A new study proves that the old adage "use it or lose it" is definitely true when it comes to fitness.

After just two weeks of sedentary behavior, formerly fit people had:

• A decline in heart and lung health
• Increased waist circumference
• Greater body fat and liver fat
• Higher levels of insulin resistance

"The study showed that two weeks of reduced physical activity—from approximately 10,000 steps per day down to 1,500 per day—caused changes in health markers that are associated with type 2 diabetes and cardiovascular disease," said study author Kelly Bowden Davies. She's a lecturer at Newcastle University and the University of Liverpool in the United Kingdom.

But the good news from the study is that the body seems to quickly bounce back once you start moving again.

"It's important to note that when people resumed their normal activity levels after this period, the negative health changes were reversed," she said.

The researchers recruited 28 healthy, regularly active adults. Eighteen were women. The average age of the study volunteers was 32. Their average body mass index (BMI)—a rough measure of body fat based on height and weight measurements—was just over 24. A BMI under 24.9 is considered normal weight.

The study volunteers had been quite active, normally clocking about 10,000 steps daily. Bowden Davies said most of this was just from daily activity, rather than structured exercise. She said they usually participate in no more than 2 hours of structured exercise weekly.

The researchers asked the volunteers to cut their activity drastically. They dropped an average of just over 100 minutes a day, the researchers said.

After two weeks of couch potato life, the study volunteers underwent a battery of testing. These results were compared to findings measured when the study started.

Bowden Davies said cardiorespiratory fitness levels dropped by 4% in just 2 weeks.

Waist circumference rose by nearly one-third of an inch. Liver fat increased by 0.2%. Total body fat went up by 0.5%. Insulin resistance increased and triglyceride (a type of blood fat) levels went up slightly.

Fourteen days after resuming activity, these measures all bounced back, the investigators found.

"Even subtle increases in activity can have a positive effect on health. Moving more and breaking up sedentary activity is encouraged," Bowden Davies added.

Dr. John Osborne, an American Heart Association spokesman, said this was a very interesting, and somewhat surprising, study. The findings validate advice he gives his patients. "If you can be a shark or a turtle, be a shark—always moving. This study showed you can lose the benefits of exercise very quickly, but the good news is that when they became sharks again, all the benefits came right back."

Another expert who reviewed the study, Dr. Edmund Giegerich, chief of endocrinology and vice chairman of medicine at New York-Presbyterian Brooklyn Methodist Hospital in New York City, was also somewhat surprised by the magnitude of changes that happened in just two weeks.

Giegerich said the study confirms how important it is to stay active.

"Going from being sedentary to more active can help a great deal in preventing the onset of type 2 diabetes. Just try to be more active. You'll feel better, and if you're trying to lose weight, it can help a little. You don't have to run a marathon. Walking is fine. Just get up and get moving," he advised.

Both experts pointed out that the study was small, and in a larger group, the findings might be different. The study was also only done for a short period of time.

Bowden Davies, Osborne and Giegerich all suspect that if people who are at a lower fitness level stop almost all of their activity that the results might even be worse.

The study was presented Wednesday at the European Association for the Study of Diabetes meeting, in Barcelona. Findings presented at meetings are typically viewed as preliminary until they're published in a peer-reviewed journal.