Less exercise, and not rise in calorie intake, is behind the rising obesity rate in the U.S., found researchers at the Stanford University School of Medicine.

Researchers state that physical activity during leisure time decreased significantly for both males and females between 1994 and 2010. This decrease in exercise levels is linked to the rise in obesity rates.

The study is published in the journal American Journal of Medicine and is based on data from National Health and Nutrition Examination Survey (NHANES).

Data analysis revealed that number of women who reported low to no physical activity during leisure time rose from 19.1 percent in 1994 to 51.7 percent in 2010. For men, the numbers went up from 11.4 percent in 1994 to 43.5 percent in 2010. Between 1994 and 2010, average Body Mass Index increased throughout all age groups, especially among women in the 18-39 age group.

Researchers didn't look at the specific increase in fat, carbohydrate and protein consumption. However, they say that the total calorie intake has been more or less stable in the past 20 years.

"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," explained Uri Ladabaum, MD, Associate Professor of Medicine (Gastroenterology and Hepatology) at the Stanford University School of Medicine.

"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," Ladabaum said in a news release.

The study wasn't restricted to just BMI calculations. Researchers even looked at abdominal obesity, which is considered to be a better indicator of health than BMI. Waist circumference of 88 cm (34.65 in) or higher in women and 102 cm (40.16 in) or greater in men is considered unhealthy.

Researchers found that the average waist circumference rose by 0.37% per year for women and 0.27% per year for men. Once again, women were more likely to be affected by the unhealthy lifestyle.

The study also showed that fifty percent of the working people in the study reported no physical activity.