Decades of research have produced a simple, clear message: Being active is healthy, and physical inactivity is not. Exercise is necessary to stay fit and to stave off America's biggest killers – heart disease, diabetes and cancer. So if people get their 45 minutes of exercise in at the gym each day, they'll stay healthy for life, right? Wrong. An increasing amount of evidence suggests that modern, sedentary lifestyles, even for those with definite exercise routines, may put people at an increased risk of cancer.

A new analysis of existing research suggests that nearly 49,000 cases of breast cancer and almost 43,000 cases of colon cancer might be avoided if people simply spend less time being sedentary. That research was presented today at the American Institute for Cancer Research meeting in Washington, D.C. And scientists say it's not just about spending more time at the gym, but spending less time just sitting. In fact, many Americans, even the ones who exercise daily, are leading what researchers would call a sedentary lifestyle. Most people spend a majority of the day being inactive – sitting at a computer, commuting to work, eating meals, watching television.

Dr. Neville Owen, who studies the effects of sedentary behavior at the Baker IDI Heart and Diabetes Institute in Melbourne, Australia, estimates that these activities add up to an average of a whopping 15.5 hours of each day spent just sitting. He said the connection between sitting and cancer lies in physiological changes that occur when the body is inactive for long periods of time. "When you're sitting, the big muscles, especially in lower part of body, are completely unloaded. They're not doing their job," Owen said. That inactivity prompts changes in the body's metabolism, Owen said, and produces a number of biological signals, what scientists call biomarkers, which are linked to cancer.