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...