Less exercise is behind the increase in waistlines across the U.S., and not more calories, according to a new study.

The American Journal of Medicine reports that the lack of leisure-time activity is behind weight gain, especially among young women.

A new analysis of information from the National Health and Nutrition Examination Survey (NHANES) shows that there has been a marked decrease in physical activity with a subsequent increase in Body Mass Index (BMI) among Americans over the past 20 years, with caloric intake remaining consistent. Experts agree that it is this decline in exercise that has led to the increase in obesity across the country, particularly among young women.

Lead study investigator Uri Ladabaum, MD, MS, Associate Professor of Medicine (Gastroenterology and Hepatology), Stanford University School of Medicine and colleagues looked at NHANES data from the past two decades, and discovered that the number of women in the U.S. who reported a lack of exercise increased from 19.1 percent in 1994 to 51.7 percent in 2010. During this same time period, the average BMI also increased, despite daily caloric intake remaining steady. The most significant jump in weight gain was found in women between the ages of 18 to 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. 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,” said Ladabaum in a statement.

According to the Centers for Disease Control and Prevention (CDC), over one-third of U.S. adults are obese. Obesity-related health conditions include heart disease, stroke, diabetes and cancer. The medical costs associated with obese people was $1,429 higher than those of normal weight in 2008.

Ladabaum and colleagues are encouraging Americans to engage in regular physical activity, and are calling for leaders to provide safe places for such physical activity to occur in order to help Americans take back control of their health.

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