Research conducted at Harvard first linked TV watching to obesity more than 25 years ago. Since then, extensive research has confirmed the link between TV viewing and obesity in children and adults, in countries around the world. And there’s good evidence that cutting back on TV time can help with weight control-part of the reason why many organizations recommend that children and teens limit TV/media time to no more than two hours per day. This article briefly outlines the research on how TV viewing and other sedentary activities contribute to obesity risk, and why reducing screen time and sedentary time are important targets for obesity prevention.
TV Viewing and Childhood Obesity
Studies that follow children over long periods of time have consistently found that the more TV children watch, the more likely they are to gain excess weight. Children who have TV sets in their bedrooms are also more likely to gain excess weight than children who don’t. And there’s evidence that early TV habits may have long-lasting effects: Two studies that followed children from birth found that TV viewing in childhood predicts obesity risk well into adulthood and mid-life.
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Several trials designed to reduce children’s TV use have found improvements in body mass index (BMI), body fat, and other obesity-related measures. Based on this evidence, the U.S. Task Force on Community Preventive Services recommends that communities roll out behavior-change programs aimed at curbing screen time, since there’s “sufficient evidence” that such programs do help reduce screen time and improve weight.
Some of these successful TV-reduction trials have been delivered through the schools: The Planet Health trial, for example, used middle school classroom lessons to encourage less TV viewing, more activity, and improvements in diet; compared to the control group, students assigned to receive the lessons cut back on their TV time, and had lower rates of obesity in girls. Another trial found that third- and fourth-graders who received an 18-lesson “TV turnoff” curriculum cut back on TV time and on meals eaten while watching TV, compared with children in the control group, and they had a relative decrease in BMI and other measures of body fatness. TV “allowance” devices, which restrict TV watching to a set number of hours per week, may help limit children’s screen time, and in turn, help with weight control.
TV Viewing and Adult Obesity
There’s convincing evidence in adults, too, that the more television people watch, the more likely they are to gain weight or become overweight or obese. And there’s emerging evidence that too much TV watching also increases the risk of weight-related chronic diseases. For example, the Nurses’ Health Study followed more than 50,000 middle-age women for six years. For every two hours the women spent watching television each day, they had a 23 percent higher risk of becoming obese and a 14 percent higher risk of developing diabetes. A more recent analysis that summarized the findings of this study and seven similar studies found that for every two hours spent watching TV, the risk of developing diabetes, developing heart disease, and early death increased by 20, 15, and 13 percent, respectively.
TV reduction trials have focused largely on children, not adults. But a small pilot study in 36 men and women suggests that an electronic TV “lock-out” device could help adults with weight control. Half of the volunteers were assigned to use a lock-out device that would cut their TV viewing time by half; the other half were assigned to a control group with no limits on TV. The volunteers who used the lock-out device watched less television and burned more calories each day, and they had a greater reduction in BMI than the control group. The difference in BMI did not reach statistical significance, however. (22) Given the study’s small size, more research is needed to confirm these results.
How Does TV Watching Increase the Risk of Obesity? A Closer Look at Food Marketing
Researchers have hypothesized that TV watching could promote obesity in several ways: displacing time for physical activity; promoting poor diets; giving more opportunities for unhealthy snacking (during TV viewing); and even by interfering with sleep.
Many studies show that TV viewing is associated with greater calorie intake or poorer diet quality, and there’s increasing evidence that food and beverage marketing on television may be responsible for the TV-obesity link. The effects of TV viewing on physical activity are much smaller than on diet, so they don’t seem to play as strong a role. Some research findings that support the food marketing-TV-obesity link:
- The thousands of food-related TV ads that children and youth see each year are primarily for high-calorie, low-nutrient foods and drinks, according to a comprehensive review of the evidence by the Institute of Medicine (IOM). Food marketing influences children’s food preferences and purchase requests, and marketers rely on this “pester power” to influence what parents buy.
- Branded foods, drinks, and restaurants are often featured in TV shows and movies (the ad industry term for this is “product placement”), and these product placements are overwhelmingly for unhealthy foods. An analysis of food brands that appeared in prime-time television programming in 2008 found that children and teens saw roughly one food brand per day, and three out of four of these brand appearances were for sugary soft drinks.
- Laboratory studies find that TV food ads influence food consumption. In one experiment, for example, children who watched cartoons with food commercials ate 45 percent more snack food while viewing than children who watched cartoons with non-food advertising.
- More evidence that exposure to food ads, rather than watching television itself, contributes to obesity comes from a study that tracked the TV viewing habits and change in BMI of 1,100 young children over a five-year period. The more hours per day of commercial TV children watched at the start of the study, the more likely they were to have a relative increase in BMI at the study’s end. There was no link between non-commercial TV watching and change in BMI.
Source: Harvard University, T.H. Chan School of Public Health