Sitting target: How long periods of inactivity slash years off your life

Michael Jensen is talking to me on the phone, but his voice is drowned out by what sounds like a vacuum cleaner. Or maybe it's a lawnmower. I'm used to bad connections, but Jensen isn't using Bluetooth on a busy motorway. He's in his office at one of the United States' top medical-research facilities.

"I'm sorry," he says, when I ask about the noise. "I'm on a treadmill."

I'd had a similar experience with David Dunstan, an Australian researcher who talked to me on his speakerphone as he walked around his office at the Baker IDI Heart and Diabetes Institute, in Melbourne.

It's not that Jensen and Dunstan are hyperactive. Rather, both are exercise researchers looking into the link between sitting down and premature death. What they have found is clearly disturbing enough for them both to make sure they spend most of the day on their feet.

Jensen explains that he and his colleagues at the Mayo Clinic in Rochester, in the American state of Minnesota, were studying weight control when they discovered that some people "spontaneously start moving round and don't gain weight" when they have overeaten. These people don't dash to the gym - they just walk more, hop up from the couch to run errands or find other excuses to get on their feet. "This really got us thinking about this urge to move," Jensen says, "and how important that might be for maintaining good health."

That led them to a field known as "inactivity research", which reveals that inactivity, particularly in the form of sitting, is really bad for your health. It might sound like an obvious statement, but the killer point is this: inactivity is bad for you even if you exercise as well. Going to the gym is not a licence to spend the rest of the day on your backside.

In 2010, a team led by Alpa Patel of the American Cancer Society in Atlanta, in the US, analysed the data from a 14-year study of 123,000 middle-aged adults. When they compared mortality rates between those who spent six hours a day or more sitting and those who reported three hours or less - and taking into account other factors such as diet - they found something surprising. Extra time on the couch was associated with a 37 per cent higher mortality rate for women and 17 per cent higher for men. It is not clear why there is such a big gender difference.