Endurance athletes face a nutritional challenge that recreational gym-goers don’t: maintaining performance across hours of sustained effort, often in varying environmental conditions, while managing gut comfort, hydration, electrolyte balance, and energy availability simultaneously. Getting endurance nutrition right can mean the difference between bonking at mile 18 and finishing strong.
Understanding “the bonk”
“Hitting the wall” or bonking occurs when muscle glycogen stores are depleted. Since the brain runs almost exclusively on glucose, glycogen depletion also impairs cognitive function, decision-making, and coordination. For most athletes, this happens between 90 and 120 minutes of sustained moderate-to-high intensity exercise without carbohydrate supplementation. Preventing it is a matter of fueling consistently during exercise, not just loading before it.
Intra-workout carbohydrate strategies
Research shows that the gut can absorb approximately 60g of glucose per hour from a single carbohydrate source. However, using multiple carbohydrate types — glucose and fructose together — utilizes separate intestinal transport pathways, allowing absorption of up to 90g per hour without gastrointestinal distress. This is the science behind “2:1 glucose to fructose” ratio products and explains why mixing carbohydrate sources during long events is advantageous. Real food options: sports gels with different sugar types, banana with dates, or a rice cake with jam.
Fat adaptation vs. carbohydrate dependency
Some endurance athletes pursue low-carbohydrate, high-fat diets to enhance fat oxidation — the idea being that fat stores are nearly limitless even in lean athletes, while glycogen stores are finite. The evidence is mixed. Fat-adapted athletes do show improved fat oxidation at moderate intensities, but multiple studies find impaired performance at higher intensities where carbohydrate oxidation is physiologically necessary. Most elite endurance coaches now recommend a periodized approach — primarily training with carbohydrates, occasionally incorporating low-carb sessions to enhance metabolic flexibility, and always racing with adequate carbohydrate availability.
Gut training matters as much as fueling strategy
The ability to absorb carbohydrates during exercise is trainable. Regularly consuming carbohydrates during training runs and rides — even at doses lower than race-day targets — upregulates intestinal glucose transporters and reduces gastrointestinal distress. Athletes who practice fueling during training perform significantly better fueling during competition than those who “save” fueling strategies for race day.