Overeating may not be the only culprit in the obesity epidemic; the changed landscape of the American workforce may have lent a hand, researchers say.
As jobs drifted from agriculture and manufacturing into service-based careers, the average workday energy expenditure sagged by more than 100 calories, Timothy Church, MD, PhD, MPH, of Louisiana State University, and colleagues reported in PLoS One.
"This reduction in occupational energy expenditure accounts for a large portion of the observed increase in mean U.S. weight over the last five decades," they wrote.
Many studies have indicated that increased food intake is largely responsible for the obesity epidemic, especially since leisure time activities haven't changed much over the past half-century, the researchers said. Still, there is little longitudinal population-based data for assessing the true causes of the explosion of expanded waistlines.
Church and colleagues examined energy expenditure for occupations in the U.S. private industry since 1960 using data from the U.S. Bureau of Labor Statistics and correlated that with mean body weights over time, derived from the National Health and Nutrition Examination Survey.
In general, there was a progressive decrease in the proportion of employees in goods producing and agriculture. On the flip side, there's been a growth in employees in service occupations, marking a shift away from jobs that require moderate intensity physical activity to those largely involving sedentary behavior, they said.
The researchers saw that in the early 1960s, almost half the jobs in private industry in the U.S. -- 48% -- required at least moderate intensity physical activity, although that figure has now fallen to 20% (P<0.001).
Also during the last five decades, mean daily energy expenditure at work has dropped 140 calories per day for men and 124 calories per day in women.
When the researchers conducted energy-balance models comparing predicted weights based on decreasing expenditure with actual NHANES data on weight, they saw that their estimates closely matched the actual changes.
For instance, if energy output fell 142 calories per day in men, the mean weights of 76.9 kg (170 lbs) in 1960-1962 would have ballooned to a mean of 89.7 kg (198 lbs) by 2003-2006 -- which closely matched the actual mean weight of 91.8 kg (202 lbs) reported in that year's NHANES.
"Given that it is unlikely that there will be a return to occupations that demand moderate levels of physical activity, our findings provide further strong evidence of the public health importance of promoting physically active lifestyles outside of the workday," they wrote.
They also noted that the reduction of 100 calories per day in job-related energy expenditure could be compensated for by meeting 2008 federal physical activity recommendations of 150 minutes per week of moderate intensity activity or 75 minutes per week of vigorous intensity activity.
The study was limited because the researchers could not account for the changes involved in labor associated with manufacturing jobs, as technological advances have also reduced the physical activity required to do those jobs. Thus, the findings may be an underestimate, they said.
On the other hand, they did not have data on the specific details of different types of jobs, such as the actual amount of time spent sitting or how employees get to work.
The researchers reported no conflicts of interest.
- Reviewed by Dori F. Zaleznik, MD Associate Clinical Professor of Medicine, Harvard Medical School, Boston and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner