> CICO, or "calories in, calories out", is obviously real.
But it's a very bad proxy for anything related to weight. Water is 8.8lbs per gallon and yet has 0 calories so you ignore your entire water maintenance cycle in the accounting. It also assumes that every single molecule of calorie-bearing food is metabolized homogeneously in your body which isn't the case. Your digestive system will happily pass unwanted calories without absorption (especially fat) and tends to store extra water along with carbohydrates (4x by weight).
It's a very simplistic idea that is only true in absolutes. It's a restatement of the 2nd law of thermodynamics which applies to literally everything and is so general and vague as to be useless.
But it's a very bad proxy for anything related to weight. Water is 8.8lbs per gallon and yet has 0 calories so you ignore your entire water maintenance cycle in the accounting. It also assumes that every single molecule of calorie-bearing food is metabolized homogeneously in your body which isn't the case. Your digestive system will happily pass unwanted calories without absorption (especially fat) and tends to store extra water along with carbohydrates (4x by weight).
It's a very simplistic idea that is only true in absolutes. It's a restatement of the 2nd law of thermodynamics which applies to literally everything and is so general and vague as to be useless.