A new study suggests that under-exercising, rather than overeating, may be at the heart of America's obesity epidemic.
Their analysis uncovered no evidence that American's have increased their daily calorie count in the same time period.
"We wouldn't say that calories don't count, but the main takeaway is that we have to look very carefully at physical activity. The problem is not all in the intake of calories," said Dr. Uri Ladabaum, a professor of gastroenterology at Stanford Medical School. Ladabaum is also the lead author of the study that will be published in the August issue of the American Journal of Medicine.
The study relies on data collected by the National Health and Nutrition Examination Survey from 1988 to 2010, which Ladabaum and his team used to look for trends in obesity, abdominal obesity, physical activity and caloric intake in the last two decades.
The most startling finding in the study is the drop in the amount of exercise Americans do in our free time. The researchers found that from 1988 to 2010, the percentage of adults who reported doing no exercise in their free time grew dramatically from 19% to 52% in women, and from 11% to 43% in men.
"We suspected there was a trend in that direction, but not that magnitude," said Ladabaum. "People can get exercise in other ways, but most people don't walk or bike to work, and most people are not in jobs that require physical activity."
At the same time, the researchers found that the prevalence of obesity increased from 25% to 35% in women, and from 20% to 35% in men. In that time period the proportion of normal weight men and women dropped, while the proportion of overweight men and women remained the same.
But here comes the surprising part: The researchers did not find any evidence that people were ingesting more calories on a daily basis in 2010 than they were in 1988.
"The one caveat here is that the amount of calorie intake was based on self report, so it is possible people were not recalling correctly what they ate, or not reporting correctly," said Ladabaum.
Ladabaum notes that the study can tell us only that a major drop in time spent exercising occurred at the same time as a rise in obesity, not that one caused the other.
"The study looks at trends and certain associations, but does not prove any cause and effect between these," he said.
He also wants to make clear that the fact that the average caloric intake did not change substantially does not mean that caloric intake has been optimal at the population level or at the level of individuals.
"We simply did not detect a substantial increase over time," he said.
Even with all those caveats, however, this study could still be used to inform policy on managing America's obesity epidemic.
"Even though it is very difficult to prove directly that public health interventions promoting physical activity will make a difference, I think they will," he said. "This study should serve as a reinforcement of the message that we need to think of a multi-component solution where diet is a big part of it, and physical activity is a big part as well."
However, there is a big difference between telling people that they should be exercising and actually getting them to do it.
"The finger wagging Puritan in me wants sedentary folks to get off the couch and exercise, but my public health background cautions me to go beyond the data tables and look at the lives of Americans today," writes Pamela Powers Hannely, managing editor of the American Journal of Medicine in an editorial.
She notes that single mothers in particular, may have difficulty figuring out how to work exercise into their already hectic lives. She also wonders if this may explain why the rise in obesity has been most prevalent in women between the ages of 18 to 39.