> No, not necessarily. Several studies have shown that different people can get different amounts of energy out of the same food, depending amongst others on their gut micro biome, though stress also seems to play a role. It’s never going to be that simple.
Sure, so they just need to compute their at-rest calorie consumption differently, and from there the rest is the same.
> Conventional thermodynamics don’t work when you consider a full human, which is a very out-of-equilibrium and not-isolated system. Conservation of energy does not tell you anything about the efficiency of the energy extraction process.
This is like saying that even if you don't refuel your car it will never stop, because different cars have different mpg ratings. A human is indeed a closed system when you consider the works it outputs and the calories it ingests, unless I somehow missed a newfound capacity for photosynthesis. The fact that it might be a bit harder to compute calorie requirements than what might be naively done does not allow you to just dismiss everything else.
> Sure, so they just need to compute their at-rest calorie consumption differently, and from there the rest is the same.
Not at all, because different gut biomes are more efficient than others at processing some kinds of foods. Famously, some populations are more effective at processing fish than others. Really, "amount of calories" works on average, but people with food issues tend not to be average, otherwise they would not be outliers.
You cannot take any assumption for granted. If it were so easy, we'd have solved it. It's only very recently that we started to grasp the role of the microbiome, and we are far from having explored it all.
> A human is indeed a closed system when you consider the works it outputs and the calories it ingests, unless I somehow missed a newfound capacity for photosynthesis.
You missed a lot of things. Radiative heating and convective heat exchange, for one. Our body spend a lot of energy to heat up when it's cold, and try really hard not to do anything when it's hot. This works differently for different people; I tend to heat up fairly efficiently and I am very rarely cold; my GF is the other way around. Obviously, this is also environment dependent.
Plus, we eat all the damn time. How can you seriously be arguing that we are a closed system? Again the problem is not that it's slightly harder. The problem is that it is multifactorial, that we don't know all the factors, and that the importance of each factor varies with genetics, history, and the environment.
> The fact that it might be a bit harder to compute calorie requirements than what might be naively done does not allow you to just dismiss everything else.
Sure, so they just need to compute their at-rest calorie consumption differently, and from there the rest is the same.
> Conventional thermodynamics don’t work when you consider a full human, which is a very out-of-equilibrium and not-isolated system. Conservation of energy does not tell you anything about the efficiency of the energy extraction process.
This is like saying that even if you don't refuel your car it will never stop, because different cars have different mpg ratings. A human is indeed a closed system when you consider the works it outputs and the calories it ingests, unless I somehow missed a newfound capacity for photosynthesis. The fact that it might be a bit harder to compute calorie requirements than what might be naively done does not allow you to just dismiss everything else.