The principle that seems to work really well for endurance athletes can be summarized by; “Train on fat. Race on carbs”.

The fuel limitation to long duration (2+ hours), sustained, high output exercise will be the amount of glycogen (a form of stored glucose derived from carbs) the athlete has on board. By becoming more fat adapted the athlete will be able to conserve his or her very limited glycogen stores by using a higher percentage of fat as fuel at all intensities. This fat adaptation strategy is accomplished by the methods covered in these articles https://uphillathlete.com/?s=Fat.

However, during the big day(s) of your event you NEED to keep the available glucose levels in the blood topped up. Because, when they run low you will be forced to slow dramatically.

Steve, Mark and Scott had ample fat stores for the duration of the Slovak direct climb and were very well adapted to conserve glycogen. What they were not able to do, and no one can, was have enough stored glycogen to last 60 hours. By regularly topping up the carb fuel tank during the climb they could prevent a complete let down when the glycogen fuel tank ran dry.

Scott

My, admittedly lay person, understanding is that during heavy exercise the insulin is spike and resultant glucose swings are much reduced and not a factor like they are at rest or low level activity. My understanding and my experience is that during heavy exercise the gels give an almost immediate boost to blood glucose levels. This glucose is also immediately used by the muscles which damps the insulin response. This makes sense to me but I am no physiologist/biochemist. I do not know of any studies of insulin levels during heavy exercise when glucose is taken in. One of my favorite feeds in long races and big days in the mountains has been de-fizzed Coca Cola with a caffein tablet dissolved in it. The theory was, back in the day, that the caffeine liberated more fatty acids for fuel and the sucrose went immediately to to work in the muscles. Lots of sugar combined with the caffeine seems to really helped me to feel strong in the late stages of long efforts. Not very scientific but…. hey it works.

Scott