One of the most common beliefs about the world's rapidly growing waistlines is that we're growing larger because our jobs are no longer physically demanding - that it's our desks' faults. I've little doubt that those rare folks who still burn calories in their day jobs do in fact burn more calories than most. But the question is: Do those additionally burnt calories make those folks lighter than those of us who drive desks?
No doubt, on paper burning an extra 500 calories during a sweaty workday ought to have at least the same impact as not eating 500 calories through dietary discretion - maybe even more. Maybe more, because while all foods have calories, the quality and types of foods affect how many available calories there are for the body's stockpiles, while exercise is just a straight burn.
Yet studies looking at physical activity and weight don't seem to show major associations; even massive amounts of additional daily physical activity don't seem to translate into massive differences on the scale.
Take, for example, a study published last month in the American Journal of Clinical Nutrition. Researchers analyzed the impact of physical activity on weight in a cross-sectional analysis of nearly half a million men and women in China. Participants provided researchers with self-reported amounts of daily physical activity that, in turn, were broken down to account for activities due to work, housework, active transportation and recreation.
The results were rather underwhelming. Simply put, even huge amounts of additional physical activity failed to provide any dramatic benefits to weight. Without getting too bogged down in statistics, the researchers quantified physical activity in terms of something called "metabolic equivalent task hours" (MET-h/d). The average study participant was getting 22 of these a day. In terms of weight, the researchers found that a person needed to accumulate an additional 14 MET-h/d to affect a pound of body weight.
To put that number in a bit of perspective, you'd amass 3 MET-h/d by walking 3 miles in an hour. To put this all another way, to lose 10 pounds from physical activity would require you to be six times more active than the average person in this study; to lose 50 pounds would require you to be 30 times more active.
Breaking down this data further also helps to vindicate our desks. In this study, while factory workers were indeed more physically active than those with desk jobs, the average difference was around 10 MET-h/d - which would translate to a factory worker being about three-quarters of a pound lighter than a desk jockey as a result of his or her more physically demanding job.
What people sometimes seem to forget about physical activity is that it often leads people to eat more. Whether by means of the hunger generated by the exercise itself and subsequently larger portions or by means of reward - like many who hit the gym, exercise is rewarded with indulgences that we feel we've "earned" - calories burned through extra physical activity regularly lead us to add them back into our diets.
At the end of the day, there is no behavior more important to your health than regular exercise and physical activity, but please don't kid yourself into thinking it's your desk's fault. We're eating more, all of us, and if you want to see your weight change, you simply can't take your eyes off of your plates.
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Yoni Freedhoff, MD, is an assistant professor of family medicine at the University of Ottawa, where he's the founder and medical director of the Bariatric Medical Institute--dedicated to non-surgical weight management since 2004. Dr. Freedhoff sounds off daily on his award-winning blog, Weighty Matters, and you can follow him on Twitter @YoniFreedhoff. Dr. Freedhoff's latest book, The 10 Day Reset Solution: Why Everything You've Been Taught About Dieting is Wrong and How to Fix It, will be published by Random House's Crown/Harmony in 2014.