Most starches turn into sugar within minutes of entering your body. Fiber can slow the absorption, but also create GI issues. Fat-adapted athletes have no GI problems eating fats on a run over 5 hours. Their GI tract is adapted for processing them efficiently, as well as judicious carbs. The mix is ideal for both energy density, moderate throughput and overall digestibility. Nut butters are a good example of single-source quality food for these.
Your premise is not right wrote:
Wildhorse wrote:Walmsley is having GI issues in longer races since he famously shoots for 600 kcal/hour of carbohydrate, etc.
You're confusing sugar with carbohydrates.
Fat ingestion would cause digestive issues in a race, lower the availability of energy
and reduce performance. For an example of this, look what happened to Sage in this race.
The solution is to eat more complex carbohydrates, and to lower the high
concentrations of sugar. That is what Tim Tollefson does, and it works well for him.
This slight change is easy to make. Minor adjustments can remedy the issue.