Today's couch potato lifestyle has been blamed for skyrocketing rates of obesity, but the cause of this trend may have more to do with the potato than the couch. An analysis of 20 years of published data on people's daily energy expenditure indicates that overeating, rather than a sedentary existence, is the major cause of the industrial world's obesity epidemic.
What is certain about obesity is that it is ultimately caused by an imbalance in people's energy budgets. When you take in more calories than you burn, your body squirrels the excess away as fat. The imbalance can stem from too much food, too little physical activity, or a combination of the two. Studies of self-reported exercise and eating habits have suggested that daily physical activity has decreased in recent decades, while daily caloric intake has remained steady. But people's accounting of their own behaviors is notoriously inaccurate.
To get more reliable data, biologists Klaas Westerterp of Maastricht University in the Netherlands and John Speakman of the University of Aberdeen in the U.K. turned to a technique called the doubly labeled water (DLW) method. Over 2 weeks, subjects are given trace amounts of water molecules whose hydrogen or oxygen atoms contain extra neutrons. The body draws on the oxygen atoms for metabolism, expelling some of them in carbon dioxide. By tracking the ratio of heavy hydrogen and oxygen in the urine, researchers can estimate a person's overall rate of metabolism.
The researchers analyzed published data from a 20-year DLW public health study of 366 Maastricht residents led by Westerterp. They compared these data with results from published studies conducted in the United States and the developing world. DLW "is the gold standard," says Speakman, "so we pooled all the data available."
The results indicate that people are burning just as many calories as they ever did. The daily expenditure of energy was similar across all studies throughout the period, whether the subjects were in Europe, the United States, or the developing world. Over the same 20-year period, obesity prevalence has doubled in the Netherlands and more than tripled in the United States. "We're not saying that exercise doesn't make a difference," says Speakman. "If you train for marathons, then of course you'll get fitter. But for average people, the daily physical activity hasn't changed. In the time we spend watching television today, people probably listened to radio in the 1950s and read books in the 1920s," he concludes. "This work suggests that the obesity epidemic has been largely driven by increases in food intake.”
"The study is provocative," says Loren Cordain, a physiologist at Colorado State University in Fort Collins. "But I wouldn't hang my hat on it," he says, because the DLW studies used different experimental procedures, making them hard to compare. Cordain also worries that the 366 people studied in Maastricht might not be representative of the entire country. Speakman counters that "we know of no reason they wouldn't be representative."