[relueeuler]

Your description above is not how the body works. To support a mass M must require a quantity of energy E obtained from food. The attempt to alter this model with “loose” ideas about “burn rates” therefore does not make sense.

The mathematics of weight loss is laid out clearly in 20 minutes here:

https://m.youtube.com/watch?v=vuIlsN32WaE

[hsn915]

Let's suppose (it's probably true) that there's an absolute minimum amount of calories your body must use per day to stay alive today.

Is that amount even sufficient to stay alive every day? Maybe your body can reduce some organ function for some amount of time, but it can't keep it shutdown forever.

ok, let's assume we're talking about the minimal burn rate that can sustain your body without shutting down any organ function what so ever.

Does that mean your body is burning exactly that amount of calories every day like a clock?

Certainly not. It's probably burning a lot more to help you move and think and deal with the daily stressors.

When you cut calories what generally happens is you feel lethargic. As if you can't muster the energy to do what you want to do and which you could previously do without problems.

I'm not into "studies" but I'm pretty sure there are some studies that show people who cut calories after a while their body adapts and lowers their base metabolic rate.

[naasking]

Yes, when calories are cut below a certain level, your body downregulates "NEAT" (non-exercise activity thermogenesis), which includes activities like fidgeting, walking around a lot or walking instead of driving, taking stairs instead of elevators, etc. This actually accounts for a considerable number of calories.

Furthermore, a mass M needing energy E is too simplistic because it neglects environmental factors. A mass M in sub-Saharan Africa or a tropical jungle will require considerably more than E for maintenance than the same mass M in a relatively mild climate. This is because your body expends energy to regulate homeostasis (sweating and increased heart rate to cool the body in hot and humid conditions), so more difficult environments require more energy.

That said, "calories in vs. calories out" is still true, you just have to consider that your "calories out" responds to the number of calories in. Drop them too low too quickly, and your NEAT drops to maintain energy balance. But, you can force your body to expend more of that energy by requiring yourself to walk (increasing NEAT), or putting yourself in uncomfortable environments (like saunas), or my forcing yourself into strenuous exercise regimes (weight lifting, HITT, etc).

[stonemetal12]

For the studies type:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4803033/

Total energy expenditure is constrained not additive.