THURSDAY, Nov. 8 (HealthDay News) -- All those hours Americans spend in their office chairs or on their sofas may be packing on a particularly unhealthy form of fat around the heart, a new study suggests.

What's more, the fat stayed in place even when people undertook regular exercise, according to a study reported this week in Los Angeles at the annual meeting of the American Heart Association.

CT scans of more than 500 older Americans found that excess time spent sitting "was significantly related to pericardial fat around your heart," said study lead author Britta Larsen, a postdoctoral researcher in the department of cardiovascular epidemiology at the University of California, San Diego.

There have been numerous large studies recently suggesting that when it comes to its deleterious health effects, sitting is not just the absence of physical activity -- it has effects on the body that go beyond lack of exercise.

According to Larsen, that means that "even if you run every day but then you sit for eight hours a day, the sitting is still doing something bad for your health." She also noted that studies have found sitting to be detrimental to health even after scientists factored out excess weight gain.

"So, we wanted to see if sitting was related to the distributionof fat, because different types of fat are worse than others," Larsen said.

The study looked at data on 504 Californian adults, average age 65. In particular, Larsen's team examined CT scan data that showed how much of certain types of body fat were deposited in each participant's body.

"We looked at subcutaneous fat, which is stuff on the outside [for example, a "pot belly"]; then visceral fat, which is around your organs; intramuscular fat, which is actually in your muscles; intrathoracic [chest cavity] fat; and pericardial fat, which is around your heart," Larsen said.

The participants were also asked about the amount of time per week they spent sitting and how much time they had spent being physically active.

The study found that the more time spent sitting, the bigger the area of fat deposited around a person's heart, Larsen said. She explained that pericardial fat "is strongly related to cardiovascular disease. It gets in the way of heart function, it clogs up your arteries -- you don't want it there."

Prolonged sitting was not significantly related to any of the other types of fat, the study found.

There was also bad news for people who sit a lot but assume that they can exercise away all that pericardial fat. According to the study, regular exercise was notrelated to a lessening of pericardial fat, although it did help reduce visceral fat around the organs, which is strongly tied to diabetes and metabolic disease.

All of this means that for people who want to prevent the buildup of unhealthy fat deposits, exercise may not be enough.

The study "really emphasizes that [sitting and exercise] are two distinct behaviors," Larsen explained. "In order to really be healthy you need to focus on both -- get enough exercise but also not sit for 10 hours per day like most of us do."

She agreed that is probably a tall order for millions of office-bound Americans. Still, simple innovations, such as "standing desks," or getting up for a stroll every hour or two at work can help mitigate sitting's effects.

"Sitting really is bad for your heart," Larsen said. "So we are hoping that the workplace, especially, becomes more standup-friendly."

One expert said the study raises interesting issues, but it's too early to link sitting with heart disease and heart attack.

"Other studies have indicated that pericardial fat is associated with cholesterol blockages, particularly in people who are less obese," noted Dr. Stephen Green, associate chairman in the department of cardiology at North Shore University Hospital in Manhasset, N.Y.

However, he added, "in this study there was no direct linkage between sitting and heart attacks, or sitting and cholesterol blockages."

Study author Larsen also stressed that the study could only point to an association between sitting and pericardial fat, it could not prove a cause-and-effect relationship.

Dr. Sripal Bangalore, an assistant professor of medicine at NYU School of Medicine in New York City, added that the study may help unravel the mystery of why a "couch potato" lifestyle is so unhealthy.

"The current study shows that a sedentary lifestyle may have a differential increase in fat accumulation based on activity level," he said. "It is very intriguing and should be confirmed in future studies," Bangalore added.

"I guess until that time, it is time to sit less and be more active," said Bangalore, who is also director of research at the Cardiovascular Outcomes Group at NYU Langone Medical Center.

Findings presented at medical meetings are typically considered preliminary until published in a peer-reviewed journal.

More information

There's more on physical activity and the heart at the American Heart Association.