Previous research has shown that people who don't move around much are prone to heart disease and type 2 diabetes but the reason is not known.
Researchers from the University of Missouri in the US tried to tease out the precise role of inactivity in causing ill health, The Mirror reported.
The team devised a novel approach: they stopped a group of very active people from exercising as usual.
The study got them to cut the number of steps they took each day by at least half.
The researchers wanted to find out whether physical laziness would stop the body from being able to control blood sugar which is the key disease-inducing factor for diabetes and heart disease.
Volunteers were fitted with glucose measuring devices so that their blood sugar could be checked continuously through 24 hours.
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They were asked to move about as little as possible but eat normally.
To examine their basic blood sugar control, these healthy people were told they could walk and exercise as normal for three days.
During these three days their blood sugar didn't spike at all after eating, a sign that they had perfect control over their blood sugar and they were ideally sensitive to insulin.
For the second part of the experiment, the volunteers became virtually sedentary and the time spent exercising fell to about three minutes.
The study found that during those three inactive days, blood glucose levels spiked after every meal (loss of blood sugar control) with the peaks being 25 per cent higher than during active days.
The peaks also grew each successive day, the study found.
In other words, blood sugar went more and more out of control, the longer the subjects remained inactive.