One reason being sedentary is so bad for health may be that it promotes "hardening" of the arteries with calcified deposits, a new U.S. study suggests.

Research with middle-aged volunteers found that each additional hour of sedentary time was linked to 12 percent higher odds of having calcium buildup in the coronary arteries, an early sign of coronary heart disease.

"This is one of the first studies to help tease out the ways in which sedentary time relates to heart disease risk, by evaluating this early marker of atherosclerosis in the heart arteries," said study coauthor Julia Kozlitina of the University of Texas Southwestern Medical Center in Dallas.

The researchers analyzed data on more than 2,000 participants in the Dallas Heart Study who had measures of physical activity based on wearable tracking devices and had coronary artery calcium scans.

Participants' average age was 50 years old and about half were black. Overall, the volunteers spent between one hour and 11 hours per day sedentary, and spent between zero and 200 minutes a day doing moderate to vigorous physical activity, with an average of 29 minutes.

About one-quarter of people in the sample had some detectable coronary artery calcium, Kozlitina said.

Participants who were the most sedentary tended to be older, to have diabetes, high blood pressure and higher body mass index. They also were more likely to have coronary calcium, the study team reports in the Journal of the American College of Cardiology.

Accounting for age and other factors, the researchers linked extra hours of sedentary time to higher risk of having coronary artery calcium. Time spent exercising was not tied to the likelihood of coronary calcium, however.

A single week of physical activity monitoring may not be representative of lifetime exercise habits, and can't necessarily prove that being sedentary causes coronary artery calcium to accumulate, only that the two factors are linked, the authors point out.

"The most interesting finding from this study is that sedentary time, but not moderate to vigorous physical activity, was associated with coronary artery calcium," said Qibin Qi of the Albert Einstein College of Medicine in New York, who was not part of the study.

"Besides exercise in the gym and a walk during lunch break, breaks in sedentary behavior might help," he said by email. "That means getting up from your desk job to move around once in a while (e.g., get a cup of tea) could be beneficial. Future studies will need to look at the optimal length and frequency of breaks from sedentary time."

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Exercising and being sedentary may both influence cardiovascular disease, but by different pathways, Qi told Reuters Health.

Maintaining a healthy diet, not smoking and keeping alcohol intake light or moderate also help prevent cardiovascular disease, Qi said.

"Try to take a one to five minute break every hour; stand up; walk up a flight of stairs instead of taking the elevator; etc.," Kozlitina said by email. "All of this helps in a small way."