Two interesting studies were published in the International Journal of Obesity this week. The first one examined how energy expenditure has changed in North America and Europe since the 1980s. It is generally assumed that the obesity epidemic is fueled partly by increased food intake and partly by a more sedentary lifestyle, but this assumption remains largely unproven. The present study aimed to directly measure energy expenditure, beginning when the technology to do so was developed (1980s).

Using three independent methods, the researchers show that energy expenditure has not decreased over the past two decades. In fact, people now use slightly more calories each day than we used to. But the authors of the study are quick to point out that energy expenditure and overall motor activity are not the same thing. This becomes clear in the second paper, which has one author in common with the first.

In the next study, scientists measured the correlation between motor activity and body composition (body fat %). They report that indeed higher body fat percentage is correlated with decreased locomotion. If you look closely, this does not contradict the first paper.

The difference between the two studies is that one measures locomotion (movement) and the other measures energy expenditure directly, independent of movement. Locomotion is related to total energy expenditure, but the relationship is highly dependent on the body mass of the individual. To put it plainly, bigger people use more energy to do the same activity as smaller people. So while it is clear that obesity is associated with moving less, these individuals are generally burning the same number of calories as leaner individuals who move more.

Even more telling is that the amount of energy people use in North America and Europe is similar to the amount used by native, indigenous populations and also animals in the wild. This makes it very difficult to argue that decreased energy expenditure is contributing to the obesity epidemic. So what is truly fascinating about these two papers is the conclusion that can be drawn about the cause of obesity: we are eating too much.

Evidence is mounting that people in Western cultures are vastly underestimating their energy intake, which is causing a tremendous rise in obesity prevalence. Based on this information it seems like dietary habits should be the first point of intervention if we want to reverse this trend.

Thoughts?