Watching television for hour upon hour obviously isn't the best way to spend leisure time - inactivity has been linked to obesity and heart disease. But a new study quantifies TV viewing's effect on risk of death.

Researchers found that each hour a day spent watching TV was linked with an 18 percent greater risk of dying from cardiovascular disease, an 11 percent greater risk of all causes of death, and a 9 percent increased risk of death from cancer.

The study, released in Circulation, a journal of the American Heart Assn., looked at health data among 8,800 men and women older than 25 who were part of the Australian Diabetes, Obesity and Lifestyle Study. Participants recorded their television viewing hours for a week, and researchers separated the results by amount of viewing: those who watched less than two hours of TV a day, those who watched two to four hours a day, and those who watched more than four hours a day.

The subjects also had oral glucose tolerance tests to determine blood sugar and gave blood samples to establish cholesterol levels at the beginning of the study. People with a history of cardiovascular disease were not included. In a follow up about six years later, 87 people had died due to cardiovascular disease and 125 of cancer.

Researchers found a strong connection between TV hours and death from cardiovascular disease, not just among the overweight and obese, but among people who had a healthy weight and exercised.

People who watched more than four hours a day showed an 80 percent greater risk of death from cardiovascular disease and a 46 percent higher risk of all causes of death compared with those who watched fewer than two hours a day, suggesting that being sedentary could have general deleterious effects. The numbers were the same after the researchers controlled for smoking, high blood pressure, high cholesterol, an unhealthy diet and leisure-time exercise.

"What we showed was that irrespective of a person's exercise level, sitting for four or more hours watching television was linked to a significant increase in risk of death compared to watching lower amounts of TV," said Dr. David Dunstan, lead author of the study and professor and head of the Physical Activity Laboratory at the Baker IDI Heart and Diabetes Institute in Victoria, Australia. "The message here is that in addition to promoting regular exercise, we also need to promote avoiding long periods of sitting, such as spending long hours in front of the computer screen."

To him, the results weren't unexpected. "When we're in that sitting posture, we're not using our muscles, and we know from extensive evidence that muscle contractions are important for the body's regulatory processes, such as the ability to break down glucose and use it as energy." That can cause insulin resistance, which can trigger a spike in blood sugar levels, possibly leading to type 2 diabetes.

Dr. Prediman K. Shah, director of the cardiology division of the Cedars-Sinai Heart Institute, agreed. He pointed out that muscles become deconditioned when not used, triggering harmful physiological changes. "If your activity is slowing down, you metabolize cholesterol less and synthesize it more," he said.

Even sporadic exercisers who sit for long periods need to increase their daily activity.

"The physical activity we do over a 24-hour period is important," says Dr. Gerald F. Fletcher, a cardiologist at the Mayo Clinic in Jacksonville, Fla., and a spokesman for the American Heart Assn.

That means taking the stairs instead of the elevator, gardening, walking the dog . . .

"For couch potatoes, sitting on your duff is hazardous to your health," Shah said. "The bottom line is keep moving."