Carbohydrate Loading Is Not Just 'Eating More Pasta'. The Protocol That Actually Maximises Race-Day Glycogen.

Most endurance athletes eat more carbohydrates the day before a race and call it carbohydrate loading. What they are actually doing is topping up depleted stores from normal training. True glycogen supercompensation — pushing muscle glycogen to its absolute ceiling — requires a structured 36–72 hour protocol that most amateur athletes have never actually executed.

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The Bergström Foundation

The physiological basis of carbohydrate loading was established by Jonas Bergström and Eric Hultman in 1967 using the muscle biopsy technique they developed. Their key finding:

Muscle glycogen content in untrained individuals at rest averages 80–100 mmol/kg wet weight. Following glycogen depletion (hard training to exhaustion), a subsequent high-carbohydrate diet produced glycogen concentrations of 200+ mmol/kg — more than double the starting value. Glycogen synthase activity — the enzyme responsible for glycogen polymerisation — was dramatically upregulated in the depleted state, creating a temporary window of enhanced uptake capacity.

This phenomenon — supercompensation — was the physiological mechanism behind the classic "depletion-loading" protocol. The original Bergström protocol used a 3-day depletion phase (exhaustive exercise plus very low carbohydrate) followed by a 3-day loading phase. Effective, but accompanied by significant performance impairment, fatigue, and mood disturbance during the depletion phase.

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The Modified Protocol: Same Ceiling, Less Suffering

Sherman et al. (1981) demonstrated that trained athletes — who have chronically elevated glycogen synthase activity and expanded glycogen storage capacity — can achieve supercompensation *without* a depletion phase. The modified protocol:

Days 1–3: Normal training with moderate carbohydrate (5–7g/kg/day) Day 4: Single moderate-intensity session to glycogen depletion (~70 minutes at ~65% VO2max) Days 5–6: Rest or very light activity + high carbohydrate loading (9–12g/kg/day) Race day: Normal pre-race carbohydrate meal (1–4g/kg, 1–4 hours before start)

Maximum glycogen stores in trained athletes following this protocol: 500–600 mmol/kg in the primary working muscles — sufficient to fuel approximately 90–120 minutes of race-pace effort before depletion. For events lasting beyond this window, exogenous carbohydrate during the race is required regardless of loading status.

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Glycogen Storage Capacity: The Often-Missed Ceiling

Muscle glycogen capacity varies significantly by training status:

• Untrained individual: ~300–400 mmol/kg; total stored glycogen ~300–400g
• Trained endurance athlete: ~500–600 mmol/kg; total ~400–600g
• Elite with years of aerobic training: up to 700 mmol/kg in specific muscle groups

Training itself expands glycogen storage capacity over months to years — another reason aerobic base building precedes event-specific preparation. Athletes who load without an adequate aerobic base are working with a smaller tank.

Critical point often misunderstood: only muscle glycogen in the working muscles is available during exercise — liver glycogen (~80–100g) supports blood glucose maintenance but is not directly delivered to muscle. Loading leg glycogen is relevant for running and cycling; it provides minimal direct benefit for upper body effort.

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Race-Day Carbohydrate: The Fructose-Glucose Blend

For events lasting 2+ hours, exogenous carbohydrate during the race extends performance beyond the 90-minute glycogen ceiling. The ceiling for carbohydrate oxidation from a single source (glucose or maltodextrin) is approximately 60g/hour — limited by SGLT1 intestinal transporter saturation.

Adding fructose alongside glucose allows simultaneous use of a second transporter (GLUT5), elevating total oxidation rate to 90g/hour (2:1 glucose:fructose ratio) or even 120g/hour (in trained athletes using 1:0.8 ratio with isotonic delivery and gut training). This is why professional cycling and running products targeting the final hours of long events contain glucose-fructose blends rather than single-source carbohydrates.

Gut training — progressive exposure to race-pace carbohydrate intake during training — is essential for tolerating 90g+/hour without GI distress. Attempting race-level carbohydrate intake for the first time on race day is a common mistake in amateur competition.

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The Loading Day Meal Plan: What 10g/kg Actually Looks Like

For a 70kg athlete: 700g of carbohydrate on peak loading days. The practical challenge is volume — carbohydrate-dense foods also carry fibre and bulk. Strategies to hit the target without excessive GI distress:

• Prioritise low-fibre, high-density sources: white rice, white pasta, bread, potatoes (without skin), bananas, sports drinks, rice cakes, bagels, honey
• Reduce fat and fibre on loading days — fat competes for GI space; fibre slows gastric emptying
• Distribute across 5–6 meals: 700g/6 meals = ~115g per eating occasion, a manageable quantity
• Liquid carbohydrates: fruit juice, sports drinks, and maltodextrin beverages significantly increase loading capacity without the satiety signal of solid food

For athletes building loading-day meal plans that hit specific carbohydrate gram targets across different food preferences and meal frequencies — the macro meal generator at winsport.uk/tools/nutrition/macro-meal-generator allows custom calorie and macro targets across 3–6 meals. Setting a high-carbohydrate split (e.g. 75% carbohydrate) at your loading-day calorie target generates a practical food distribution framework across the critical 48-hour pre-race window.

Do you prescribe a structured loading protocol for your athletes — or is race-week nutrition left to individual preference and habit?