Just two weeks of normal office life without exercise can put people on the path to diabetes, researchers have found.
Sitting at a desk all day, driving to work and lazing on the sofa at weekends triggered ill- health in all the participants of a new study.
The research team at Liverpool University warned that Britons' increasingly inactive lives could damage their long-term health, potentially leading to conditions such as type 2 diabetes, heart disease and strokes.
However, the experiment also showed the ill-effects can be reversed with simple steps like taking the stairs instead of the lift, getting off a bus a stop early and going to the supermarket instead of shopping online.
Lead researcher Dr Daniel Cuthbertson warned a sedentary lifestyle can "sow the seeds for future disease".
He said: "'Through various advances, our society has become a lot more sedentary. "Our grandparents did washing manually, they did manual labour, people were physically active.
"Now many of us are based at desks tapping away - we don't even go out to do our shopping.
"Sedentary behaviour has a number of adverse health effects that over an extended period of time could be harmful."
Researchers followed 45 people with an average age of 36 who did not regularly do any active exercise like jogging or going to the gym but all walked at least 10,000 steps a day as part of their daily lives.
They were asked to become inactive for two weeks. This involved getting public transport or driving to work, taking lifts or escalators instead of stairs, cutting their steps down to around 1,500 a day and spending the weekend at home, mainly watching TV or playing computer games.
Researchers checked their activity levels using a tracker on their arms. All were also asked to stick to their normal diets and keep a food diary to show they had not changed what they ate.
After two weeks, tests showed participants had increases in their fat levels and waist sizes, and showed signs of muscle loss and lower cardio-respiratory fitness.
Their bodies were also less able to respond to the hormone insulin - a symptom which can be a precursor to diabetes.
After participants had resumed their normal activity levels for 14 days, the negative effects were reversed.
Dr Cuthbertson added: "Even in two weeks, the transition from busy bee to couch potato provoked subtle changes which over months or years would predispose people to certain diseases."