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Glycogen Depletion Calculator | Nutrition Tool

Estimate your glycogen stores, depletion rate, and bonk risk based on body weight, sport, intensity, and carb loading status. Based on Bergstrom et al. 1967.

Thomas Prommer
Built by an engineer who chases finish lines and is obsessed with data. Thomas Prommer — technology executive who has worked with Google, Apple, Nike, Adidas, Netflix and other global brands. Also an Ironman finisher, HYROX Pro Division competitor, and marathon runner. These tools combine engineering rigor with real race experience.
Thomas Prommer
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Glycogen Depletion Calculator

Estimate how fast you burn through glycogen stores based on your weight, sport, intensity, and carb loading status. Know your bonk risk before race day.

Methodology

Muscle glycogen is estimated at approximately 7 g per kg of lean mass (Bergstrom et al. 1967), with liver glycogen contributing 80–100 g. Lean mass is approximated from total body weight using a population-average body fat assumption. Carb loading (2–3 days of high carbohydrate intake) increases muscle glycogen by 50–100%, reaching up to 12 g/kg lean mass. Depletion rates scale with intensity: low effort burns roughly 0.5 g/min, moderate effort 1–1.5 g/min, and high-intensity racing or intervals 2–3 g/min. Mixed sports such as triathlon and HYROX apply a 10% depletion multiplier to reflect the multi-modal demand on glycogen. Time to depletion is calculated from total stores divided by the per-minute burn rate. Carb intake recommendations to extend performance are based on the current maximum gut absorption rate of 60–90 g/h for single-source carbohydrates.

FAQ

What is bonking or hitting the wall?

Bonking (also called hitting the wall) occurs when muscle and liver glycogen stores are critically depleted. The brain and muscles both depend on blood glucose, so when liver glycogen is exhausted, blood sugar drops and performance collapses — often suddenly. Symptoms include extreme fatigue, inability to maintain pace, confusion, and involuntary slowing. It is preventable with adequate pre-race fueling and on-course carbohydrate intake.

How does carb loading increase glycogen stores?

Carbohydrate loading — eating a high-carb diet (8–12 g/kg body weight per day) for 2–3 days before an event while tapering training — increases muscle glycogen by 50–100% above normal resting levels. Research by Bergstrom et al. (1967) first demonstrated that trained athletes can store up to 700–800 g of muscle glycogen after loading, compared to 400–500 g at normal levels. The extra stores provide a meaningful buffer for events lasting more than 90 minutes.

Why does intensity determine depletion rate so strongly?

At low intensities, the aerobic system draws heavily on fat oxidation, sparing glycogen. As intensity rises above roughly 65–70% of VO2max, the contribution of carbohydrates increases rapidly because fat metabolism cannot supply ATP fast enough. At race pace or during intervals, almost all energy comes from glycogen and blood glucose, driving depletion rates up to 2–3 g per minute. This is why pacing conservatively early in a long race dramatically extends glycogen availability.

How do I use these results alongside the Race Nutrition Calculator?

Use the Glycogen Depletion Calculator first to understand your starting stores and how quickly you will deplete them at your target intensity. Then open the Endurance Fueling Planner to build an hour-by-hour carbohydrate and fluid strategy. The two tools complement each other: glycogen depletion tells you the urgency of fueling, while the fueling planner tells you exactly what to consume and when.

What are the signs of glycogen depletion during exercise?

Early signs include heavier-than-expected leg fatigue, declining power or pace at the same perceived effort, and increased hunger. As depletion deepens, athletes experience shakiness, light-headedness, difficulty concentrating, and a sudden involuntary drop in pace. In severe cases, disorientation and muscle tremors occur. Many athletes mistake these for general fatigue rather than a fueling problem, which is why tracking estimated depletion time before the event is valuable.

Why do mixed sports like HYROX or triathlon deplete glycogen faster?

Multi-discipline events require rapid transitions between different muscle groups and movement patterns. Cycling primarily loads the quads and glutes; running recruits the hamstrings, calves, and hip flexors; HYROX adds upper-body strength stations. Each discipline draws on its own local glycogen pools, and the cumulative demand exceeds what a single-sport effort of the same duration would produce. The 10% multiplier used in this calculator reflects that additional demand and the higher average intensity typical of competition-format mixed events.

Results are estimates based on population-average physiology. Individual glycogen storage capacity, fat oxidation capacity, and gut tolerance vary significantly. Consult a sports dietitian for personalized race nutrition planning.