Ege

Two clarifications that follow from this and that people immediately ask about.

No, breathing harder on purpose does not help. The rate is set by how much you are metabolising, not the other way round. Deliberate over-breathing just changes your blood chemistry and makes you dizzy.

The mass you lose in a day is mostly not fat. After a long ride you might be several kilograms lighter on the scale, and nearly all of that is water and glycogen. Fat oxidation over a very hard day amounts to a much smaller number, and the scale figure recovers within a day or two as you rehydrate.

That is worth knowing because it is the main reason day-to-day weight readings are so noisy relative to actual change.

There is a nice everyday demonstration of the principle: the racket arm of a lifelong tennis player has measurably denser bone than the other arm.

Same person, same genetics, same diet, same age. The only variable is decades of load on one side. It is about as clean a natural experiment as you get, and it shows the adaptation is local and driven by mechanical demand rather than by anything systemic.