American adults are no more likely to be meeting national guidelines for physical activity than they were a decade ago, according to a University of Iowa study published last Friday in JAMA Network Open.
But they are spending more time sitting, the study reports.
Those are troubling trends. Both insufficient physical activity and prolonged sitting have been individually linked to an increased risk of a number of chronic illnesses, including heart disease, type 2 diabetes and certain cancers, as well as to early death from all causes.
And the more we sit, the more we need to exercise. Research has suggested that to offset long stretches of sitting, we may need to engage in at least an hour of moderate-intensity physical activity daily, and even that may not be enough.
“Our findings highlight a critical need for future public health efforts to aim for not only an increase in physical activity but also a reduction of sedentary time,” the authors of the current study write.
The U.S. government issued its first national health guidelines for physical activity in 2008, which it updated in 2018. Those guidelines recommend that American adults engage in 150 to 300 minutes (2.5 to 5 hours) a week of moderate-intensity — or 75 to 150 minutes a week of vigorous-intensity — aerobic physical activity each week.
Examples of moderate-intensity aerobic activity include brisk walking (at least 3 miles per hour), biking (up to 10 miles per hour) and general gardening, according to the American Heart Association. Examples of vigorous-intensity exercise include jogging, biking faster than 10 miles per hour, and heavy gardening (continuous digging and hoeing).
The 2018 guidelines also address sedentary behavior, concluding that if American adults spend less time sitting, the overall health of the country would improve. But the guidelines do not give any specific advice on how much sitting is too much.
How the study was done
The authors of the current study wanted to see if the 2008 guidelines had influenced Americans’ behavior. They analyzed data collected from a representative sample of 27,000 people who took part in the Centers for Disease Control and Prevention’s National Health and Nutrition Examination Survey (NHANES) from 2007 through 2016. Among the questions on the survey were ones about the intensity, duration and frequency of the participants’ physical activity, not only during their leisure time at home, but also at work or while commuting to work or other destinations.
The survey revealed that the proportion of Americans who reported meeting the national aerobic activity guidelines stayed essentially the same during the eight years after its initial publication, rising only slightly (and statistically insignificantly) from 63.2 percent in 2007-2008 to 65.2 percent in 2015-2016. (Both of those numbers are higher than estimates from previous studies that looked at the data, but that’s because the earlier studies included only leisure activity.)
The survey did find, however, that certain subgroups of people had increased the amount of time they spend exercising: women, blacks, nonsmokers and people who were neither underweight nor overweight.
Although Americans weren’t exercising more in 2016, they were sitting more. In 2007-2008, they spent an average of 5.7 hours per day sitting or lying down while watching television, using a computer, playing video games, reading or socializing. That increased to 6.5 hours by 2015-2016.
The subgroups who spent the greatest amount of time being sedentary were people with at least an undergraduate college degree and those who were overweight or obese.
The survey data also revealed that the proportion of American adults who spend six or more hours a day sitting had increased from 16.1 percent to 18.8 percent.
That’s almost one in five adults.
Move more, sit less
The study comes with several limitations. Most notably, it relies on self-reported data, which can be inaccurate. It also doesn’t include findings regarding muscle-strengthening activities, which are also part of the government’s physical activity guidelines. (The recommendation is to do such exercises twice a week.)
Still, NHANES data is collected with plenty of quality controls, and has been shown to be reliable in the past, the study’s authors point out.
Their findings have “significant health implications,” they add, and should serve as a strong reminder that we not only need to get more exercise, we also need to remember “sedentary behavior is not just the inverse of physical activity.”
“We hoped the guidelines [in 2008] would be something that gave people a new message about recognizing the importance of being more physically active,” Wei Bao, the study’s senior author and a molecular epidemiologist at the University of Iowa, told Time magazine reporter Alice Park. “But it turned out that there wasn’t much change. And we found a clear increase in the sitting time. This means we need to be more aggressive in finding ways to provide people opportunities to sit less.”
We also need to help people find safer, easier ways of being more physically active — whether or not the total amount meets the recommended levels.
As the 2018 guidelines note, “Some physical activity is better than none. Adults who sit less and do any amount of moderate to vigorous physical activity gain some health benefits.”