A study of more than 200,000 Australians adds to the growing body of evidence that people who sit the most die the soonest. It also found that you can't exercise this effect away, though exercise does help reduce it greatly.

The study's simple message is that spending more time standing and less time sitting prolongs life.

But while the death risk was much lower for anyone who exercised five hours a week or more, it still rose as these active people sat longer.

It is now well accepted that too much sitting is unhealthy. Studies in the last few years have found that death risks rise when people watch spend more leisure time in front of a computer screen or TV or simply sit too much.

The current study took a more direct approach, looking at the relationship of total daily sitting time to the likelihood of dying within the next three years, seeking to put a number on just how harmful prolonged sitting is.

Its most striking finding was that people who sat more than 11 hours a day had a 40% higher risk of dying in the next three years than people who sat less than four hours a day. This was after adjusting for factors such as age, weight, physical activity and general health status, all of which affect the death risk. It also found a clear dose-response effect: the more people sat, the higher their risk of death.

The results are part of the Sax Institute's 45 and Up Study, the largest ongoing study of healthy aging in the Southern Hemisphere. It compared the self-reported daily sitting time of 222,497 Australian adults 45 years or older with their likelihood of death in the next three years.

Healthy or sick, active or inactive, the more people sat, the more likely they were to die in the next three years. Physical activity did reduce this risk significantly: the 40% higher death risk found when comparing people who sat the most to those who sat the least increased to 100% when comparing those who sat the most and exercised the least to those who sat the least and exercised the most. But while the death risk was much lower for anyone who exercised five hours a week or more, it still rose as these active people sat longer.

In other words, people still need to exercise, but it's also important to spend less time sitting.

An accompanying editorial suggests that the evidence is now strong enough that doctors should prescribe reduced sitting time to their patients. But there's no reason that people can't be proactive and write their own prescription here.

It's been estimated that the average adult spends 90% of their leisure time sitting. So there's plenty of room for improvement.

An article on the study appears in Archives of Internal Medicine.