Get off the couch!

The average American may expend as little energy as a person who sleeps 24 hours a day by the year 2020, following a worldwide trend of decreasing activity.

That’s one prediction from UNC-Chapel Hill researchers whose study found a global decline in activity levels. They foresee a continuing decrease in activity worldwide. When viewed in the context of physical activity levels throughout human evolution, the global decline in the past few decades is particularly abrupt.

Faculty members Barry Popkin and Shu Wen Ng conducted the study. They used extensive data from the 1960s onward to determine how people around the world spend their time and how they move in their daily lives. The resulting publication, “Time use and physical activity: a shift away from movement across the globe,” will be published in the August issue of Obesity Reviews.

Both researchers are on the faculty of UNC’s Gillings School of Global Public Health.  Popkin, Ph.D., is W.R. Kenan Jr. Distinguished Professor of nutrition, and Shu Wen Ng, Ph.D., is a research assistant professor of nutrition.

Television, computer games, texting

“We have understood for some time that children and adults in the United States are increasingly spending more time in front of televisions and in other sedentary activities such as playing computer games, using computers and texting on cell phones,” said Ng, who is the study’s senior author. “This study shows that the same shifts have also occurred in China, India, Brazil and the United Kingdom. In fact, we find adults in the U.K. are more sedentary than those in the U.S.”

Popkin noted that the introduction of home technology that includes rice cookers, refrigerators, stoves, washing machines and microwaves is global, reducing the time traditionally spent producing food and completing housework. Similar technological changes have led to less walking, more use of cars and buses, and in general, have lowered activity spent in travel across the world.

Historically, Ng said, adults have been most active in their jobs. Now, she says, “whether you live in China, India or the U.S., computers and many forms of automation remove physical exertion at work. Changes in the types of work people do have greatly reduced our overall activity levels over the past half-century.”

The study uses repeated nationally representative studies on time-use from the United States, the United Kingdom and China, along with more limited nationally representative time allocation data from Brazil and India, to document rapid declines in physical activity. This is particularly true in China and Brazil, the countries with the two highest absolute and relative rates of decline in total physical activity and some of the higher increases in sedentary time.

For the two countries, declines in activity were driven largely by reductions in movement at work, at home, and to a lesser degree, in travel or transportation. This is not surprising given that in the past few decades, the Chinese and Brazilians have been shifting from agriculture into manufacturing, service and other sectors, increasing use of machines and labor-saving technology in the workplace, and acquiring greater access to home technologies (e.g., electrification, piped water, appliances), as well as motorized vehicles.

Bleak projections

The study makes bleak projections, given continuation of trends, for the levels of activity in the five countries in 2020 and 2030. Using a physiological measure called metabolic equivalent of task (MET) that describes the amount of energy spent in accomplishing a task, the study determines that by 2020 the average American adult will expend about 190 MET hours per week. In comparison, a person who slept 24 hours in a day would expend 151 MET hours per week, and an active adult who did vigorous activity for 30 minutes to an hour each day, but otherwise had a desk job, would expend between 240 and 265 MET-hours per week.

People in Britain will reach the 190 MET hours level by 2030. Those in China and Brazil will continue on a steeper downward trend, reaching the U.S. and U.K. physical activity levels by 2030. The situation in India appears less severe, but the average of the levels masks the stark socioeconomic dichotomy likely to continue in India, with wealthier Indians leading lifestyles similar to those of the British.

These changes will have significant implications for health outcomes, health-care costs, and overall functional well-being of societies around the world.

Read more.

Published July 16, 2012.