There are many adherents to the Calories In/ Calories Out (CICO) theory that constantly bleat about “It all comes down to the First Law of Thermodynamics”. The First Law of Thermodynamics refers to a law of physics where energy cannot be created or destroyed in a closed system and is ALWAYS true. It certainly holds true in the complex world of human physiology, but is completely irrelevant. What the CICO people think it means is that if you reduce calories intake, you must lose weight. Of course, it means nothing of the sort. So, let’s see why. The main reason is that the most important part of CICO is NOT the Calories In, but the Calories Out and the Insulin. Here’s our representation of the human body.