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How much time does it really take for a healthy person to become measurably less healthy? Less than you might think.

According to new research from the University of Liverpool, just two weeks without regular physical activity can lead to muscular and metabolic changes that could potentially increase one’s risk of diabetes, heart disease, and possibly even premature death.

The new study is being presented this week at the European Congress on Obesity in Porto, Portugal, and has not yet been published in a peer-reviewed scientific journal. The findings are preliminary, but the authors say they still highlight the dangerous consequences of sedentary behavior—even for a short period of time.

To investigate just how much a two-week break from physical activity might impact healthy young adults, researchers recruited 28 men and women, average age 25, who didn’t work out regularly but walked about 10,000 steps a day. The participants had an average body mass index of 25, which is considered borderline between normal and overweight.

Before the study began, researchers measured the participants’ fat and muscle mass, mitochondrial function (a measure of how well they regulate energy and recover from exercise), and physical fitness. The participants were then asked to wear an activity tracker for two weeks, and to reduce their daily activity by more than 80%—to about 1,500 steps a day. They were also told not to alter their food intake over that time.

During those 14 days, the time people spent doing moderate-to-vigorous activity dropped by an average of 125 minutes a day—from 161 minutes to just 36. Daily sedentary time increased by an average of 129 minutes.

Not surprisingly, when the participants were re-checked after those two weeks, they had gained weight and lost muscle mass. Total body fat had also increased—especially fat around the abdomen, which is a major risk factor for developing chronic disease.

The researchers also noticed other changes that were less expected. Participants were unable to run for as long, or at the same intensity, as they could previously. They also experienced a decrease in insulin sensitivity, an increase in fat accumulated in the liver, and an increase in triglycerides (one component of cholesterol).

“We thought that we would see some subtle changes,” says co-author Dan Cuthbertson, PhD, reader and consultant for the University of Liverpool’s Institute of Ageing and Chronic Disease. “But when everything you measure gets worse in such a short time period, including these important risk factors for heart disease and type 2 diabetes, it is actually quite surprising.”

The results were also surprising because the study involved relatively young and healthy individuals, says lead author and graduate student Kelly Bowden-Davies. “If even those people were at risk, you have to think about what that means for patients who are older or less healthy, or who have other risk factors, like a family history of disease.”

The authors acknowledge that the changes were small, but they were statistically significant. If a sedentary lifestyle was continued for longer than two weeks, they say, those changes would likely become more pronounced.

And although the study participants drastically cut back their daily activity, Bowden-Davies points out that they were still going about their daily lives. “They still went to work or university, or looked after their children,” she says. “So this is a typical example of what some individuals are doing in society.” Even for people who are regularly active, it’s not hard to imagine how some lifestyle change—like a new job or a longer commute—could trigger this type of reduction in walking and other types of regular exercise, the authors say.

But there’s good news from the study, too: When participants resumed their normal activity after their sedentary period, their health measures returned to normal over the following two weeks.

“The effects were entirely reversible—so it’s fine if you’re fit and well and you go on holiday for two weeks and then you get right back to normal,” says Cuthbertson. “But the problem is that many people don’t reverse back to these levels of activity, and then perhaps the effects will accumulate.” The longer people are inactive, the harder it is to get back into shape, he adds, especially for those who already have health issues.

Cuthbertson also stresses that even if people can’t get to the gym or break a sweat regularly, just staying active—by sitting less and walking more—can help ward off the types of changes observed in this study. “Simply being less sedentary and maintaining a high step count has very clear health benefits,” he says.

The bottom line? Consequences from inactivity can occur sooner than people might realize, say the study authors, so it’s important to keep moving and avoid long periods of sitting. But if you have been getting less physical activity than normal, it’s also not too late to regain what you might have lost.