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We know we’re supposed to exercise regularly — but how regularly? How long does it take for a lack of exercise to have measurable negative effects on our health? 

As it turns out, less time than anyone expected: three to five days, according to Leryn Reynolds, Ph.D., an associate professor of exercise science at ��㴫ý. In better news, as little as one workout can bring some improvement. 

“The overarching goal of my lab is to look at the physiological links between physical activity or inactivity and cardiometabolic health,” Dr. Reynolds said. “Specifically, right now, we're looking at how quickly short-term inactivity can impact your blood vessel health, as well as postprandial blood glucose, or glucose levels after you eat a meal.” 

For the past three years, Dr. Reynolds has conducted research supported by a $154,000 American Heart Association grant on how short-term alterations in physical activity or inactivity impact cardiometabolic health in older adults.  

To evaluate these effects, Dr. Reynolds did something a little counterintuitive: She asked groups of older and young adults who were regularly active to stop exercising. 

The idea wasn’t for the participants to confine themselves to bed, but to choose more sedentary activities. On average, participants ended up taking just over 5,000 steps per day, which is what the average American takes, she said. 

In addition to believing that being physically inactive would reduce endothelial function and result in higher blood glucose, Dr. Reynolds and her team thought adults over the age of 55 would show more accelerated or severe effects compared to younger adults. 

“Surprisingly, we did not find that,” she said. Instead, people with similar activity levels showed similar effects regardless of how old they were, suggesting that the amount of activity people get has a greater effect on cardiometabolic health than aging.  

“There have been studies showing that if you are chronically sedentary, your blood vessel function is going to be lower, and your blood glucose levels are going to be higher,” Dr. Reynolds said. “My group was the first to show that this does actually happen very quickly.” 

She often explains this idea using the example of a short holiday break, she said. When there’s a celebration over a long weekend, whether it’s Thanksgiving or the Fourth of July, people often eat more and move less — and even that small amount of time is enough to show measurable changes in cardiometabolic health. 

“One question that I get asked a lot is, ‘Well, it's only three days, or it's only five days. Do these folks truly have an increased risk of cardiovascular disease or diabetes?’ And no, I don't think so,” Dr. Reynolds said. “I'm not trying to say that if you're inactive for five days, you're going to have a heart attack, or you're going to develop Type 2 diabetes. But I do think that it shows just how quickly these physiological responses happen and highlights the need for being physically active every single day.” 

So how much is enough? Dr. Reynolds defers to the American College of Sports Medicine guidelines, which recommend healthy adults aim for at least 150 minutes of moderate-intensity aerobic physical activity a week — with the caveat that her research shows more frequent, shorter bouts of exercise are better than a “weekend warrior” approach of doing it all at once. 

“Some people are never going to hit 10,000 steps per day,” she said. “But there is evidence out there to suggest that if you can just increase your physical activity levels by 500 steps a day on average each week, that results in a 14% lower risk of developing heart disease. 

“So if every couple of weeks you increase that average by 500 steps per day, you might not ever be able to get to an average of 10,000 steps, but you’re still improving your overall cardiometabolic health.” 

One of Dr. Reynolds’ favorite ways to meet her activity goals? Walking her Labrador retrievers, Maggie and Sugar. 

Aside from evidence to suggest that dog owners have higher physical activity levels in general, she said, her research shows one more benefit: dogs aren’t known for letting their humans go three days between walks.