Last month, researchers affiliated with the World Health Organization and the London School of Hygiene and Tropical Medicine reported that, worldwide, people's waistlines are expanding, with the total combined weight of human beings on Earth now exceeding 287 million tons. A third of that global human biomass exists in North America, although we account for only 6 percent of the world's population.
The study was widely publicized, especially after the BBC used its findings to develop a diverting online tool that lets users compare their biomass to that of people in other nations. (I learned that I have the B.M.I. of your average, middle-aged Eritrean.)
The study, however, did not address possible underlying causes of the ever-growing weight of nations.
But a group of groundbreaking new reports, being published online as a series today in The Lancet, suggest that voluntary physical inactivity, a practice once confined mostly to North America and parts of Europe, is spreading rapidly to the rest of the world and likely contributing materially to global gains in tonnage and declines in health.
Consider the findings of perhaps the most sobering of the new studies, which looked at the extent to which sedentary lifestyles are colonizing the world.
Led by Pedro C. Hallal, a professor at the Federal University of Pelotas in Brazil, the researchers turned to a large body of data about activity that the W.H.O. has been collecting in recent years. To gather the data, the W.H.O. provides questionnaires to people in various nations that ask, in effect, how much they exercise and otherwise move in their daily lives, an admittedly inexact way to measure activity (people misremember or, for cultural or other reasons, prevaricate). But it's the best global information currently available.
The latest figures suggest that the world's population has become disturbingly inactive. According to the researchers' calculations, 31.1 percent of the world's adults, or about 1.5 billion people, are almost completely sedentary, meaning that they do not meet the minimum recommendation of 150 minutes of walking or other moderate activity per week, or about 20 minutes a day.
Teenagers are faring even worse. More than 80 percent of young people ages 13 to 15 worldwide are not getting the hour a day of vigorous exercise recommended for their age group.
Unsurprisingly, North America and Europe lead the world in not exercising, with 43.3 percent of Americans and 24.8 percent of Europeans not reaching the low recommended threshold. But the world is catching up or, rather, joining us in sitting down. More than 30 percent of Russians are inactive nowadays; ditto in the Middle East; and about 27 percent of Africans are sedentary.
(Although in general, the richer the nation, the less active it is, the most sedentary nation on Earth is the tiny island of Malta, population 419,000, 72 percent of whom almost never voluntarily move around much.)
The consequences for global and personal health are punishing and likely to grow more so, reports another first-of-its-kind study in the Lancet series. In it, the authors conclude that sitting around most of the day has become as deadly as smoking or obesity.
Specifically, using data from W.H.O. and other large population studies worldwide, the researchers determined that inactivity is linked to about 6 percent of all instances of heart disease on Earth; 7 percent of Type 2 diabetes cases; and 10 percent of all breast and colon cancers, including among people who don't smoke and are of normal weight.
About 5.3 million people a year die from diseases tied to physical inactivity, the authors calculated.
By comparison, about 5.1 million die annually because of smoking, as an accompanying comment article points out.
"It seems clear from our data that physical inactivity, on a global scale, is at least equivalent to smoking and obesity," in terms of its deleterious impact on people's health, says Dr. I-Min Lee, a professor of epidemiology at the Harvard School of Public Health, who led the study.
Here, too, the United States is among global leaders, with 10.8 percent of all premature deaths in the United States now related to too little exercise. But other nations are outstripping us. More than 18 percent of deaths in Saudi Arabia are likely linked to physical inactivity, Dr. Lee's group concluded; the same goes in Serbia. In Britain, where the Summer Olympic Games next week will showcase the glories of the human body in motion, almost 17 percent of the nation's deaths are linked to a lack of exercise.
What can be done globally to slow or reverse the trend toward physical inertia? Other studies and comments in the Lancet series propose both governmental and personal action, including building more parks, promoting bike commutes and providing financial incentives, like lower insurance premiums, to encourage exercise, as well as using cellphone apps and other technologies to nudge people toward more movement.
But perhaps the most intriguing and potentially viable suggestion in the Lancet series is to start emphasizing just how deadly physical inactivity can be. "I don't think most people really understand that not exercising," even if someone is otherwise healthy, "appears to be just as unhealthy" as smoking or being severely overweight, Dr. Lee says. A sedentary life is shorter than it should be.
On the other hand, inactivity is easier for most of us to combat than a nicotine addiction or intransigent obesity, she adds. "It only takes a 20- or 30-minute walk most days of the week," she says. "With rare exceptions, everyone, everywhere can manage that," and five million lives around the world could be extended.
