Research conducted at Harvard first linked TV watching to obesity more than 25 years ago. (5) 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. (6–10) Children who have TV sets in their bedrooms are also more likely to gain excess weight than children who don’t. (11,12) 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. (13,14)
Several trials designed to reduce children’s TV use have found improvements in body mass index (BMI), body fat, and other obesity-related measures. (15–18) 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. (19)
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. (17) 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. (16) 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. (18)
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. (20) 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. (21) 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. (3)
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. (23)
Many studies show that TV viewing is associated with greater calorie intake or poorer diet quality, (24–27) 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). (28) 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. (23) 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. (29)
- Laboratory studies find that TV food ads influence food consumption. (30,31) 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. (30)
- 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. (32)
In the wake of the IOM report, Coca Cola, McDonald’s, and 15 other major food and drink companies pledged to self-regulate food advertising during U.S. television shows aimed at children under the age of 12, through the voluntary Children’s Food and Beverage Advertising Initiative (CFBAI) that launched in 2006. But loopholes exist. The guidelines don’t cover, for example, general audience prime-time shows, such as American Idol, which are often viewed by young children, and don’t cover teens; while TV food and drink advertising to children ages 2-11 decreased from 2004 to 2008, advertising to adolescents (12-17) and adults (18-49) rose substantially. (33) There’s no check to make sure that companies comply with their guidelines-and no sanctions if they don’t. In fact, a recent review of the sugary drink advertising market found that children’s and teens’ exposure to sugary soda ads doubled from 2008 to 2010, with Coca Cola (a CFBAI member) and Dr. Pepper Snapple Group (not a member) leading the way. (34)
Another problem with this U.S. food industry approach is that there are no overarching nutrition standards for what constitutes a “healthy” food or drink-and the future of such standards is a matter of hot political debate. (35) In 2009, the U.S. Congress directed the Federal Trade Commission (FTC) to form an Inter-agency Working Group on Food Marketed to Children, to develop voluntary nutrition standards for foods and drinks marketed to children, and also to define what types of marketing would be covered by the standards. But the proposed standards, released in April 2011, have been met by strong resistance from the food and beverage industry and have been stymied by Congress. (36)
Other Sedentary Behaviors and Obesity
Other sedentary behaviors-computer/Internet use, video game playing, sitting at work, driving, and the like-have have not been studied as extensively as TV watching. But there is evidence that these other forms of “sit time” can contribute to obesity.
Computer/Video Game/Internet Use
Some studies in children and teens suggest that computer, video game, and Internet use are associated with excess weight, (37–39) although not all studies have found an effect. (6,40) More and more television content is moving from TV screens to computer and smartphone screens, however, so it’s possible that clearer effects will emerge as “Generation M,” as it has been dubbed, (41) spends more and more time immersed in these new media forms. Furthermore, food and beverage companies are becoming more sophisticated and targeted in their use of digital marketing and social media across these platforms, and public health advocates have called for stronger government regulation and industry self-regulation. (35)
A newer breed of video games, so-called “active video games,” requires players to move around to control the screen. A recent small trial suggests that trading sedentary video games for active video games may help curb BMI and body fat in overweight kids, but this finding awaits confirmation by other studies. (42)
Total Time Sitting
There’s evidence that spending too much time sitting-at work or at home-increases the risk of becoming obese, (21,43) and may also increase the risk of chronic diseases and early death. (20,44,45) Of course, people who spend a lot of time sitting may spend less time being active; but physical activity levels don’t seem to explain the sitting-health risk relationship. What is unclear is whether sitting itself is the culprit, or whether sitting is just a marker of another unhealthy aspect of lifestyle-such as TV watching-that is primarily responsible for these observations. It’s also possible that other types of modern sedentary behaviors promote overeating in different ways: Reading or working on the computer, for example, may increase people’s stress and lead to overeating, while listening to music may distract people from noticing whether they are hungry or full. (46)
Driving/Riding in a Car
There have been few long-term studies on whether passive forms of transportation, such as driving or riding in a car, contribute to weight gain. One eight-year study conducted in eight provinces in China found that men who acquired a car gained four pounds more than men who did not acquire a car, and had double the odds of becoming obese. (47) Active commuting by walking or bicycling, in contrast, does offer people an opportunity to fit exercise into their days and may be a promising strategy for weight control.
The Bottom Line: Limit TV and “Sit Time,” Increase “Fit Time” to Prevent Obesity
Overall, there is little doubt that time spent watching TV is an important risk factor for obesity-and a modifiable risk factor. There’s evidence that excessive marketing of unhealthy foods and beverages on television contributes to the TV-obesity link. It remains to be seen whether there’s enough political will to implement stronger regulations or a ban on junk food TV advertising to children, though such regulations, if implemented, would likely be effective-and cost effective. (48)
But in the meantime, there are other ways to curb exposure to TV and media:
- Parents can limit children’s screen time to no more than two hours per day, and a TV or computer ‘allowance’ device may help with setting limits. The American Academy of Pediatrics also recommends no screen time for children under the age of 2. (23)
- Making children’s bedrooms TV-free and Internet-free-by removing TV sets or connections, or not putting them there in the first place-can also help children stay within the two-hour limit, as can turning off the TV during meals.
- Schools, child care centers, and after-school programs can have policies that limit recreational screen time, and are also excellent venues for rolling out screen time reduction programs,such as Planet Health (17) and Eat Well and Keep Moving. (54)
- Healthcare providers can counsel parents to limit their children’s screen time and to become advocates for stricter regulations on TV/media food and beverage advertising to children.
Staying active helps with weight control, as does limiting sedentary activities-recreational computer time, driving, and the like. So a good strategy is to replace “sit time” with “fit time”-walking or biking for part or all of a workday commute, instead of driving, or playing in the park instead of playing video games. People’s physical and social surroundings have a strong influence on how active they are, however: Without bike lanes or bicycle racks, it’s hard to bike to work, and if neighborhoods or parks are not safe, children won’t have a place to play. Creating environments that support active lifestyles will take policy change at the federal, state, and local levels, as well as the cooperation of the private sector and community groups.
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The aim of the Harvard School of Public Health Obesity Prevention Source Web site is to provide timely information about obesity’s global causes, consequences, prevention, and control, for the public, health and public health practitioners, business and community leaders, and policymakers. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The Web site’s obesity prevention policy recommendations are based primarily on a review of U.S. expert guidance, unless otherwise indicated; in other countries, different policy approaches may be needed to achieve improvements in food and physical activity environments, so that healthy choices are easy choices, for all.