Carbohydrates and Sports Performance: Glycogen, Carbo-Loading, and the Marathon Wall
Glycogen powers high-intensity exercise and runs out after 90–120 minutes — which is exactly why marathoners hit the wall. Here's the science of carbo-loading, carbohydrate intake during endurance events, the post-exercise glycogen window, and where low-carb approaches fit in sports nutrition.
By sadiqbd · June 10, 2026
The carbohydrate debate in sports nutrition is largely over — glycogen wins for endurance, protein wins for strength
Carbohydrate sports nutrition research goes back to 1967, when Birger Ahlborg and colleagues demonstrated that runners could dramatically extend their time to exhaustion by eating a high-carbohydrate diet in the days before a competition. The intervention — "carbo-loading" — is now standard preparation for marathon and longer endurance events. The science behind it explains not just the protocol but why carbohydrates are irreplaceable for certain types of exercise despite the popularity of low-carbohydrate approaches.
Glycogen: the carbohydrate storage form that powers exercise
Carbohydrates consumed in food are converted to glucose in the blood and stored primarily as glycogen in the liver and muscle:
- Liver glycogen: approximately 100g total (about 400 kcal). Releases glucose into the bloodstream to maintain blood sugar between meals and during exercise.
- Muscle glycogen: approximately 400g total (about 1,600 kcal), distributed across all muscle groups. Used locally by working muscles and cannot be directly released into the blood.
Total glycogen stores: approximately 2,000 kcal. This is enough to fuel approximately 90–120 minutes of sustained moderate-to-vigorous exercise.
"Hitting the wall": glycogen depletion in action
The marathon "wall" — the sudden and dramatic deterioration in performance most marathoners experience around the 30–35km mark — is glycogen depletion made tangible.
When muscle glycogen falls critically low, the body can no longer maintain race pace from glycogen oxidation alone and must increasingly rely on fat oxidation. Fat is available in large quantities even in lean athletes (even a very lean person has 50,000+ kcal stored as fat), but it's limited to moderate exercise intensities — ATP production from fat is slower than from glycogen and can't sustain the pace needed for competitive marathon running.
The physiological result: pace slows dramatically, muscles feel heavy, and mental clarity drops (because the brain relies on blood glucose, which is maintained by liver glycogen — also depleted by this point).
Athletes who hit the wall have run beyond their glycogen stores. Carbohydrate intake during the race (gels, drinks) partially replenishes these stores and delays the wall.
Carbohydrate loading: the science
Pre-competition carbohydrate loading (carbo-loading) aims to maximise glycogen stores beyond their typical resting level:
Original protocol (Bergström and Hultman, 1967): 3 days of high-carbohydrate diet following glycogen depletion. Achieved glycogen supercompensation (~30–50% above normal).
Modern simplified protocol: 3–4 days of moderate-to-high carbohydrate intake (~10g/kg/day) without the preceding depletion phase. Achieves similar glycogen supercompensation with less discomfort. Muscle glycogen can reach 800–900mmol/kg dry weight vs. resting ~350–450mmol/kg.
Who benefits: marathon, triathlon (Ironman), cycling gran fondos, cross-country skiing — events lasting 90+ minutes where glycogen is limiting. Little benefit for events under 60–90 minutes.
Carbohydrate intake during exercise
For events over 60–90 minutes, consuming carbohydrate during exercise sustains blood glucose and spares glycogen:
Research-based intake guidelines:
- 60–75 minutes: 30–60g carbohydrate per hour
- Over 90 minutes: 60–90g carbohydrate per hour
- Ultra-endurance (>3 hours): up to 90g/hour, using mixed glucose + fructose sources
The 60g/hour ceiling for glucose alone reflects the absorption rate of the SGLT1 transporter (glucose intestinal absorption). Adding fructose (absorbed via GLUT5) allows total carbohydrate absorption to reach 90g/hour.
Carbohydrate sources during exercise: sports gels (typically 20–25g carbohydrate), sports drinks, bananas, energy chews, medjool dates.
Post-exercise carbohydrate: the glycogen repletion window
After exercise that depletes glycogen, rates of glycogen resynthesis are highest in the 30–60 minutes immediately following exercise. The GLUT4 transporter is maximally active without requiring insulin during this window.
Optimal post-exercise carbohydrate intake: ~1.0–1.2g/kg/hour for 4–6 hours following glycogen-depleting exercise.
For a 70kg athlete: ~70–84g carbohydrate in the first hour, then continuing at similar rates for 4–6 hours if glycogen resynthesis is a priority (e.g., two training sessions in one day, or competition on successive days).
Combining with protein: adding 0.3–0.4g/kg protein to the post-exercise carbohydrate intake stimulates insulin secretion (glycogen synthesis) and initiates muscle protein synthesis simultaneously. A meal of rice + chicken or milk + banana achieves this.
Low-carbohydrate approaches for endurance: the nuance
"Fat adaptation" for endurance sports — training on a low-carbohydrate diet to increase fat oxidation capacity — is a real physiological adaptation. Fat-adapted athletes can sustain higher fat oxidation rates than carbohydrate-trained athletes.
However, the performance evidence is more complicated:
- Fat adaptation reduces carbohydrate oxidation capacity, potentially impairing high-intensity work
- The absolute pace sustainable from fat oxidation is lower than from glycogen
- Studies comparing fat-adapted and carbohydrate-fuelled athletes across race distances show comparable performance at sub-maximal intensities but potentially impaired performance at race intensities requiring glycogen
The current consensus for competitive endurance athletes: high-carbohydrate approaches remain the performance-maximising strategy for events at race pace. Low-carbohydrate training has utility for base training and developing metabolic flexibility, but not for peak race performance.
Carbohydrates and resistance training
Carbohydrate requirements for strength and power sports are lower than for endurance but not zero:
- Strength training depletes approximately 25–40% of muscle glycogen per session
- Adequate glycogen supports training volume and intensity
- Post-strength-training carbohydrate supports glycogen resynthesis and creates an insulin-mediated anabolic environment
Carbohydrate targets for strength athletes: 4–6g/kg/day — higher than sedentary needs, lower than endurance athlete needs.
How to use the Macro Calculator on sadiqbd.com
- Enter your stats and activity level
- Read the carbohydrate allocation — higher for endurance activities, moderate for strength
- Adjust based on training phase — higher carbohydrates during heavy training periods, lower during rest weeks
- Time carbohydrates around training — pre-exercise and post-exercise timing optimises performance and recovery
Frequently Asked Questions
Does everyone need to carbo-load before a marathon? Carbo-loading is most beneficial for runners targeting race pace for 3+ hours. Recreational runners running at a comfortable conversational pace for 4–5 hours use more fat and less glycogen per minute — they're less likely to deplete glycogen. Still, arriving at a marathon with full glycogen stores is always preferable.
Is low-carb better for fat loss during training? Calories in vs. out determines fat loss. Low-carbohydrate diets produce fat loss when they create a caloric deficit — as do any other dietary approaches. The choice of carbohydrate level for fat loss should primarily consider adherence, training performance, and protein adequacy.
Is the Macro Calculator free? Yes — completely free, no sign-up required.
Glycogen is the irreplaceable fuel for high-intensity exercise. The carbohydrate sports nutrition evidence is clear: load before long events, fuel during them, replace after them — and calibrate carbohydrate intake to training demand.
Try the Macro Calculator free at sadiqbd.com — find your personalised protein, carbohydrate, and fat targets for any goal and activity level.
