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New Study Suggests Decreased Physical Activity a Key Factor in the Rise in Obesity

Why is obesity more prevalent in America now than it was 25 years ago? Ask that question to anyone you know, and their answer will likely include an explanation of how Americans are eating more now than we were then. But is there is another side to the equation? Is it possible that America’s collective weight gain has been due to a decrease in physical activity, rather than an increase in calorie intake?  A new study published in The American Journal of Medicine (Ladabaum, et al, 2014) suggests that very idea, finding that while our calorie intake has remained the same over the last two decades, our physical activity has plummeted, factors that are positively associated with increased obesity.

The study was conducted using data from the National Health and Nutrition Examination Survey (NHANES) to examine trends in obesity, physical activity, and calorie intake from 1988 to 2010. Results support previous findings on the growing prevalence of obesity, showing substantial increases in BMI, waist circumference, and abdominal adiposity.

The most surprising findings from this study relate to trends in daily calorie consumption and physical activity. The authors found that Americans, on average, are eating about the same number of calories per day as they were in 1988. This is particularly interesting given the fact that the rising prevalence of obesity is commonly attributed (at least in part) to a collective change in eating habits. If this were true, however, we would expect to see a sizable increase in daily calories consumed over the period between 1988 and 2010. Instead, the study showed no significant increase in calorie consumption, and the researchers found no association between daily caloric intake and increased BMI or waist-circumference.

Based on these findings, it does not appear that excess calories are contributing to America’s obesity problem, but what about physical activity (the other side of the energy balance equation)? The study showed a major increase in the percentage of Americans reporting no leisure-time physical activity (i.e. activity beyond that which is job related) between 1988 and 2010. The percentage of women reporting no leisure-time physical activity more than doubled, increasing from 19.1 percent to 51.7 percent. Similarly, the percentage of men reporting no leisure-time physical activity nearly quadrupled, from 11.4 percent to 43.2 percent. Although they were not able to see an association between daily caloric intake and increases in BMI or waist-circumference, the researchers did find an association between decreased physical activity levels and increased BMI.

Put simply, this study suggests that Americans seem to be burning fewer calories in the form of physical activity. Does this mean decreases in physical activity are the sole culprit for the increased prevalence of obesity? There are likely multiple factors at play. In addition, the study authors acknowledge limitations of surveys that rely on self-reporting such as NHANES (which used self-reported 24-hr recalls for the data on calorie intake), as they may underestimate and/or underreport calorie intake. This could be due to a combination of participants purposefully leaving out food items from their 24-hr recalls, and simply not remembering to record some of the items they ate.

Some consumer sub-populations have been much more affected by the rise in obesity than others. For example, women between the ages of 18 and 39 showed the largest increases in waist circumference and BMI. Racial and ethnic disparities were also apparent; for example, non-Hispanic black women showed the largest increase in prevalence of obesity.

From this research, it appears that a major drop in physical activity has been a primary driver of our nation’s rise in obesity. The researchers state that these findings should not undermine the importance of calorie control in the prevention or treatment of obesity on an individual level. The results simply tell us that, on a population level, we may be putting too much focus on diet and not enough on physical activity. Although many schools and organizations have “gotten moving” in an effort to help America become more physically active, more work is needed to reverse the downward physical activity trend.

For more information on this study, view the new IFIC Foundation infographic, “Obesity and Lack of Physical Activity.” (PDF link)


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