The human body is practically a closed system. The matter exchanged through breathing, urinating and pooping is negligible compared to the total mass of the human. Additionally, you can add the mass of the breathed air, consumed/excreted food/water to a new system, which will then be closed. CICO of such a system would be absolute. But the chemically stored energy of the food and the body vastly outranges that which is thermally exchanged. Simplifying the model to only consider energy content of the eaten food as the energy input is justified. Miscellaneous energy outputs can be roughly accounted for by various efficiency factors. Calories in will correspond to the total chemical energy potential as determined through burning tests, for example (as found on food packaging). Calories out will correspond to metabolic processes and movement (typically factors accounted for) as well as any losses through digestion.  For example, the chemical potential of sugar is easily converted into energy useably by the body, resulting in very high efficiency factors (near 1), while meat is more difficult to process. Meat might therefore have a lower efficiency factor (say 0.85), meaning 15 percent of the potential energy is not extracted. Therefore calories out in the case of meat consumption is higher than calories out in the case of eating sugar.  When accounting for all energy streams, CICO has no limitations. Unless I have misunderstood you, and you meant a hormonal model to explain why fat people have poorer impulse control, and therefore consume more food than normal people...