What’s the science behind this? I’ve read things like exercising while fasted increases calories burned by 20% as opposed to not fasted. I’ve also heard numerous, numerous claims that you can ‘eat a little more’ calorically the tighter your fasting window is, and I have personally experienced this firsthand. My Resting Metabolic Rate is about 2 calories per minute, or 2,600k cal/day; that’s sitting down. Does fasting have any effect on this?
You’d have to compare the peak theoretical ATP yield rate that the major catabolic pathways of glycolysis (“sugar-burning”) and beta oxidation (“fat-/ketone-burning”), each in conjunction with the citric acid cycle, can attain; this matters since ATP is the “late-stage”, effectively final, energy storage medium used all over the place. Then you’d have to look into what affects the availability of input “fuel” – glucose via glycogenolysis and gluconeogenesis and fatty acids via lipolysis. Here you’d probably stumble upon the main benefit of fasting (vs spreading out the intake isocalorically throughout the day), which is a likely decrease in baseline serum insulin, in turn promoting lipolysis. Lastly you’d have to make an assumption about the time elapsed since the last meal, to estimate the ratio at which fat- and sugar-burning co-occur, as it’s never strictly one or the other. That would be my simplistic thought process as a layman, if I badly wanted an approximate numeric answer.
As far as studies are concerned, more often than not IF/keto and traditional calorie restriction are reported to deliver comparable results. A common complaint by the IF camp is that studies tend to be too short-lived and/or too high-carb to allow their participants to become truly “fat-adapted”. I can’t say whether and to what extent that might be the case. What I don’t wish to believe is that a “fat-adapted” individual can burn thousands of calories more per day than their “sugar-burning proletariat” counterpart. It just wouldn’t make sense. They would either have to be a walking oven, suffer from profound ADHD-HI, or generally speaking “invest” that excess energy somehow, assuming that we still believe in thermodynamics. Alternatively or additionally they could as well have unproductively circulating bloodstream lipids in excess of what can actually get oxidized.