I think this is a very good question bevause
even if I go on a high fat/low carb nutrition your body should still be capable of „using“ carbs to generate energy. Especially in situations where only carbs are easily available (high altitude expeditions etc)
The other point is that high fat diets have shown to increase fat adaption / utilisation in studies but with the drawback of using more oxygen to produce the needed ATP.
Is this a thing to consider when going to high altitude where the oxygen pressure drops below a critical point meaning less oxygen is available?