Well, here is the point where we disagree. My opinion is that CICO, despite being technically true, focuses your attention on eating and exercise as the most relevant causes of obesity. I agree with the statement "calories in = calories out" as observation. I disagree with the conclusion that the most relevant things for obesity are how much you eat and how much you exercise. And my aversion against CICO is that it predictably leads people to this conclusion. As you have demonstrated right now.

I am not an expert, but here are a few questions that I think need to be answered in order to get a "gears model" of obesity. See how none of them contradicts CICO, but they all cast doubt on the simplistic advice to "just eat less and exercise more".

• when you put food in your mouth, what mechanism decides which nutrients enter the bloodstream and which merely pass the digestive system and get out of the body?
• when the nutrient are in the blodstream, what mechanism decides which of them are used to build/repair cells, which are stored as energy sources in muscles, and which are stored as energy reserves in fat cells?
• when the energy reserves are in the fat cells, what mechanism decides whether they get released into the bloodstream again?
• (probably some more important questions I forgot now)

When people talk about "metabolic privilege", they roughly mean that some people are lucky that for some reason, even if they eat a lot, it does not result in storing fat in fat cells. I am not sure what exactly happens instead; whether the nutrients get expelled from the body, or whether the metabolism stubbornly stores them in muscles and refuses to store them in the fat cells, so that the person feels full of energy all day long. Those people can overeat as much as they can, and yet they don't get weight.

Then you have the opposite type of people, whose metabolism stubbornly refuses to release the fat from fat cells, no matter how much they starve or how much they try to exercise. Eating just slightly more than appropriate results immediately in weight gain. (In extreme cases, if they try to starve, they will just get weak and maybe fall in coma, but they still won't lose a single kilogram.)

The obvious question is what separates these two groups of people, and what can be done if you happen to be in the latter? The simplistic response "calories in, calories out" provides absolutely no answer to this, it is just a smug way to avoid the question and pretend that it does not matter.

Sometimes this changes with age. In my 20s, I could eat as much as I wanted, and I barely ever exercised, yet my body somehow handled the situation without getting much overweight. In my 40s, I can do cardio and weightlifting every day, and barely eat anything other than fresh vegetables, and the weight only goes down at a microscopic speed, and if I ever eat a big lunch again (not a cake, just a normal lunch) the weight immediately jumps back. The "calories in, calories out" model neither predicts this, nor offers a solution. It doesn't even predict that when I try some new diet, sometimes I lose a bit weight during the first week, but then I get it back the next week, despite doing the same thing both weeks. I do eat less and exercise more than I did in the past, yet I keep gaining weight.

Now, this is generally known that age makes weight loss way more difficult. But the specific mechanism is something more than just eating more and exercising less, because it happens even if you eat less and exercise more. And if this works differently for the same person at a different age, it seems plausible that it can also work differently for two different people at the same age. In the search for the specific mechanism, the answer "calories in, calories out" is an active distraction.