A study looked at the studies on fueling ultra-endurance events. Ultra-endurance is defined as activities (walking and running) which take at least 6 hours. The study was (Eric Williams. Nutritional implications for ultra-endurance walking and running events. Extreme Physiol Med. 2016; 5: 13).
Given that the majority of an ultra-endurance athlete’s training is spent engaged in lengthy durations of aerobic activity, many of these athletes are well adapted to utilizing lipids via oxidative phosphorylation
Fat burners! But during the event itself how hard are they hitting it?
When the athlete is exercising at the standard marathon pace that requires 80–90% of maximal oxygen consumption (VO2 max) or above, carbohydrate will be his or her primary fuel source and could provide up to 96% of the energy being expended.
This is an issue with Low Carbohydrate diets since glycogen stores are reduced greatly. This is also why Phinney’s endurance tests are done at 62% of VO2max.
The paper had a nice graphic which shows the elements involved in performance in marathons.
Each of these would be interesting to look at in detail.