CICO is food nutritionist, food lobbyist mantra, posing physiology and nutrition as a hard science like physics. In the Minnesota starvation experiment, it was shown that a reduction calorie intake cause a reduction in energy expenditure of the body. Therefore, calories in calories out is defacto irrelevant when considering a complex system like the human body. A system regulated by hormones, such as insulin, which regulate body weight. in retrospect, it seems rather amusing to think that the human body will choose to burn fat when food is constantly going in, when fat is an emergency source of energy for prolonged periods of time without food.
https://www.youtube.com/watch?v=APZCfmgzoS0
https://www.youtube.com/watch?v=zcMBm-UVdII