At the height of summer, naps at the beach can be alluring, and many of us may find ourselves tempted to take prolonged vacations from exercise.
But two new, admonitory studies involving both older and younger adults who temporarily cut back on their physical activity indicate that the metabolic consequences of not moving much for a few weeks can be pervasive and persistent, lingering to some extent even after people start moving around normally again.
Physical activity is, of course, good for us and our metabolisms. Among other effects, contracting muscles burn blood sugar as fuel and, in response to signals from the hormone insulin, also store some of it for future use. Over the long term, these conditions help our bodies to stave off soaring blood sugar, insulin resistance and Type 2 diabetes.
But what happens when, as a result of choices or circumstances, we do not exercise or move around much for a period of time?
In some past studies with healthy, active young people, often college students, the consequences have been swift but reversible. When these volunteers have taken to their beds and chairs for days on end in the interest of science, they often have developed heightened blood sugar and some early symptoms of insulin resistance.
But within a day or two of returning to their normal activities, their metabolisms usually stabilized and blood sugar and insulin levels dropped.
Many of us, though, are not robust young college students, and whether the impacts of becoming inactive, even for a short period of time, are likely to be as ephemeral for us has been less clear.
So for one of the new studies, which was published in June in Diabetologia, researchers at the University of Liverpool in England and other institutions asked 45 adult men and women to abruptly start sitting more.
The volunteers had previously been active, walking for more than 10,000 steps on most days, according to monitors that they wore for several days at the study’s start. They also had been metabolically healthy, testing showed, and free from diabetes, although some had near relatives with the condition.
During the study, the volunteers simply stopped moving much, cutting their daily steps to below 2,000 and sitting for more than three and a half additional hours each day, a routine that they continued for two weeks.
The researchers then rechecked their metabolisms and body compositions and asked them to return to their former activity levels for another two weeks, after which the tests were repeated.
The results proved to be consistent if worrisome. The volunteers almost all had developed what the scientists called “metabolic derangements” during their two weeks of being still. Their blood sugar levels had risen, insulin sensitivity declined, cholesterol profiles become less healthy, and they had lost a little muscle mass in their legs while gaining fat around their abdomens.
Thankfully, most of these derangements were reversed once the men and women became active again.
But for unknown reasons, a few of the volunteers did not return to quite the same level of exercise they had engaged in before. They now completed fewer minutes of vigorous activity each week than previously and had some slight but lasting symptoms of insulin resistance, even after two weeks of moving normally.
The consequences of sudden inactivity were more severe and, in their way, poignant in the other new study, which was published in July in The Journals of Gerontology.
It focused on overweight people past age 65 who already were at risk of developing diabetes because they had high blood sugar. But they otherwise were healthy and active, walking about 7,000 or 8,000 steps each day.
Now, as in the other study, they sat, reducing their steps to below 1,000 a day for two weeks, after which, for a final two weeks, they moved about normally.
Like the adults in the other study, these older volunteers quickly developed worse blood sugar control during their two weeks of barely moving. Insulin resistance climbed. Some developed changes in muscle tissue indicating that they might soon begin to lose muscle mass, and a few had to be removed from the study because they had edged into full-blown Type 2 diabetes after becoming inactive.
For most of the men and women who remained in the experiment, their undesirable metabolic changes were not fully reversed after two weeks of moving about again.
The upshot of these findings is that a few weeks of inactivity could leave us less well, perhaps for a prolonged period of time, with the health consequences amplified by increasing age, said Chris McGlory, a research fellow in kinesiology at McMaster University in Canada, who led the study of older people.
“It’s not uncommon for older people to become sick or injured and wind up hospitalized or housebound for several weeks, or for someone who’s younger to just decide to take a few weeks off” from regular exercise and physical activity, he said
“But if it’s at all possible,” he added. “Don’t stop moving.”
By Gretchen Reynolds © 2018 The New York Times