Exercise Now, Sit in Front of the TV Later
If we spend an hour working out, that’s one hour less during our day that we can potentially spend being sedentary.
But we may, consciously or not, find other opportunities during the day to move less, undermining our best intentions and the potential health benefits of the exercise, according to an interesting new study of how people use their time, minute by minute, on days when they exercise and when they do not. Time management is always tricky, involving constant trade-offs, not all of which are voluntary or even conscious.
If we shop for groceries, for example, we may not have enough time later to visit the gym. Or if we binge-watch sitcoms, those accumulated hours of sitting cannot be devoted to any other activity.
But while many past epidemiological studies and experiments have looked at how we apportion our days — how many hours we spend, on average, sleeping, sitting, working, eating, exercising and so on — few have closely examined how we balance one activity against another and the extent to which some activities elbow others aside.
Those details matter, because if, for instance, we hope to use exercise to increase how many calories we burn during the day, we need to avoid becoming less active over all on exercise days.
In other words, on days when we exercise, we need to avoid sitting more or skipping physical activities we might normally do, like taking the stairs or cleaning the house, during non-exercise hours.
But do we?
To find out, researchers from the National Cancer Institute turned to an existing and elaborate database of information about more than 1,000 healthy, middle-aged and older men and women.
The data had been collected over several years in conjunction with the AARP. (The current study, which will be published in September in the American Journal of Preventive Medicine, was funded by the National Cancer Institute.)
When the participants joined the study, they completed a variety of health tests and questionnaires, including a bogglingly detailed look at their lives on the previous day.
To complete this form, the men and women were asked to answer multiple questions about how they had spent each minute the day before, including how long and in what ways they had moved and how, when and where they had sat. Had they been watching television or been seated at a desk? Had they been in a car?
Each of the men and women in the study completed this recall questionnaire every other month for a year. At least one of their questionnaires covered a workday (unless they were retired) and another a weekend.
The researchers then gathered all of the information for each of the participants and began mapping out their days.
In general, the participants turned out to be fairly sedentary, which makes them typical of most of us. Most of the men and women reported sitting for about 9 or 10 hours a day.
Much of their sitting occurred at home, especially in front of the television.
But about two-thirds of the group did say that they exercised, at least sometimes.
When they worked out, their reported sessions lasted for about an hour, on average, and were moderately intense, such as brisk walking.
Not surprisingly, on those days when people exercised, they rejiggered other aspects of their lives.
In particular, they spent less time completing “light” activities, such as household chores, shopping and unhurried walking.
Taking time from these kinds of incidental physical activities to devote to more-formal exercise might seem laudable.
But, interestingly, when the scientists also calculated each person’s daily energy expenditure, using existing equations about the energy costs of different activities, they found that people expended more total energy — meaning they burned more calories — on days when they exercised than days when they did not.
But not by much. In fact, their total daily expenditure was increased by only about half as much as would have been expected, given the number of calories they were burning during exercise, because they moved less in other ways on those days.
Meanwhile, the amount of time that most of the people in the study reported spending watching television barely budged, whether they exercised or not.
“It was surprising” to see how much of the time people devoted to exercise seemed to have been “chipped away from other physical activities,” but not from the hours spent sitting in front of the TV, says Charles Matthews, a senior investigator at the National Cancer Institute who led the study.
Of course, this study relied on people’s memories of their daily activities, which are suspect, no matter how diligently someone fills out the questionnaires.
But the patterns were consistent among the 1,000 men and women, Dr. Matthews says, and suggest that we might want to take a close and critical look at how we spend our days, especially if we expect exercise to help us to control our weight and improve our health.
Using an activity tracker or keeping a careful log of how and when you move and sit throughout the day “could be useful,” he says. “Many of us probably are not aware of how we actually apportion our time.”