New research from Stanford shows that physical activity -- or lack thereof -- may be a bigger driver of the obesity epidemic than diet is.
The researchers looked at national survey results of people's health habits -- including diet and exercise -- from 1988 to 2010. The stunner was the increase in people who reported no leisure-time physical activity.
In 1988, 19 percent of women were inactive. By 2010, that number had jumped to 52 percent.
For men, the rate nearly quadrupled, going from 11 to 43 percent in the same time period.
But what didn't change was the number of calories people consumed. In other words, people were eating about the same but exercising significantly less.
Dr. Uri Ladabaum, a gastroenterologist at Stanford University Medical Center, led the study. He said that the research can only suggest an association between inactivity and increasing obesity, but that people should not decide diet is irrelevant to obesity.
"We would not want to suggest in any way that caloric intake is not important," Ladabaum said. "But it raises the question of how much of the change in obesity prevalence might be related to physical activity."
The researchers analyzed results from the National Health and Nutrition Examination Survey, conducted by the Centers for Disease Control. Participants reported their activity over the last month and their diet over the last 24 hours. Ladabaum said the team did not find a difference in total caloric count or breakdown by protein, carbohydrate and fat, over the 22-year study period.
The prevalence of obesity increased from 1988-2010, from 25 to 35 percent of women and from 20 to 35 percent of men.
The data included survey results from more than 17,000 participants between 1988-1994 and about 5,000 people each year from 1995 through 2010.
The study was published in the American Journal of Medicine.