Your thoughts on this study? It says being keto adapted increases body fat burning,but decrease endurance becose fat requires more oxygen to make same amount of ATP than carbs.
https://examine.com/nutrition/low-carbing-for-endurance/
Is it hard fact of chemistry that fat will always require more oxygen to make given amount of ATP than carbs,or is it possible that after long period of adaptation,it can become just as efficient?
You might want to check out The Art and Science of Low-Carbohydrate Performance, by Volek and Phinney. They have been studying these matters (for decades in Dr. Phinney’s case), and have a lot of useful stuff to say. Yeah, your oxygen requirement changes, and that’s one way to tell whether an athlete is burning glucose or fat, but it sure doesn’t seem to hamper performance any. From a number of Dr. Phinney’s presentations to LCDU events, I have a feeling he’d find a polite way to call this study bunk, lol! 
It would be nice to see a similar study with a much longer period than 3 weeks. Yet it does seem to be a fact that it takes more oxygen to burn fat and get the same amount of energy. Argues for increasing that good old V02 max as much as one can.
Here’s a description of the different ways we make energy:
Does anybody know how much more oxygen is needed to make same amount of energy? Is it 5%?
Dont get me wrong,I love keto but lets not be like vegans who completly dismiss any negatives about their diet and pretend its flawless.
If you need more oxygen,I am highly sceptical that it wont affect performance.It means you need to pump more blood,this will impose performance decrease in activities that push the cardio vascular system to he limit.
Heart have limited pumping capacity,lungs have limited oxygen intake,you can increase your red blood cell count but you cant push that forever,your blood will become too thick.
Also,fast twitch muscles run exclusively on glucose,coupled that with smaller intra muscular and hepatic glycogen stores,I cant imagine how performance is not going to suffer.
Some muscles,like arms have more fast twitch muscles than legs,that mean endurance running isnt going to see as much performance drop as boxing would.
I can only reiterate my first post. I think you will find the answers you want in that book.
You can make energy from glucose anaerobically - without additional oxygen. But even when aerobically burned it is less energy dense than fat and requires additional water for storage. It’s silly to compare them on the basis of oxygen consumption, they serve different purposes. Think of glucose as the match and your fat as the candle.
If I recall correctly, being a fat burner actually allows one to continue burning fat to a higher heart rate threshold than burning carbs.
I would agree with PaulL and read “The Art and Science of … Performance.”