Lack of leisure-time physical activity, not caloric intake, may be to blame for increased obesity in the United States according to a recent study.

Researchers from Stanford University in California found that in the past 20 years there has been a sharp decrease in physical exercise and an increase in average body mass index (BMI), while caloric intake has remained steady. They theorized that a nationwide drop in leisure-time physical activity, especially among young women, may be responsible for the upward trend in obesity rates.

Women who reported no physical activity jumped from 19.1 percent in 1994 to 51.7 percent in 2010. For men, the number increased from 11.4 percent in 1994 to 43.5 percent in 2010. During the period, average BMI has increased across the board, with the most dramatic rise found among young women ages 18-39.

"These changes have occurred in the context of substantial increases in the proportion of adults reporting no leisure-time physical activity, but in the absence of any significant population-level changes in average daily caloric intake," Uri Ladabaum, lead investigator of the study, said in a statement. "At the population level, we found a significant association between the level of leisure-time physical activity, but not daily caloric intake, and the increases in both BMI and waist circumference."

For the study, researchers analyzed data from the National Health and Nutrition Examination Survey from the last 20 years. They looked at the escalation of obesity in terms of both exercise and caloric intake, and tracked the rise in abdominal obesity, which is an independent indicator of mortality even among people with normal BMIs.

While increased caloric intake is often blamed for rising rates of obesity, no association between these was found in this study; in contrast, an association was found between the trends over time for lack of physical activity and high BMI numbers.

"Our findings do not support the popular notion that the increase of obesity in the United States can be attributed primarily to sustained increase over time in the average daily caloric intake of Americans," Ladabaum concluded. "Although the overall trends in obesity in the United States are well appreciated and obesity prevalence may be stabilizing, our analyses highlight troublesome trends in younger adults, in women, and in abdominal obesity prevalence, as well as persistent racial/ethnic disparities."

Although there is no easy answer in the ongoing battle against obesity, researchers said identifying the link between the drop in physical activity and increased BMIs, as well as the groups particularly affected, can assist public health officials to develop targeted, effective interventions. 

The findings were recently published in the American Journal of Medicine.