Originally Posted by
redlude97
What is your FTP? That is ~200-300w avg for 5 hours. At Z2 you might be close to 50% fat oxidation, but at close to 100% FTP(~90% V02max) you are almost 0% fat. The distribution of the ride will dictate that percentage. There are certain programs that will estimate that based on your distribution in zones assuming your fat burning capabilities are avg. As others already pointed out the number is closer to 1500g as intramuscular glycogen is not transportable like liver glycogen. You can also get some glycogen replenishment even while fasted from gluconeogenesis of glycerin freed from triglyceride(fat) oxidation. A well trained cyclist can probably do a 5 hour ride fasted, and a pro tour rider can probably do that at 2-300W, but unless your FTP is north of 300W it is unlikely you can do so at that avg power.
I suspect that you are correct - my guess at fat burning percent is probably way off (and probably varies greatly depending).
Regarding my FTP a few years ago when I measured it (20 min test) it was around 260. But I hit age 70 soon and back in 2018 I had a 6 month period of half or less my normal riding, my fitness fell off a cliff, and now despite going back to former training levels, 260W is and will remain a memory. I seem to be a solid 15W (maybe even 20) less than I was in 2017.
I did a couple of 30 minute Fartlek sessions the other day and they were both neither easy nor 100% of what I could do. And they both came out at 425'ish KJ per half hour (850KJ per hour). Back in 2016 I did a solo century at 820 KJ/hour - like I said - memories only.
dave