In our previous posts we outlined some guidelines for fueling and hydration during ultra endurance sports. While fueling recommendations do not differ between running and cycling, there are important differences between the sports that cannot be ignored.
Typically, runners will undergo more physical impact and mechanical related disturbances to the GI tract and body. This may impact the rate at which carbohydrate (CHO) and overall energy intake may be tolerated, even by experienced athletes. Therefore, ultrarunners may not always be able to take in as much CHO as cyclists. Recent evidence also suggests some endurance and ultra endurance runners are not able to oxidize the entire amount of CHO when ingesting 90 g/h while running (2 h steady state run), indicating that 90 g/h may be unrealistic for some athletes and may result in greater GI symptoms. Futhermore, the reduced ability to completely utilize high rates of exogenous CHO may be more prevalent in recreational versus elite runners who have been training with high CHO for a longer period of time.
Higher carb intakes have been correlated with a faster finish time during an ironman event; but, at the same time were associated with greater prevalence of GI symptoms. On the other hand, elite ultra runners consuming between 60-120 g CHO/h reported no GI symptoms during an event after completing a gut training protocol. It should also be noted that general CHO recommendations for athletic performance are overwhelmingly based on research in elite male cyclists so they may not translate to the diverse population of ultra endurance athletes. It has been suggested that, for endurance and ultra endurance sport (@~60% Vo2max), a CHO intake rate of ~1 g/kg body mass-h may be more tolerable for some athletes compared to 90 g/h, particularly when intensity is higher and blood flow to the gut is reduced.
Less experienced runners may need to train with lower CHO intake rates with the goal of slightly increasing over time as the gut allows, compared to highly trained endurance and ultra endurance athletes. For these runners and any other ultra endurance athletes (including cyclists) who struggle to consume 90 g/h, it makes sense to individualize the fueling strategy and work towards CHO intake rates ~1 g CHO/kg-h.
PMID: 30056753, 30056755, 35058795; 21775906, 32403259