The Calories In/Calorie Out (CICO) model of weight loss is gets brought up frequently if you hang around in certain communities -low carb, vegan, etc. Any place where you may find people trying to adopt a lifestyle change in order to lose weight, really. Some people say that the discussion starts and ends with this way of understanding what is going on, while many low-carb people describe that eating that way helps them to control their hunger, and Jason Fung has given analogies that insulin and ghrelin levels behave as parts of a control system to determine how your body uses the energy you are taking on board.
It is quite interesting to watch these discussions coming from a place of having a background in fluids because, frankly, this is a long-solved issue for us. I am not telling you that I have the answer, but, if this was a fluids problem I would tell you that both things can be true at the same time. Meaning that the insulin, ghrelin, and blood sugar levels, and other quantifiable aspects of your metabolism could be satisfying multiple conditions at once. In fact, they almost certainly are. Medicine isn’t my field so I can only speculate from this, but I will give some color and analogies here from the engineering point of view.
Asking “why is my weight so high?” and then pointing to the CICO method isn’t outright false, but it is incomplete, in the sense that using this rule alone is not sufficient to describe the observed behavior of the system. A straightforward analogy here is the temperature of your house during the winter. It’s cold outside and you have a heater putting energy in the air in your house. As always, basic energy conservation is held here; basically, there is some rate of energy being put into the house and some rate of energy being taken out of the house. At some equilibrium temperature, these rates are equal (on average) and that is what the temperature of the air in your house will be. But these statements just relate to the truth about what’s happening with the energy and heat fluxes in the system. All true facts. But they won’t help you figure out why the air temperature in your house is a certain value. And to bring the analogy back, that’s what is missing in the CICO description of weight loss.
When the ambient temperature isn’t to your liking, you can set a target temperature for your HVAC system to keep the inside air at, so another player has entered the game of informing us about what is happening with our house air temperature. It will add its own set of rules to the system behavior while respecting the rule for energy balance. The final air temperature that is observed is the result of the energy balance being satisfied, along with the control system rules. But in understanding the role of each rule in the system, we note that, to within reasonable limits, the temperature in the house it determined by the control system behavior and not by the fact that an equilibrium temperature is the result of having an internal air temperature at which the rate of heat leaving the system is the same as the rate at which it enters.
Thus, to bring this discussion to an end, the point I’m making is that it would be appropriate if the weight of a person was determined by some individual or set of control system behaviors. And indeed, this is basically the additional consideration that Dr. Jason Fung and other insulin model advocates are trying to bring up when they raise this point. To change the weight of your body, you must change the “weight” setting of the control system which is in driving system behavior. To a first approximation, insulin appears to be this thing to set this. Reduce the average insulin in your body, and you can change the weight setting that you want to be at. The real upshot of this is that it is a way to make decisions about what to eat, where lower glycemic index foods require lower insulin to process by your body. This is the logic behind the ketogenic diets which have been so popular recently. The CICO rule still applies, but by adding the control system understanding of the problem, the right decisions become easier to understand.