[WorldMaker]

CICO is an oversimplification that generally assumes that "all calories are equal" on both sides of the equation. On the one side it assumes uniform density of energy in foods and is based on a lot of rough estimates from burning foods in ovens. On the other side, most of our concepts of how much calories we "burn" in a day or given activity and how we use those calories in the complex biology of our bodies is not very far divorced from "assume the body is an ideal spherical furnace" based a lot on CO2 exhaled and temperatures raised. It is a greatly over-simplifying model on both sides of the equation.

Of course, greatly over-simplified models are still useful. CICO as a useful first approximation of a diet still has its uses and its places where it is more useful than some alternative models.

I think food calories and the way we talk about them (like food "contains" them, always burning them) feel a lot to me like the last bastion of Phlogiston Theory in any of the sciences. Chemistry has moved away from the "Calorie" as an approved unit of measure for the more accurate/more reliable "Joule", but also to remove some ties to old Phlogiston baggage.

I think most people would laugh at this idea pushed to its current Physics extreme that food should be measured in Joules by Relativity's infamous E = mc^2 mass-to-energy conversion ratio and that we should assume that the human body is some efficiency percentage of an ideal spherical fusion reactor. Joules In/Joules Out, right?. Why does it sound more accurate to so many as a model when it is "heat particles"/Calories?

(Which again, isn't a call to entirely toss the model, it serves many as a first approximation well enough. But it seems past time to develop better, more targeted models.)