They found that replacing sedentary behavior, such as watching television, with even low-intensity activity—such as standing or walking to cook or do laundry—increased the chances of healthy aging. And at work, replacing some of the time spent sitting with simple movements, such as standing or walking around the office, can improve health.
“These findings indicate that physical activity does not need to be high-intensity to be potentially beneficial for various aspects of health, which have particularly important implications for public health because older adults tend to have limited physical capacity to engage in moderate-to-vigorous physical activity,” Molin Wang, associate professor of epidemiology at the Harvard T.H. Chan School of Public Health and an author of the study, wrote in an email.
For the purposes of the study, healthy aging was defined as surviving to at least age 70 without major chronic disease and without impairment in subjective memory, physical function or mental health. The data suggest, however, that the relationship between light activity and healthy aging continues into the 80s and 90s, Wang said.
Replace TV time with physical activity or sleep
The researchers used responses from a Nurses’ Health Study of 45,176 women that began more than 20 years ago. Respondents were asked questions such as “On average, how many hours per week do you spend standing or walking at home?” or “On average, how many hours per week do you spend standing or walking at work or outside the home?” Participants were on average 59.2 years old and free of major chronic diseases when the study began in 1992. They were followed for 20 years.
The study incorporated isotemporal substitution modeling to assess the potential effect on healthy aging of replacing one hour of one behavior with the equivalent duration of another. They found, for example, that every two hours spent sitting and watching television was linked to a 12% decrease in the odds of healthy aging. Conversely, every two hours per day of light physical activity at work was linked to a 6% increase in the odds of healthy aging.
Replacing TV time with light physical activity at home also increases the chances of healthy aging. For those who sleep seven hours a night or less, replacing TV time with sleep also benefits health.
“What we found is that if you replace sedentary behaviors with any activity, I mean, even light physical activity, like standing or walking, or doing household chores, it’s better than just being a slacker for an extended period of time,” said Frank Hu, professor and chair of the department of nutrition at the Harvard T.H. Chan School of Public Health and an author of the study.
And replacing TV time with light physical activity, or sleep for some people, at any age, would be beneficial, Wang said.
The researchers adjusted for several variables, including age, income, family history of cancer, myocardial infarction and diabetes, baseline hypertension and high cholesterol, menopausal status and use of postmenopausal hormones, and diet.
The study results show an association between sitting and watching television compared to. light physical activity and the chances of healthy aging, not a cause-and-effect relationship. The researchers also wrote that because their study population was limited to nurses in the United States, “the results may not be generalizable to other populations.”
Hu says the isotemporal substitution model used is important because it recalls the concept of opportunity cost in economic theory or the idea of isocaloric substitution in food and nutrition. These models recognize that people have a limited amount of time in a day or a limited number of calories they will consume, and that when they choose one thing over another, they must do something else.
Watching TV and eating junk food
The problem with TV is not just that it is a sedentary habit. While watching it, people tend to eat more junk food and drink more sugary drinks, thus increasing their calorie intake, Hu explained.
Scott Lear, a professor of health sciences at Simon Fraser University in Vancouver, said that while the study’s findings aren’t groundbreaking, the researchers made some important distinctions. They made it clear that health is also affected by what people do when they’re sedentary and where they do it, not just by whether they’re inactive. The health impacts are different for people who sit at home or at work, and for people who sit watching TV or who read or write.
“Watching TV has been linked to an increased risk of premature death, type 2 diabetes, heart disease and certain cancers, and watching TV is a little different than just sitting and reading a book,” said Lear, who was not involved in the study.
The fact that sleep was included in the study is also new, Lear said, noting that it is an underappreciated health behavior. It may seem like a sedentary behavior, but the brain is very active when we sleep, performing functions that affect our overall health.
He compared the brain to a desk, and at the end of the day, all the events of the day — going to the grocery store, having to run to the bus, talking to someone at work — are like files scattered on the floor that need to be picked up and organized in the morning. That’s what the brain does when we sleep, he explained.
Sleep is also when we clear out toxins that build up in the brain throughout the day, and studies have shown a link between the buildup of these metabolic wastes and the risk of dementia later in life, Lear said.
“We often talk about physical activity and healthy eating. Those are the two things we talk about the most. But sleep is extremely important,” Lear said.
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