A study from Stanford University reported in The American Journal of Medicine suggests the rise in obesity in the US is likely due to increased sedentary lifestyles across the nation, and not eating too many calories.

The researchers came to this conclusion after studying data for the last 20 years from the US National Health and Nutrition Examination Survey (NHANES) that shows there has been a sharp decline in levels of leisure time physical activity among Americans - especially among young women - accompanied by an increase in average body mass index (BMI), while calorie consumption has remained somewhat steady.

Lead author Uri Ladabaum, Associate Professor of Medicine in Stanford University School of Medicine, and colleagues analyzed trends in obesity, waistline obesity, physical activity and calorie intake in American adults up to 2010.

In 2010 over half of American women reported no leisure-time exercise

More than half (51.7%) of female adults in the US reported no leisure-time physical activity in 2010. This proportion is nearly treble what it was in 1994, when 19.1% of adult American women reported doing no exercise.

For men, while the proportion who reported no leisure-time physical activity in 2010 was lower than for women, at 43.5%, this is nearly four times the 11.4% of men who said they did not exercise in 1994.

When they analyzed the data by subgroups, the team found women, and black and Mexican-American women in particular, showed the greatest decreases in reported exercise.

BMI and average waist size have also climbed steadily

Meanwhile, across the same period, the US saw average body mass index rise by an average of 0.37% per year, with the most dramatic increase being in young women.

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A person is considered abdominally obese if their waist circumference is 88 cm (34.65 in) or more for a woman, and 102 cm (40.15 in) or more for a man.

The team also looked at changes in abdominal obesity, which some consider an independent risk factor for death, even among people with normal BMIs - thus being "apple-shaped" is considered riskier than "pear-shaped" for the same height and weight.

A person is considered abdominally obese if their waist circumference is 88 cm (34.65 in) or more for a woman, and 102 cm (40.15 in) or more for a man.

The researchers found that average waist size went up by 0.37% per year for women and 0.27% for men.

They found that abdominal obesity has gone up in both normal-weight and overweight women, while for men it only went up in overweight men.

Prof. Ladabaum says these changes have occurred in the absence of significant changes in calorie consumption:

"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."

Although he and his colleagues did not investigate the types of food consumed, they were able to calculate the total daily calorie, fat, carbohydrate, and protein consumption over the period. They found these have not changed significantly over the last 20 years.

"It remains controversial whether overweight alone increases mortality risk," says Prof.Ladabaum, "but the trends in abdominal obesity among the overweight are concerning in light of the risks associated with increased waist circumference independent of BMI."

'Troublesome trends in younger adults'

While increased calorie intake is often blamed for the current obesity epidemic in the US, the researchers say they found no evidence of this in their study, as Prof. Ladabaum concludes:

"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."

He also warns that that while it looks like obesity rates appear to be levelling out in the US, their "analyses highlight troublesome trends in younger adults, in women, and in abdominal obesity prevalence, as well as persistent racial/ethnic disparities."

In January 2014, Medical News Today reported how a study from Kansas State University suggests less sitting and more moving improves health and quality of life. The researchers showed people who do this have a lower risk for chronic diseases, including heart disease, diabetes, stroke, breast cancer, colon cancer and others.