There is a popular view on the subject of trying to reduce the amount of fat on one’s body, which is that the first law of thermodynamics states that if you eat fewer Calories than you burn, you must lose weight. This is, of course, false, since the laws of thermodynamics have to do with energy, not with weight. What is true is that if you consume fewer Calories than you expend, the total energy in your body must go down. But the laws of physics say nothing whatever about where that reduction in energy will come from.

As far as the laws of physics are concerned, it’s entirely possible that the total energy reduction will come from your glycogen stores, your muscles, and your internal organs. Heck, as far as the laws of thermodynamics are concerned, you can actually increase your fat stores on a Calorie deficit, so long as the energy added to them comes from someplace else in your body. Indeed, I’ve read about some interesting experiments on hibernating ground squirrels where after surgically removing fat, the squirrels (who were eating nothing because they were hibernating) added to their now-depleted fat stores.

The problem, I think, is that the people who love to talk about Calories-in-Calories-out or the laws of thermodynamics have an extremely mechanical view of the human body—by which I mean they think of it as if it’s the kind of machine that human beings build. They think of fat stores like the gasoline tank in a car. When human beings build a machine, we tend to have only one place where energy comes from because that makes it much easier to build. Though even in truth, even our more complicated machines have multiple energy storage sites. The typical car has the mechanical system driven by the engine and also an electrical system driven by the car’s battery. And the two systems are linked because the battery is recharged (and the electrical system can be partially powered by) the alternator (a kind of generator) which is powered by the engine.

So to be more accurate, the people who talk about thermodynamics tend to talk about the human body as if it’s the kind of machine that they would design, if they knew enough about machines to design them. It never occurs to them that the human body can break down every part of itself and will if it deems it necessary. It also never occurs to them that it can significantly down-regulate its metabolism. It never occurs to them how many forms down-regulating the metabolism can take, from feeling colder at the same temperature to reducing the amount of exercise taken with a feeling of lethargy, to reducing immune function, to slowing down healing, and still other things.

It also, I think, never occurs to these people the degree to which the body will abandon almost everything in the pursuit of food if it feels that it’s starving, and that includes things like sleeping more than four hours a night and being able to carry on a modern job.

That all this never occurs to them makes it much easier, I think, to talk about the laws of thermodynamics in the context of reducing the amount of fat on the body.