Americans spend billions of dollars every year on costly fad diets and expensive weight-loss programs built around sensible eating plans. But new research suggests the real culprit in the nation's ever-expanding waistline isn't diet alone, but a lack of exercise.
Sedentary lifestyles are the primary cause of rising obesity rates in the U.S., according to a new Stanford University analysis of information from the long-running National Health and Nutrition Examination Survey (NHANES).
The study, published in the American Journal of Medicine, found a sharp decrease in physical exercise and an increase in average body mass index (BMI) over the past two decades, while caloric intake has remained roughly the same.
Investigators concluded that a nationwide drop in leisure-time physical activity, especially among young women, may be responsible for the upward trend in obesity rates, Medical Xpress
By analyzing NHANES data from the last 20 years, Stanford researchers found:
- The number of American women who reported no physical activity jumped from 19.1 percent in 1994 to 51.7 percent in 2010.
- For physically inactive men, the number increased from 11.4 percent in 1994 to 43.5 percent in 2010.
- Over the course of the study, the 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," said lead investigator Uri Ladabaum, M.D., and associate professor of medicine at 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."
The investigators found total daily calorie, fat, carbohydrate, and protein consumption has not changed significantly over the last 20 years, yet the obesity rate among Americans is continuing to rise.
Researchers also tracked the rise in abdominal obesity, which is an independent indicator of mortality even among people with normal BMIs. Abdominal obesity is defined by waist circumference of 34.65 inches or greater for women and 40.16 inches or greater for men. Data showed that average waist circumference increased by 0.37 percent per year for women and 0.27 percent per year for men.
The group most affected by increased rates of abdominal obesity was women.
"The prevalence of abdominal obesity has increased among normal-weight women and overweight women and men," observes Dr. Ladabaum. "It remains controversial whether overweight alone increases mortality risk, but the trends in abdominal obesity among the overweight are concerning in light of the risks associated with increased waist circumference independent of BMI."
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