Blow your snark out your butt. I know what a closed system is. You, apparently, do not. A human is not a closed system.
“the laws of physics apply to everything.”
Not when misapplied. A human is not a car, and your body is not a gas tank. Assuming, of course, you crap sometimes. You body does not use or store all energy coming in. There is a reason some people can eat very little and not lose weight, while others can eat a lot and not gain - and the difference is NOT exercise.
you can ABSOLUTELY model the human body as a closed system. all you need to do is define the boundary of the system such that it is closed w.r.t the energy balance equation and then proceed to account for all energy in and all energy out. i’m not surprised you don’t understand this... as a biologist, you likely never studied physics in any depth, so stop trying to mislead people with your ignorance. sure, everybody has different metabolic rates. if you’re eating very little and not losing weight, it’s because your TDEE is low. you do know what TDEE is, right? although that can be estimated with reasonable error ranges, there will always be people with medical conditions and other factors that affect their metabolism and cause their TDEE calculations to be off. this doesn’t mean the physics is wrong... it simply means that the estimates and assumptions for calculating your their specific TDEE values are off. that’s why it takes time and careful tracking of calories in and calories out over some period of time to close in on a more accurate TDEE number.
the fact that you think the laws of physics only applies to some things and not others puts you squarely in the “nutter” category. i’m done here. go educate yourself. come back and apologize when you realize that you don’t know what you think you know. or not. i don’t really care. just stop deliberately misleading people and telling them that science can’t explain all of this. it can and it does.