Too much sitting might sabotage the benefits of exercise.
This comes as bad news to the legions of office workers who spend their days tethered to a desk but dutifully hit the gym regularly to offset the effects of having been so sedentary. No one is suggesting that they quit doing that — any exercise is good — but a small but worrying study suggests that the fix might not be quite that easy.
The findings, reported in the Journal of Applied Physiology, suggest that inactivity may alter our bodies in ways that are not just unhealthy on their own but also blunt the healthfulness of exercise.
We know, of course, that physical activity is beneficial and being sedentary, for the most part, is not.
But the biological interplay between inactivity and exercise has been puzzling. Is sitting unhealthy for us primarily because we are not exercising when we are sitting? Or does sitting have its own unique effects on our bodies and, if so, could those outcomes somehow alter or even overpower the positive contributions of exercise?
Those questions prompted scientists at the University of Texas at Austin to look into the matter.
They began by checking the health and aerobic fitness of 10 physically active male and female graduate students. They fitted them with activity monitors to measure how much they normally moved.
Then they asked the volunteers to stop moving around so much and instead confine themselves to fewer than 4,000 steps a day and at least 13 hours of remaining seated. The volunteers complied, sitting, almost uninterrupted, for four days in a row. They also changed their diets slightly, consuming fewer calories, so that they would not gain weight, which might have changed their metabolisms, separately from the sitting.
On the morning of the fifth day, the volunteers visited the university’s human performance lab. There they were given a large breakfast shake composed of half-and-half and melted ice cream, after which their blood was monitored for triglycerides, blood sugar and insulin for the next six hours. The idea, said Edward Coyle, a professor of kinesiology and senior author of the study, was to see how their metabolisms would respond to this outlandishly fatty, sugary meal after their days of enforced idleness.
Then the volunteers repeated the four-days-of-sitting routine. This time, however, they exercised briskly for an hour on treadmills before they were given the high-fat shakes.
To no one’s surprise, in the first set of tests, the students displayed high levels of triglycerides and blood sugar and low insulin sensitivity. What no one saw coming was that the students’ triglycerides and blood sugar levels were no better on the morning after they had run.
These results suggest that being sedentary for long periods may create conditions inside our bodies “that make us resistant to the usual metabolic improvements after acute exercise,” Coyle said.
In other words, if we sit too much, our workouts may lose some of their punch.
This study was small, short-term and narrowly focused, which means, Coyle said, further research is needed. Nonetheless, he said, the data indicate that “it is a very good idea not to sit all day.”