Just two weeks of a couch potato lifestyle increases the risk of serious disease, British research has suggested.
Experts found that 14 days of sitting around reduces muscle mass, increases body fat and raises the potential for high cholesterol.
Taking 10,000 steps a day - widely regarded as a target for maintaining good health - should be something people strive for, they said.
The team, from the University of Liverpool, followed a group of 28 healthy people of a normal weight with an average age of 25.
Participants usually took 10,000 steps per day or more but did not have more than two hours of structured exercise - such as going to the gym or playing sport - per week.
For 14 days, people wore a SenseWear armband, which lets researchers track levels of physical activity, steps, sleep and lifestyle.
The group also had health checks on things such as fat, muscle mass and physical fitness at the start and the end of the study.
For the research, people were told to reduce their activity levels by more than 80 per cent to about 1500 steps per day.
They were also told to eat their normal meals and keep a food diary.
Over the course of the study, exercise levels dropped from a daily average of 161 minutes to 36 minutes.
At the same time, the amount of time spent sedentary - such as sitting down - increased by an average of 129 minutes.
The results showed significant changes to the body, including loss of muscle mass (average loss 0.36kg) and increases in total body fat, with central body fat going up by about 1 per cent.
There was also an increase in liver fat and an increase in bad cholesterol markers.
Overall, cardio-respiratory fitness levels also declined.
Dr Dan Cuthbertson, who led the work and is presenting his findings at the European Congress on Obesity in Porto, Portugal, said any physical activity - even walking - is better than being sedentary.
"This doesn't need to be structured exercise - it could be things like getting off the bus a stop earlier or walking to the shops instead of driving," he said.
"If you think of a typical mum at home who is always busy and on the go but doesn't go the gym regularly, there are still significant health benefits in what she's doing."
He said people who do not exercise risk obesity and illnesses such as Type 2 diabetes.
