Computers can make you fat - and not just from lack of exercise.
Research shows that staring at a computer screen makes us work up an appetite for sweet treats like chocolate cake and biscuits.
It is thought that the mental stress of computer work triggers changes in blood sugar and hormone levels that trick the brain into thinking it has worked off lots of calories that need replaced.
Computer games and TV watching have the same effect, the International Congress on Obesity in Stockholm heard.
Researcher Jean-Philippe Chaput suggests that office workers who want to keep trim should take regular screen breaks - and even consider typing while standing up.
Dr Chaput, of the University of Copenhagen, started to research the subject after noticing how is supervisor regularly snacked on chocolate biscuits when doing computer work.
The researcher asked one group of female students to condense some text into an on-screen summary and another group to simply relax for 45 minutes.
Those doing the computer-based task burnt just three more calories than the others - but ate much more food when given access to a buffet afterwards.
In fact they took in an extra 230 calories - the equivalent of a two-fingered Kit Kat and a Cadbury's Twirl. Chocolate and fatty foods were particularly popular.Experiments showed that the computer work made blood sugar levels fluctuate wildly and hunger hormones to rise. As a result, the students gorged on food they didn't need.
In contrast, physical exercise leads to far fewer fluctuations - meaning we are less prone to overeat afterwards.
Dr Chaput said: 'Working on your computer is sedentary and doing nothing increases your weight. But use of them also increases your appetite as well so there is a double whammy in effect.'
The popularity of the games has led to them being increasingly used in anti-obesity drives in schools and prisons.
But Dr Chaput said: : The next thing is to look at new computer games like the Wii.
'The message is to use it to increase energy expenditure. If they over-compensate by eating more then they don't benefit.
Bad for your health ?
Working at a computer makes us work up an appetite for sweet treats like chocolate cake and biscuits.
'Maybe we are thinking we are doing a good thing for kids and teenagers but maybe they over-compensate by eating more and we need to know that.'
In the meantime, he suggests that people who use computers a lot take regular - and active - screen breaks.
Physical exercise - and the more strenuous the better - counteracts the hunger pangs, or hyperphagic effects, of sitting at a computer.
'In my view the solution is not to stop using computers and those things but maybe to compensate this excess mental activity by having good physical activity regimen to prevent the hyperphagic effects of mental work.'