Does Diminished Work Activity Explain Our 50-Year Overweight Trend?

Daily work-related energy expenditure over the last half-century in the U.S. has decreased by over 100 calories.  This may well explain the increase in body weights we’ve seen, according to a 2011 article in PLoS ONE.

I sorta hate to open this can o’ worms, but it’s important.  As a population, are we fat because we eat too much or because we burn too few calories in physical activity?  Or is it a combination?  The correct answer may help us learn how to reverse the trend.

Methodology

Authors of the study at hand estimated the amount of energy (calories) necessary to perform various jobs, then noted changes in numbers of people employed in those jobs over time.  In the early 196os, for example, nearly half of U.S. jobs required at least moderate intensity physical activity, compared to less than 20% demanding that degree of energy now.  The authors note the dramatic shift from manufacturing to service-type jobs over the last 50 years.  Service jobs, like mine, often entail a lot of sitting and standing around. 

They chose to ignore how much energy we expend in exercise, figuring what we do in a 40-hour work week overwhelms the 1-2 hours of  exercise we may do.

Researchers’ Findings and Conclusions

They found that work-related daily energy expenditure has decreased by over 100 calories over the last half-century, which (in the authors’ view) would account for a significant portion of the increased body weight we’ve seen.  Since physically demanding jobs are unlikely to see a resurgence, the authors advocate physically active lifestyles away from workplace. 

Discussion

Surveys indicate that only one in four of us fulfill the federal physical activity guidelines: 150 minutes a week of moderate intensity activity or 75 minutes a week of vigorous intensity activity.  When activity is actually measured with an accelerometer, only one in 20 achieve that lofty goal.  We over-estimate how much we exercise, and under-estimate how much we eat.

(If you want to emulate a Paleolithic lifestyle, you should probably shoot for an hour of exercise daily, not 20 minutes as above.)

The researchers cite studies showing significantly increased average per capita calorie consumption in the U.S. over the last several decades.  Some experts estimate the caloric increase is in the range of 500 a day for adults; the authors here think that’s too high but don’t offer a specific alternative. Looking at one of their references (Hall et al), they must think the increase is closer to 200 calories a day, comparing 2005 to 1975.

Several studies suggest that average daily energy expenditure has not decreased in developed countries, at least from the 1980s to the present.   A strength of the current study at hand is that it spans about 50 years, up to 2008.

My sense is that both calorie consumption (too much) and physical activity (too little) contribute to our overweight problem that started 40 or 50 years ago.  Excessive consumption is the predominant factor.  To “exercise off”  the calories in a Snickers candy bar, you’d have to jog for an hour.  If you’re watching your weight, you’ll have more success if you just skip the Snickers.

In case you couldn’t tell,  I still believe in the “calories in/calories out” model of overweight and obesity, aka “the energy balance equation.”  At the same time, I believe certain foods  are more fattening than others: concentrated sugars and refined starches.

Steve Parker, M.D.

References:

Church, T.S., et al.  Trends over 5 decades in U.S. occupation-related physical activity and their associations with obesity.  PLoS ONE, 2011, 6(5): e19657.  doi: 10.1371/journal.pone.0019657  

Swinburn, B., et al.  Increased food energy supply is more than sufficient to explain the U.S. epidemic of obesity.  American Journal of Clinical Nutrition, 2009 (90): 1,453-1,456.  

Hall, K.D., et al.  The progressive increase of food waste in America and its environmental impact.  PLoS ONE, 2009, 4(11): e7940.  doi: 10.1371/journal.pone.0007940