Over this past winter, I regularly did 4 hour endurance rides (usually 0.65-0.75 IF) on the trainer and experimented with different fueling before and during. I have always been an all day steady rider who I believe has a high fat oxidation rate, as I tend to get stronger at the end of long rides/races relative to the field.
On the low end, I consumed no calories for almost 2 hours into one rides, then 200-300 per hour of fats and natural sugar (i.e. RX/larabars, nuts plus a banana, etc.). On the high end, I had a balanced breakfast of yogurt, fruit, oats, and nuts before, then consumed ~250-300 calories per hour of real food (typically clif bars, bananas, RX bars, cashew clusters, bread). I really dislike taking sports nutrition and gels/drink mix unless it’s absolutely necessary because of intensity- I’ve found that I need to double my calorie intake to keep going for multiple hours running on pure sugar.
Could I complete those endurance rides starved or with very minimal calorie/carb consumption? Yes. Was it optimal for performance? Not a snowball’s chance in hell, and here’s why. I was much much more beat after those long rides without proper fueling- my legs would be dead, sometimes my stomach even felt queasy, and I was drained for not just that day but often the next. I would usually binge eat at lunch then have very little motivation and energy to do anything the rest of the day. It was harder to back up those rides with another session the next day, especially one with intensity.
Conversely, the rides that I fueled properly I felt much better after. Aside from my undercarriage hurting from being locked in one place for 4+ hours, I felt like I could’ve kept pedaling all day. I was much less likely to binge after, and my energy for the day (and productivity) were much higher. It also was much easier to come back the next days for some sweet spot work, or a long run, or something with intensity.
Does that mean you should aim for 90g carbs/hour on endurance rides? Absolutely not IMO, that’s excessive and not necessary when you’re riding at a low enough intensity to burn fat. From a health standpoint, I personally don’t like downing straight sugar unless it’s immediately used because I’m pushing near threshold for hours straight. But not fueling or underfueling those rides is much more detrimental not just that day but in the subsequent days.