Sports & Gaming · Dietetics
Glycogen Depletion Calculator
Estimate how long until glycogen stores are depleted during exercise based on intensity, body weight, and pre-exercise carbohydrate status.
Calculator
Formula
Time to depletion (minutes) equals total glycogen stores (grams) divided by the rate of glycogen utilisation (grams per minute). Glycogen stores depend on body weight and carbohydrate loading status. Glycogen burn rate scales with exercise intensity expressed as a fraction of VO2 max.
Source: Adapted from Jeukendrup & Gleeson (2010), Sport Nutrition, 2nd ed., Human Kinetics; and Coggan (1991), Medicine & Science in Sports & Exercise.
How it works
Glycogen is stored in two main depots: skeletal muscle (~80% of total) and the liver (~20%). Muscle glycogen provides fuel directly to working muscles, while liver glycogen maintains blood glucose for the brain and other tissues. Total stores range from roughly 300 g in a depleted, lighter athlete to over 700 g in a heavier, carbohydrate-loaded athlete.
The rate at which glycogen is used scales exponentially with exercise intensity relative to VO2max. At low intensities (<50% VO2max) fat oxidation dominates and glycogen is spared. Above 70% VO2max, carbohydrate becomes the primary fuel and glycogen burns rapidly. The exponential relationship used here is derived from published substrate oxidation data and approximated as: rate (g/min/kg) = 0.000267 × e^(0.0435 × intensity%).
Any carbohydrate consumed during exercise (gels, drinks, bars) is absorbed and oxidised, directly offsetting glycogen depletion. Subtracting this exogenous carbohydrate supply from the gross burn rate gives the net depletion rate. Dividing total stores by the net rate yields time to depletion in minutes.
Worked example
Example: A 70 kg cyclist with a normal mixed-diet carbohydrate status rides at 70% VO2max and consumes 60 g of carbohydrate per hour (1 g/min).
- Muscle glycogen: 70 kg × 4.5 g/kg = 315 g
- Liver glycogen: 80 g
- Total stores: 395 g
- Gross burn rate: 0.000267 × e^(0.0435 × 70) × 70 = ~2.57 g/min
- Net burn rate: 2.57 − 1.0 = 1.57 g/min
- Time to depletion: 395 ÷ 1.57 ≈ 252 min (4.2 hr)
Without the carbohydrate intake the same athlete would deplete stores in about 154 min (2.6 hr), illustrating the dramatic effect of fuelling during exercise.
Limitations & notes
This calculator uses population-average glycogen storage estimates and an empirical burn-rate equation. Individual variation in glycogen storage capacity, fat adaptation, training status, and metabolic efficiency can cause actual depletion times to differ significantly. Liver glycogen values are especially variable and are influenced by overnight fasting and meal timing. The model assumes a constant steady-state exercise intensity; interval training or variable-pace efforts will alter depletion patterns. Exogenous carbohydrate absorption is limited to roughly 60–90 g/hr and entering values above this range will overestimate the sparing effect. Results should be used as a planning guide, not a precise physiological measurement.
Frequently asked questions
What is glycogen and why does it matter for endurance sports?
Glycogen is the stored form of glucose found in muscles and the liver. It is the primary fuel for moderate-to-high intensity exercise. When stores run out, pace drops dramatically — an event known as 'hitting the wall' in running or 'bonking' in cycling.
How much glycogen does the average person store?
A typical 70 kg person on a normal mixed diet stores roughly 400–500 g of glycogen: about 300–400 g in the muscles and 80–100 g in the liver. Carbohydrate loading can raise total stores to 600–700 g or more in trained athletes.
How does exercise intensity affect glycogen use?
Glycogen use increases exponentially with intensity. At 50% VO2max the rate is modest (~1 g/min for a 70 kg athlete), but at 85% VO2max it can exceed 3–4 g/min, meaning stores can be exhausted within 90–120 minutes at race pace without fuelling.
How much carbohydrate should I consume per hour to delay depletion?
Most research recommends 30–60 g/hr for events lasting 1–2.5 hours, rising to 60–90 g/hr for longer efforts. The gut can typically absorb up to ~60 g/hr of a single carbohydrate source, or ~90 g/hr using multiple transportable carbohydrates (e.g., glucose + fructose blends).
Does fat adaptation reduce glycogen depletion?
Yes. Athletes who train in a fat-adapted state or follow low-carbohydrate diets can upregulate fat oxidation, sparing glycogen especially at lower intensities. However, above ~75% VO2max, carbohydrate remains the dominant fuel regardless of fat adaptation status.
What is carbohydrate loading and how does it help?
Carbohydrate loading is a strategy of consuming a high-carbohydrate diet (8–12 g/kg/day) for 1–3 days before a competition. This supersaturates glycogen stores, potentially extending time to depletion by 20–30% compared with a normal diet, which is meaningful for events lasting more than 90 minutes.
Last updated: 2025-01-30 · Formula verified against primary sources.