I would caution against relying too much on the physics of “calories in, calories out”. While it is iron-clad and incontrovertibly true that it is physically im

I would caution against relying too much on the physics of “calories in, calories out”. While it is iron-clad and incontrovertibly true that it is physically impossible to increase in mass if you consume fewer calories than you expend, “calories in” is more complicated than simple choice, and “calories out” is more complicated than a simple formula. Lowering one’s “calories in“ requires consistent exertion of willpower; willpower is strongly affected by blood glucose levels; lowering “calories in” lowers blood glucose - i.e., choosing to go hungry depletes the fuel required to maintain that choice. This makes it different to most other choices. (An evocative driving metaphor: “you can choose to avoid highways, you can choose to avoid toll roads, you can choose the scenic route, but choosing to avoid gas stations is quite a different matter”.) Likewise, “calories out” is not always something you can just calculate once based on your height and weight and activity and then aim to undershoot. Though it varies from person to person, the human body will generally lower its energy expenditure when it detects a calorie deficit, manifesting as physical lethargy and brain fog. For some individuals, restricting calories might very well “underclock” their body and/or brain below the point where they can keep doing their job. There are also genuine environmental factors like whether you’ve contracted Adenovirus‐36 (https://www.nature.com/articles/s41366-021-00805-6 https://www.nature.com/articles/s41366-021-00805-6) (Personal disclaimer: 5’8” male who’s consciously gone 150lb -> 180lb -> 140lb -> 190lb -> 165lb with minimal effort in the last decade.)