Sitting at a desk at work doesn't seem to increase the risk of heart disease, and yet lounging in front of the TV at home does.

Previous studies have concluded the more sedentary people are, the more likely they are to develop heart disease and suffer an early death.

But researchers from Columbia University's Irving Medical Center found it wasn't quite that simple. They tracked the lives of 3,592 African Americans, who are at higher risk of diabetes and heart disease anyway, and found that only sitting at home increased the chances further. Those who watched the most TV every day—four hours or more—had a 50 per cent increased risk of heart disease compared to those who watched TV for two or fewer hours every day.

But sitting at a desk at work didn't raise the risk at all, the researchers found after following the group for nearly nine years, and it didn't seem to matter just how long people were at their desks.

The one mitigating factor for those who watched the most TV was engaging in 150 minutes or more of strenuous exercise every week. Those who also exercised saw their risk disappear, the researchers found.

The researchers aren't sure why sitting at home and the office should carry different risks. Lead researcher Keith Diaz said: "It may be that most people tend to watch television for hours without moving, while most workers get up from their desk frequently. The combination of eating a large meal such as dinner and then sitting for hours could also be harmful."

Even if you don't do strenuous exercise, getting up from the easy chair and taking a short walk could also lower your risk, he added.