Friday, September 12, 2008

Inactivity and Weight Gain

Most of the papers I read in the field pay lip-service to some familiar stories: thrifty genes; calories in, calories out; energy density; fat intake; gluttony and sloth.

It may sound counterintuitive, but how do we know that inactivity causes overweight and not the other way around?  In other words, isn't it possible that metabolic deregulation could cause both overweight and a reduced activity level? The answer is clearly yes. There are a number of hormones and other factors that influence activity level in animals and humans. For example, the "Zucker fatty" rat, a genetic model of severe leptin resistance, is obese and hypoactive (I wrote about it here). It's actually a remarkable facsimile of the metabolic syndrome. Since leptin resistance typically comes before insulin resistance and predicts the metabolic syndrome, modern humans may be going through a process similar to the Zucker rat.

Back to the paper. Dr. Nicholas Wareham and his group followed 393 healthy white men for 5.6 years. They took baseline measurements of body composition (weight, BMI and waist circumference) and activity level, and then measured the same things after 5.6 years. In a nutshell, here's what they found:
  • Sedentary time associates with overweight at any given timepoint. This is consistent with other studies.

  • Overweight at the beginning of the study predicted inactivity after 5.6 years.

  • Inactivity at the beginning of the study was not associated with overweight at the end.

In other words, overweight predicts inactivity but inactivity does not predict overweight. With the usual caveat that these are just associations, this is not consistent with the idea that inactivity causes overweight. It is consistent with the idea that overweight causes inactivity, or they are both caused by something else.