Are We Fat Because We Eat Too Much, Or Lack Physical Activity?

Are we fat because we eat too much, or lack physical activity?

Most people would say, “It’s both.” Most people would be wrong, at least in terms of populations rather than individuals.

Obesity results from a protracted imbalance between energy intake (calories we eat) and energy expenditure (physical activity and resting metabolism).

Overweight and obesity have increased significantly over the last 25 years in most of the developed world. Is it because we started eating more, or that we have so many energy-saving devices that we now expend less energy on physical activity? If we are less active due to technologic advances, yet keep eating as much as in the past, we will gain weight as the excess calories are stored as fat.

Technologic advances over the last 150 years have allowed us to transform from a labor-intensive agrarian economy to one based on services and information. Computers, in particular, have made it much less labor-intensive to get our jobs done. For example, when I was a hospital intern 30 years ago, I made multiple daily trips from the patient care floors downstairs to Radiology to look at x-ray films. Now, the “films” are at my fingertips on computers close to the bedside.

Have trends in technology over the last 25 years continued to reduced the energy expenditure needed to get through our days? Alternatively, are we exercising less? Either explanation would lead to weight gain if caloric intake remained the same.

Researchers in 2008 studied populations in Europe and North America, examining trends in physical activity energy expenditure over time, since the 1980s. Energy expenditure was evaluated with a highly accurate method called “doubly labelled water.” They found that physical activity energy expenditure actually increased over time, although not by much. They conclude that the ballooning waistlines in the study populations are likely to reflect excessive intake of calories.

(All I have is the abstract of the article. I’ll try to get the full article and report back here if anything additional is interesting.)

So according to Westerterp and Speakman, the problem has not been lack of physical activity. We’re simply eating too much.

On the other hand, a 2011 study found that daily work-related energy expenditure decreased by over 100 calories in the U.S. over the last 50 years.  That could certainly contribute to our expanding waistlines.

Steve Parker, M.D.

Reference: Westerterp, K.R., and Speakman, J.R. Physical activity energy expenditure has not declined since the 1980s and matches energy expenditures of wild mammals. International Journal of Obesity, 32 (2008): 1256-1263. Published online May 27, 2008. doi: 10.1038/ijo2008.74

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