A lot of nutritionists, diet book authors, and health experts love to point out that losing (or gaining) weight is a simple matter of thermodynamics: calories in – calories out = net calories. If net calories are positive, you gain weight; if net calories are negative, you lose it.

And, at a very high level, that math is correct. But, like with most real-world processes distilled to simple models, it’s also missing a whole lot of important detail.

In short, there are three main problems with the calories in / calories out formula: we don’t accurately define ‘in’, we don’t accurately define ’out’, and we don’t accurately define ‘calories’. Other than that, it works great.

Over the next few days, I’ll be unpacking each of those three definitions in separate posts, to see if we can better understand how people lose and gain weight. As you might expect, the reality is a bit more complicated than a six word equation. And the devil – as well as our salvation – is all in the details.