitting with just a few minutes of light exercise, a person can lower his or her cancer risk. Owen's study found that one- to two-minute breaks from sitting can reduce levels of molecules in the body that are linked with cancer risk.

[Don't Sit Tight: 6 Ways to Make a Deadly Activity Healthier]

This research reveals that there's more to physical activity than working out on a treadmill. Alice Bender, a spokeswoman for AICR, noted that someone who exercises 30 minutes a day — the recommended amount of physical activity — is really only active for 3 percent of his or her day.

While getting to the gym or doing other regular exercise is still important, it is not the whole story, Bender said. The AICR recommends we take small breaks from sitting during our day to "infuse the remaining 97 percent of [our] day with short periods of activity that can protect against many cancers," she said.

Exercise and cancer

In Friedenreich's study, postmenopausal women who engaged in moderate to vigorous daily exercise had lower levels of C-reactive protein in their bodies after one year compared with women who did not engage in this level of activity. Low levels of this protein have been linked to reduced breast cancer risk.

C-reactive protein is a marker of inflammation, an immune response that normally helps your body fight off infection. Chronically high levels of inflammation may damage cells and possibly increase cancer risk.

Using data from her study and previous work on cancer indicators, Friedenreich estimated that daily exercise reduced the risk of breast and colon cancer by 25 to 30 percent.

"For many of the most common cancers, it seems like something as simple as a brisk walk for 30 minutes a day can help reduce cancer risk," Friedenreich said.

Owen's study suggested that even very brief exercise may reduce cancer risk. In the study, the one- to two-minute breaks were associated with smaller waists, less insulin resistance (an early sign of diabetes) and lower levels of inflammation— all risk factors for cancer.

Get up and move

The American Institute for Cancer Research recommends the following tips for breaking up your sitting sessions:

Set a timer on your computer to remind you every hour that it’s time to step away from your desk, and take a short walk down the hall.

Instead of emailing a co-worker, chat with him or her over a walk.

If possible, stand up and walk around during phone calls and meetings.

Keep light hand weights in your office to use while reading email or talking on the phone.

Pass it on: Sitting down for prolonged periods may increase cancer risk, researchers say.

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