Calorie in / calorie out isn’t a theory, it’s a thermodynamic corollary. I compare it to flat eartherism because “things go down” is similarly a corollary of gravity. The missed link with the latter is that down isn’t what it intuitively means. The missed link with colloquial energy-balance interpretations is “calorie in” and “calorie out” don’t mean what people think it does.

If you have a healthy metabolism, cutting calories and increasing burn—cereris paribus—should spike a starvation response. That has some perks. But it should also reduce your resting metabolism, sometimes below even maintenance levels; it should increase your absorption and sequestration of energy; it should alter your taste to make calories more appetising, and increase existential anxiety around the procuring of those. (Counterfactial: I’m someone who is fine being hungry. )

Just as one can design short bridges on a flat-earth model and be fine, one can deploy this simplistic model (it’s not a theory, that’s the vastly more complex energy-balance model) to make short-term gains. But it’s fundamentally wrong in that it builds the wrong intuitions. Similar to how thinking of our brains as a steam engine or microcomputer sort of works, in some cases, or if you’re trying to be punchy in internet comments, but is fundamentally wrong and misleading.