alt Hacker News> The only thing that GLP-1 agonists prove is that CICO does indeed work
This is incorrect, as demonstrated over and over again. For many people's bodies, consuming less will result in the body changing its metabolism to burn less, and not dipping into fat stores. Conversely, for many people's bodies, exercising more does not in fact change their metabolism and the amount of energy they burn. (There are studies that going from "zero" to "not zero" makes a meaningful difference, but "not zero" to "quite active" often doesn't.) "CICO" is not useful or actionable for many people.
That there is variance in energy expenditure both within a population and within a person over time doesn't mean that a caloric deficit doesn't work. It just means that using a single scalar value (which is usually a gross estimate) to drive your caloric intake is a poor approach.
The body has means to regulate it's energy expenditure to maintain homeostasis, and in some people it can be a hundreds of kcal difference. But if you're trying to lose body fat on a 10% estimated deficit and fail, the conclusion shouldn't be that a 20% deficit will also fail.