That the body is a complex buffered system of variable effeciency is not in dispute - nor is it intractable with the thermodynamic position. It is always reali

That the body is a complex buffered system of variable effeciency is not in dispute - nor is it intractable with the thermodynamic position. It is always realistic to achieve a thermodynamic deficit, that there is some buffer or hysterisis to the curve (i.e non linearity) is not a big deal. The key insight is that the variability in the effeciency of your metabolism is not the bounding factor. A standard deviation of a few hundred calories in the natural variability of metabolism will meaningfully increase the difficulty of weight loss, but this simply means modulating the duration of your activity taking into account the diminishing returns of increased exercise and natural variability in metabolism. Now if this is psychologically or practically easy is an entirely different question.