I have read one of Jamie’s even-more-excellent-than-usual posts, and in the comments, another post was linked that provided some interesting N=1 data. I played with it and got a nice chart showing how much carbs and fat are burned during exercise, as a function of the exercise intensity.
I graphed the energy consumption by source (fat, carbs) against the total energy consumption, both measured as a percentage of max energy consumption (which would probably close enough to the VO2max). The graphs are stacked on top of the other (carbs on top in red, fat is blue) so evidently the overall shape is a triangle:
What do we see? We start out with a 1/3 carbs, 2/3 fat mix at rest (note: rest is at about 5% of the max). I needed to posit the carb consumption. The data suggests 0, but it is well known that the brain runs on carbs, so I added this as a baseline of 150g/day (0.1g/min). Then up to about 25%, the incremental energy need is fully supplied by the fat metabolism (remember, N=1), and then it stays at this level until about 80%, with all additional energy provided by carbs. Above 80%, fat metabolism goes down, as then oxygen becomes a binding constraint, and fat needs more oxygen per unit of ATP produced.From 95% onwards, 100% of the energy comes from carbs.
Conclusion: there is no point exercising at more than 25-30% of your max if you want to burn fat – the remainder comes from carbs (or worse, muscle proteins if fasting). And remember, this is N=1, in case I havent said this.
Here is the original chart:
And here is the raw data (pre adjustment for brain carbs):
|kcal/min||%max||fat g/min||%fat||carb g/min||%carb|