One of the favourite slogans of diet advisors is “energy in equals energy out”. They even add “you can’t change the laws of physics/the laws of the universe.” I don’t know if they know what the laws of the universe say.
There are four laws of the universe. We can largely ignore the zeroth and the third in the world of dieting. The two that we need to take into account are the first and the second and neither of these says energy in equals energy out.
The first law says: “In a closed system, in thermal equilibrium, energy can neither be created nor destroyed.” Energy can be changed from one form to another, but it shall be conserved. The human body, however, is not a closed system and it is not in thermal equilibrium (although it is continually trying to be there). So, we also need to consider the second law.
The second law (entropy) has been called the law of common sense – it says that energy will be lost and energy will be used up in making available energy and we need to take both of these into account. This is the law that proves that a calorie is not a calorie, as even Weight Watchers cottoned on to with their launch of ProPoints in November 2010. The energy used up in making carbohydrate, for example, available to the body as energy vs. the energy used up converting protein to usable energy is substantially different. 100 calories of carbohydrate eaten may make 93 available to the body; 100 calories of protein eaten may make only 70 available (Ref 1). That’s a significant advantage for dieters and helps to explain the effectiveness of low carbohydrate diets.
These ‘laws of the universe’ were developed during the industrial revolution to help understand if we could make a perfectly efficient steam engine. The laws were and are all about energy, not weight. The laws say nothing about weight being conserved – we humans flit between energy and weight interchangeably in the world of dieting and our conversions and assumptions are wrong.
The laws of the universe were never intended to become the fundamental principles of dieting. They do have some relevance to dieting, but only when they are correctly applied and when all the caveats are allowed for. There is simply no law that says energy in equals energy out. Even if there were, the corollary would surely be – less energy in equals less energy out, which brings us nicely to the second piece of knowledge…
Ref 1: Jequier, “Pathways to Obesity”, International Journal of Obesity, (2002). (See reference 29 on The Obesity Epidemic)