Earlier quoted context omitted.
I want to expand a little bit more: The body can be simplified down into an input/output black-box - almost like a thevenin equivalent. But most people unaccount for lost energy in the output. No two bodies are alike and no two bodies share the same efficiency in breaking down food, absorption, conversion, and excretion. Yes, you can collect the excrement and analyze the output. But that analysis doesn't tell you muc…
You're describing difficulties with precise measurement of inputs and outputs, but I don't think the difficulty of making those measurements refutes the core idea of CICO at all. The difficulty of actually implementing CICO suggests we should have sympathy for those who fail. This doesn't refute CICO.
CI is based what people eat, but how much and how often people eat (if you don't expect perfect and infinite willpower) is based on how hungry they are, and that is hormonally controlled, based on complex feedback loop that includes insulin levels (so what you eat and how often) and activity levels.
CO includes many things you have no conscious control over, like heating (ever stopped being cold after a meal?), muscle repair and buildup, brain, etc. Only small fraction of that is exercise.
People saying "it's just CICO" are as disgusting as millionaires saying "just stop being poor, it's easy, you have to make more than you use" while knowing nothing (and not caring) about student loans, healthcare costs for non-insured people, housing crisis or anti-labor practices that suppress wages.