The institutionalised conviction that we get fat simply because we overeat is based on the kind of fallacious reasoning that would lead to a failing grade in a high-school logic class. The first law of thermodynamics tells us that energy is neither created nor destroyed, so the calories we consume must be either stored, expended or excreted. If we are getting fatter, we must be taking in more energy than we are giving out: we are overeating.

But this does not tell us which direction the arrow of causality is pointing. Do we get fat because we overeat, or is some regulatory or hormonal phenomenon driving us to fatten and in turn causing us to overeat? Saying that obesity is explained by overeating and/or sedentary behaviour is like saying that chronic fatigue syndrome is explained by a lack of energy. It sounds obvious; it tells us nothing.