Glycogen depletion is the primary metabolic limitation for endurance exercise lasting >90 minutes — the infamous "bonk" or "wall" experienced by under-fueled marathoners, triathletes, and cyclists. Carbohydrate loading (glycogen supercompensation) before endurance events and strategic carbohydrate intake during events are among the highest-evidence nutritional interventions in sports science, with well-characterized physiological mechanisms and consistent performance benefits.
Glycogen Supercompensation: Physiology and Protocols
At baseline, trained muscles store approximately 400–500g glycogen (1,600–2,000 kcal). With 3-day carbohydrate loading (8–12g CHO/kg/day for 72 hours pre-race with exercise taper), glycogen storage increases to 600–900g (2,400–3,600 kcal) — a 50–100% increase. The original "depletion + loading" protocol (7-day: 3 days low-CHO training, 3 days high-CHO tapering) is NOT superior to simple 3-day high-CHO loading in trained athletes — who can supercompensate without prior depletion. Performance impact: meta-analysis shows carbohydrate loading improves endurance exercise performance lasting >90 minutes by 2–3% — equivalent to a 3-4 minute improvement in a 2-hour race performance. Body weight increases 1–2 kg due to water bound to glycogen (3g H₂O per g glycogen) — important for weight-class athletes to account for.
Race Day Fueling: During-Exercise Carbohydrate
Carbohydrate oxidation rates during exercise: up to 60g/hour from single-source carbohydrates (glucose/maltodextrin); up to 90g/hour from multi-transporter carbohydrate combinations (glucose:fructose 2:1 or 1:0.8 ratio) — because glucose and fructose use different intestinal transporters (SGLT1 and GLUT5 respectively), doubling intestinal absorption capacity. Gut training: consuming 60–90g/hour during training acclimatizes the gut, reducing GI distress at race pace. Practical application: gels, chews, sports drinks with maltodextrin + fructose blends — consumed every 20–30 minutes during events lasting >60 minutes. For clinical nutrition programs supporting athletes, our nutrition catalog includes clinical nutrition products and supplement management supplies.
