Why your fuelling targets are higher than you expect
Most endurance athletes are under-fuelling by 30 to 50 percent. Here is the evidence.
If the carbohydrate targets in your concept//fuel plan feel high, that is intentional. The research on endurance fuelling has been consistent for two decades, and the gap between what the evidence recommends and what most athletes actually consume is substantial.
The oxidation ceiling
At moderate to high intensities, the body can oxidise roughly 60 grams of carbohydrate per hour from a single source. This is a physiological ceiling, set by the capacity of intestinal glucose transporters. Add a second carbohydrate source, specifically fructose, which uses a different transporter (GLUT5), and the ceiling rises to around 90 grams per hour.
This is the multi-transporter model. It is established in the literature and validated in field conditions. The practical result is that glucose and fructose together allow substantially more fuel to reach working muscle without gut distress, provided the gut has been trained to handle the volume.
What chronic under-fuelling costs you
Stellingwerff and Cox reviewed the performance data in 2014 and found that optimised carbohydrate intake consistently produces time trial improvements of 2 to 4 percent versus low-carbohydrate approaches in efforts over 90 minutes. In a marathon at 4-hour pace, that is 5 to 10 minutes. In a half Ironman, similar or more.
Beyond pace, under-fuelling increases perceived effort at the same output, reduces decision-making quality in the final third of a race, and elevates muscle protein breakdown. Athletes who run out of carbohydrate late in a race are not hitting a wall because of fitness. They are hitting a wall because of fuelling.
Why most runners stay below the evidence
Early endurance culture valued fasted training and conservative race fuelling. A lot of that thinking persists, but the physiology has not changed. The athletes winning at every level of the sport, from sub-elite to age group, have largely adopted higher intake targets. The data follows them.
The targets in concept//fuel are not aggressive. They reflect what the evidence says the gut can handle when trained properly. The gut training article above explains how to get there.
Jeukendrup (2010) Nutrition Reviews; Stellingwerff & Cox (2014) Appl Physiol Nutr Metab; Burke et al. (2011) J Sports Sci.
Put it into practice
Apply this to your own training.