Sedentary leisure-time behaviors — particularly TV watching — are associated with higher levels of abdominal fat, even among people who exercise regularly and maintain a normal body mass index (BMI), new research from the University of Minnesota reports.
The study also found that moderate physical activity — at the amount recommended by federal guidelines — is associated with smaller levels of abdominal fat, no matter what the person’s BMI.
“These findings are important because they have some implications for interventions to improve health,” said Kara Whitaker, the study’s lead author and a post-doctoral researcher at the U’s School of Public Health, in an interview with MinnPost.
Other studies, she pointed out, have linked excess abdominal fat — regardless of a person’s overall weight — to an increased risk of disease, particularly heart disease and type 2 diabetes. In addition, there is growing evidence that sedentary behavior is a health risk separate from lack of exercise.
“If we’re finding that television viewing is associated with abdominal fat, and we know that people with higher levels of abdominal fat are at higher risk of heart disease and type 2 diabetes, then it would be likely beneficial to people to reduce their television viewing time,” Whitaker said.
For their study, which was published online Wednesday in the journal Medicine & Science in Sports & Exercise, Whitaker and her colleagues analyzed data collected from more than 3,000 men and women, aged 42 to 59, who were participating in the Coronary Artery Risk Development in Young Adults (CARDIA) study. The participants had enrolled in CARDIA back in 1985-1986, when they were between the ages of 18 and 30.
Only two racial groups — blacks and whites — are represented in CARDIA because the study was launched to specifically identify inequities among African-Americans in heart disease risk factors and outcomes.
The data used in the U’s analysis included self-reported information on the amount of time the participants spent daily in six sedentary leisure-time behaviors (watching television, using the computer, doing non-computer paperwork, listening to music, talking on the phone and sitting in a car), as well as information on the frequency with which they engaged in various physical activities.
The measurements of abdominal fat came from abdominal computed tomography (CT) scans, which were taken of each participant in 2010.
The positive and the negative
After crunching all that data, the researchers found a positive association between abdominal fat and only one of the six sedentary behaviors — watching television. Each hour-and-a half spent watching television was associated with an increase in abdominal fat of about 3 cubic centimeters, on average.
And that finding held even after adjusting for various demographic and other factors, including BMI.
Why would television watching add to belly fat more than other forms of sedentary behavior?
“People tend to eat food while they watch television, and they might not do that as often when they’re reading a book or talking on the telephone,” said Whitaker. She also pointed out that food ads on TV, which tend to be mainly for processed snacks, might lead people to eat unhealthily at other times as well.
The data also revealed a strong link, although a negative one, between abdominal fat and physical activity. Exercising for the length of time and at the intensity recommended by federal guidelines (150 minutes of moderate-intensity physical activity per week) was associated with a decrease in abdominal fat of between 6.7 to 8.1 cubic centimeters.
Not surprisingly, television viewing and not being physically active had an additive effect.
“We found that people who were watching the most television and were the least physically active had the highest levels of abdominal fat,” said Whitaker.
Limitations and implications
The study comes with several important caveats. It’s an observational study, which means it can’t prove one thing (such as TV watching) causes another (an increase in abdominal fat). Other factors, not identified in the study, might explain the results.
The study also relied on the participants’ self-reporting of their sedentary and physical activity behaviors. Such reports can be unreliable. In addition, the participants do not represent all racial and ethnic groups in the U.S., so it’s unknown if these findings are applicable to broader populations.
Furthermore, the study looked only at leisure-time sedentary behavior, and not at time spent sitting in the workplace. U researchers are currently testing strategies in the workplace (such as standing desks) to see if such interventions can reduce sedentary-behavior-related risk factors for chronic disease, said Whitaker.
Still, the current findings are provocative, particularly as they support a growing field of research that suggests sedentary behavior is — on its own — a risk factor for disease.
“My overall recommendation would be to move more, sit less,” said Whitaker.
“But if you can only improve one thing — if that’s all you have time to do — then I would encourage physical activity,” she added. “If you are more physically active and you reduce your sedentary time, particularly television viewing, that would be the most protective.”
FMI: You’ll find an abstract of the study on the Medicine & Science in Sports & Exercise website, but the full study is behind a paywall.