Link between obesity and screen time
Overconsumption might be a key component in the link between obesity and screen time, too, according to another of the new studies. Although past research already had linked increased TV time to widening waistlines, this study dug deeper. Ninety-one 13- to 15-year-olds filled out diaries for TV, video games and computer use during a one-week period. About four to seven times a day the teens were paged to record what they were paying the most attention to at that particular moment, followed by activities receiving their second- and third-most attention. "Kids live in a multitasking world," says Harvard Medical School pediatrics professor David Bickham, lead author of the screen-time study. "We're trying to assess their technology use when they're using different forms of technology at once."
Bickham says three theories have been floated for the link between screen time and obesity: food advertising, unconscious eating and displacement—that is, the idea that the media use replaces physical activity. His team's findings lent more support to the first two variables and less to the third. They found video games and computer use had no impact on BMI (body mass index). Television did, but only if it was the main event. Background TV, for example, didn't matter.
"We're saying the level of attention may make a difference," Bickham says. "You have to pay attention to advertising for it to have the impact, and [food] advertising is much less common in computers and video games. In terms of unconscious eating, when you're watching TV, your hands are free and you're stimulating your senses with the TV, so concurrent eating is more likely to happen." Previous research (pdf) has found support for both these theories, such as a study earlier this year showing that neighborhoods with more food and soft drink outdoor advertising had higher rates of obesity. Freedhoff adds that even viewing commercials for fruits and vegetables has been shown to increase consumption of unhealthy foods. "Our hunger hormones have been honed after millions of years of dietary insecurity, so when we want to eat, we tend not to crave green leafy salads," he says.
Less physical activity is not the problem
The screen-time study did find that kids engaged in more physical activity had lower BMIs, but that does not mean that more exercise is keeping those teens lighter, Freedhoff says. "What we've seen for so many years is research looking at physical activity as the preventative or the curative solution for childhood obesity, but the data on physical activity as a means to set children's weight is abysmal," he says. "What this study confirmed is that screen time increases obesity consequent to calorie intake, not to a lack of physical activity. That's a crucial message that people don't understand—obesity is not a disease of inactivity."
The third new study, looking at the link between sleep duration and obesity in teens, further blunts the idea that physical activity accounts for much of the increase in kids' weight. Researchers tracked nearly 1,400 teenagers from ninth through 12th grade and found, like past studies, that less sleep translates to higher BMIs. By analyzing BMI distribution rather than using cutoff points, University of Pennsylvania postdoctoral fellow Jonathan Mitchell says his team detected much stronger sleep effects among already obese teens. The effect of each additional hour of sleep among teens in the 90th BMI percentile was twice as big as among those in the 10th percentile. Increasing sleep from 7.5 to 10 hours a day among 18-year-olds could shave four percentage points off the proportion of teens with a BMI over 25, the researchers predicted.