Study of Americans’ energy expenditure isa ‘wake-up call,’ say Berkeley researchers
| 17 March 2004
Americans spend nine times as many minutes watching TV or movies as they do on sports, exercise, and all other leisure-time physical activities combined, according to a sobering new analysis by Berkeley researchers.
When looking at which activities contribute more to energy expenditure among Americans, the researchers found that driving a car, watching TV, and working in an office outranked sports and heart-pumping workouts. The study is due to be released this month in its final form in the new peer-reviewed publication International Journal of Behavioral Nutrition and Physical Activity, but a draft is currently available online.
“This study provides a wake-up call for the nation, particularly in light of rising obesity rates in this country,” said lead author Linda Dong, who received her master’s degree in epidemiology at Berkeley’s School of Public Health. “A lot of people aren’t fully aware of how sedentary their lives are. This paper shows that, as a population, leisure-time physical activities are at the bottom of our priority lists.”
The authors point out that the study provides the first national- scale, quantitative analysis of energy expenditure in the United States. The paper comes during an epidemic of obesity in this country, and at a time when federal health officials are reporting that poor diet and physical inactivity are quickly gaining on smoking as a cause of preventable deaths.
The Berkeley researchers used data from 7,515 adults questioned from 1992 to 1994 for the National Human Activity Pattern Survey (NHAPS). Those surveyed were asked to report on the duration of everything they did during the prior 24 hours. More than 125,500 reports of activities were grouped into 255 categories according to the level of energy expenditure required to perform those activities.
The researchers used information on how many people reported doing each activity, how long they did it, and how much energy it took to do it (expressed in terms of “metabolic expenditure values”). From this information, it was possible to estimate total energy expended on each activity.
Nearly three hours of tube time
The study found that, outside of sleeping, the largest collective contributor to energy expenditure among the population was “driving a car,” followed by “office work” and “watching TV,” partly because so many people reported those activities. In contrast, leisure-time physical activity, such as jogging or playing basketball, accounted for only 5 percent of the population’s total energy expenditure.
When considering only the amount of time spent on an activity, the researchers found that, on average, Americans spent 170 minutes a day watching TV and movies, or nine times the number of minutes spent on all leisure-time physical activities combined. The average daily duration for driving a car was 101 minutes, nearly five times the amount spent on sports and other exercise.
The authors noted that cutting back on the time spent watching TV or movies would free up precious minutes for exercise. But they also put the blame for the increased prevalence of obesity in this country on certain distinctive aspects of American life. For example, they noted that Americans work more hours per year than their counterparts in other developed countries. According to statistics from the U.S. Department of Labor, workers in the United States logged 1,821 hours in 2001, while those in Germany labored 1,467 hours. In addition, the proportion of workers who commuted 30 or more minutes to their jobs increased from 19.6 percent in 1990 to 33.7 percent in 2000.
“People are supposed to work in 60 minutes a day of moderate physical activity, but given the way our society is now, we don’t have a lot of extra time on our hands to go out and jog,” said co-author Gladys Block, professor of epidemiology and public-health nutrition at the School of Public Health. “Maybe we ought to put more emphasis on integrating exercise and energy expenditure into our daily chores and things we have to do.”
For instance, Block suggests sneaking more energy-consuming activities into our ordinary lives, such as taking the stairs instead of the elevator, or parking the car farther away from the mall entrance. “They may not get the same cardiovascular benefit from those activities as from sustained aerobic exercise, but it may be enough to help with weight control,” she said.
Weigh more, burn more
When the researchers looked at gender, they found that, in terms of the number of kilocalories burned per kilogram of body weight, the energy expenditure of women was only slightly lower than that of men. Women spend more energy on household- related activities, such as cleaning the house or caring for children, than men do, but their total energy expenditure was similar on a kilogram-for-kilogram basis. Overall, men burn more calories throughout the day because they weigh more than women.
In fact, activities relating to household duties accounted for 33 percent of the energy expenditure for women compared with 20 percent for men.
“More women than men are still shouldering the burden of household chores,” said Dong, who is now a doctoral student in epidemiology at the University of Washington. “Men also have a larger variety of activities that contribute to their energy expenditure. That suggests that women are spending more time on certain activities, like taking care of children, than men do, and less time on leisure-time physical activity.”
Among ethnic groups, African Americans were found to have the lowest percentage of total energy expended in leisure-time activities. Even when work-related physical activity was taken into account, energy expenditure was still lower among African Americans.
“This could be a reflection of the fact that more African Americans live in low-income neighborhoods that are less likely to have parks and safe places to walk and exercise,” said Block.
Middle America did not fare well when data was broken down by region. The authors found a greater percentage of energy expenditure for sports and other exercise among people who lived in the Pacific (Arizona, California, Hawaii, and Nevada) and New England regions (Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont) compared with those in the Central states (Iowa, Kansas, Missouri, and Nebraska).
The researchers noted that the results represent a snapshot in time, and that certain activities may have changed since the survey was taken in the early 1990s. “There’s no evidence that Americans have become more physically active over the past decade,” said Dong. “If anything, we’ve become more sedentary with the rise in popularity of video and computer games.”