Decrease in physical activity in many occupations over the last 50 years, and not just a change in calorie consumption, has contributed significantly to the obesity epidemic in the United States, according to a new study published this week in the journal PLoS ONE.

The study was the work of scientists from the Pennington Biomedical Research Center, part of the Louisiana State University System in Baton Rouge, and colleagues from other research centers.

Lead researcher Dr Timothy Church, who holds a John S. McIlhenny Endowed Chair at Pennington Biomedical, told the press that:

“Yesterday’s jobs have been replaced by sitting or sedentary activity.”

“In the last fifty years, we estimate that daily occupation-related energy expenditure has decreased by more than 100 calories per day, and this reduction accounts for a significant portion of the increase in mean US body weights for women and men,” he explained.

Using data from the US National Health and Nutrition Examination Surveys (NHANES) and the Bureau of Labor Statistics to calculate mean body weight and job-related energy expenditure, Church and colleagues estimated that only 20% of jobs in US private industry today demand a moderate level of physical effort, compared with 50% in the early 1960s.

They also estimated that compared with 50 years ago, today’s average American burns 100 fewer job-related calories a day.

Using computer models, they then predicted how much extra weight today’s workers would carry from burning fewer calories at work, compared to workers of 50 years ago, and the results came quite close to what today’s workers actually weigh.

For example, from 1960-62 to 2000-06, they estimated that the job-related energy expenditure for men went down by 142 calories a day. From the NHANES data they saw that the average male worker in 1960-62 weighed 76.9 kg, so they added to this weight the effect of burning 142 fewer calories a day and arrived at an average weight of 89.7 kg for 2003-06: very close to the NHANES figure of 91.8 kg.

The results for women were similar.

Church and colleagues concluded that their findings suggest changes in calorie intake cannot be the only reason for the weight gain in the American population.

“The causes of the obesity epidemic are a hotly debated issue, particularly in regard to the relative importance of diet and physical activity,” said Church.

“Our data provides further support to the importance of including both diet and physical activity in discussions related to be both the causes and potential solutions of the on-going obesity epidemic,” he added.

In their discussion, the researchers acknowledged the strengths and weaknesses of the study. A major strength was that it used nationally representative data, on both the obesity and the occupation data. They also used a well-known and accepted method for assigning energy expenditure intensity levels to occupation categories.

However, a potential weakness is that the same method was applied across all five decades, without taking into account the possibility that due to changes in working practices and new labor-saving technology, some of the occupation categories may have shifted to different levels of intensity of energy expenditure.

But, the effect of this potential weakness is to suggest that the estimates in this study may actually be under rather than over: perhaps the daily calorie usage due to job-related activity has gone down even more than this study proposes.

Another potential weakness, and there is insufficient data to eliminate its effect, is that the focus on energy expenditure intensity was based purely on type of occupation and not other factors related to the job such as type of travel to work, total sitting time, use of stairs, and so on. Also, there may be some misclassification of energy use across types of occupation: for example, some agricultural and manufacturing workers’ jobs may be less physically demanding than some jobs in the services sector.

But the researchers are confident, given this potential weakness, that they were able to minimize the effect of such misclassification because they were very conservative in assigning the energy expenditure intensities to the various occupations.

In 2008, new federal advice on exercise suggested that men and women should do at least 150 minutes a week of moderate intensity physical activity or 75 minutes of vigorous intensity.

However, only 1 in 20 Americans appears to be achieving this level. The researchers suggest if all American workers were to reach this target, then it would make up for the reduction in job-related energy expenditure over the last five decades.

“Trends over 5 Decades in U.S. Occupation-Related Physical Activity and Their Associations with Obesity.”
Timothy S. Church, Diana M. Thomas, Catrine Tudor-Locke, Peter T. Katzmarzyk, Conrad P. Earnest, Ruben Q. Rodarte, Corby K. Martin, Steven N. Blair, Claude Bouchard.
PLoS ONE 6(5): e19657. Published online 25 May 2011.
DOI:10.1371/journal.pone.0019657

Additional source: Pennington Biomedical Research Center.

Written by: Catharine Paddock, PhD