ZACH Insights / Glycogen and carbohydrate loading
A fuel store with a limit. And a way to top it up.
Muscle stores carbohydrate as glycogen. In long, hard efforts the store runs low and pace drops. Eating more carbohydrate in the days before can raise it well above normal. This article shows when that helps, when it does not, and what the measurements say.
By Zacharias Razvi · Updated 14 November 2026 · 10 min read
What glycogen is
Chains of glucose packed inside the muscle.
Glycogen is a branched chain of glucose molecules. Muscle and liver keep most of the body's carbohydrate in this form. Muscle glycogen fuels the muscle it sits in. Liver glycogen keeps the level of glucose in the blood up, which the brain and the working muscles draw on. Both stores are refilled from carbohydrate in food, and refilling takes time. 1
The stores are small compared with the body's fat. Researchers measure muscle glycogen in small samples of thigh muscle and report it per kilogram of muscle. In trained athletes, normal values in the studies below lie around 95–110 mmol per kilogram of wet muscle. 3 4 That is about 15–18 g of glycogen in each kilogram of muscle.
General education. The article explains physiology and research. It is not an individual nutrition plan.
Model 1 · Running low
A higher start delays the empty tank.
Follow the glycogen in a working muscle during a long, steady effort. Compare a normal start with a loaded start.
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A normal store.
The muscle starts near 100 mmol per kilogram. In a long, steady effort the store falls. When it reaches a critically low level, around 25 mmol per kilogram, exhaustion usually follows. Glucose from the blood cannot fully replace the missing glycogen. 2
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A loaded store.
After loading, the muscle can start near 180 mmol per kilogram. The store still falls, but it reaches the critical level later. In events longer than 90 minutes, a higher starting level postpones fatigue by about 20%. 2
When loading helps
It depends on how long the effort lasts.
A review by Hawley and colleagues sorted the studies by the length of the effort. 2
Data · Effect of a higher starting store, by duration
Under 90 minutes, little or none. Over 90 minutes, a clear effect.
Under 5 minutes: little or no effect. 60–90 minutes: no benefit. Over 90 minutes: fatigue postponed by about 20%, time trials 2–3% faster.
How fast it fills
One day of high carbohydrate intake and rest can be enough.
The classic loading plans from the 1960s and 1970s emptied the stores with hard training and a low-carbohydrate diet before refilling them. Later work showed that trained athletes can reach high levels without the emptying phase. 3
- Who took part
- 8 endurance-trained men in a study by Bussau and colleagues. 3
- What was done
- The athletes rested and ate 10 g of high-glycaemic carbohydrate per kilogram of body mass per day for 3 days. Muscle samples were taken before and after 1 and 3 days.
- What they found
- Glycogen rose from 95 to 180 mmol per kilogram of wet muscle after 1 day and did not rise further on days 2 and 3. All fibre types filled.
- What it cannot say
- Whether the same holds for women or for untrained people, and how the higher store changes performance. Performance was not measured.
Data · Muscle glycogen before and after loading
Millimoles per kilogram of wet muscle.
Bussau, men: 95 to 180. James, women: 108 to 193 and 111 to 202. James, men: 109 to 183.
Women and men
The difference was in the dose, not in the muscle.
A study from 1995 reported that women did not reach the same high glycogen levels as men after loading. In that study the women ate much less carbohydrate relative to their body mass than the men did. 4
James and colleagues repeated the comparison and gave women and men the same amount, 12 g of carbohydrate per kilogram of lean body mass per day for 3 days. Glycogen rose from 108 to 193 in the women after menstruation, from 111 to 202 in the women before menstruation, and from 109 to 183 in the men. The authors concluded that there is no sex difference in the ability to load when the dose is comparable. 4
During the event
Eating on the way adds to the store.
A review by Burke and colleagues summarises intake during exercise. In sustained high-intensity sport lasting about an hour, small amounts of carbohydrate, even rinsing the mouth, can improve performance through effects on the nervous system. For longer events, 30–60 g per hour is an appropriate target, and events longer than 2.5 hours may benefit from up to 90 g per hour. 5
The liver matters too. Measurements with magnetic resonance spectroscopy show that eating carbohydrate at a high rate, more than 1.5 g per minute, can prevent the liver store from emptying during moderate exercise. Mixtures of glucose with fructose or sucrose are recommended to reduce stomach and gut discomfort at such rates. 6
What the evidence supports
Raising the store before events longer than about 90 minutes, and eating carbohydrate during long events. Trained athletes can fill the store within a day of rest and high intake.
What it does not support
Loading before short races or training sessions under 90 minutes, a long emptying phase first, or the idea that women cannot load.
What this article cannot tell you
The muscle measurements come from small groups of trained athletes. The review of performance is from 1997, and newer trials use different foods and timing. None of it gives an amount for one person. Body size, the event, the gut and any medical condition change the answer, and a sports dietitian can work it out.
Sources
Follow the evidence.
- Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutrition Reviews. 2018;76(4):243–259. PubMed record Supports: the role of glycogen stores and their restoration with dietary carbohydrate.
- Hawley JA, Schabort EJ, Noakes TD, Dennis SC. Carbohydrate-loading and exercise performance: an update. Sports Medicine. 1997;24(2):73–81. PubMed record Supports: no effect under 5 minutes or at 60–90 minutes, fatigue postponed by about 20% over 90 minutes, 2–3% faster time trials, and the critical level of about 25 mmol/kg.
- Bussau VA, Fairchild TJ, Rao A, Steele P, Fournier PA. Carbohydrate loading in human muscle: an improved 1 day protocol. European Journal of Applied Physiology. 2002;87(3):290–295. PubMed record Supports: 10 g/kg per day, the rise from 95 to 180 mmol/kg after 1 day and no further rise.
- James AP, Lorraine M, Cullen D, et al. Muscle glycogen supercompensation: absence of a gender-related difference. European Journal of Applied Physiology. 2001;85(6):533–538. PubMed record Supports: the earlier report on women, the lower intake in that study, and the values for women and men with 12 g/kg lean body mass.
- Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences. 2011;29 Suppl 1:S17–S27. PubMed record Supports: mouth rinsing in efforts of about an hour, 30–60 g per hour and up to 90 g per hour beyond 2.5 hours.
- Gonzalez JT, Fuchs CJ, Betts JA, van Loon LJ. Liver glycogen metabolism during and after prolonged endurance-type exercise. American Journal of Physiology. Endocrinology and Metabolism. 2016;311(3):E543–E553. PubMed record Supports: intake above 1.5 g per minute preventing liver glycogen depletion and the use of glucose with fructose or sucrose.
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Where to go from here.
All Insights · Running · Recovery
General education, not a diagnosis or an individual treatment plan. Models are schematic unless a data source is stated beside the figure. Zacharias Razvi is a final-semester physiotherapy student and is not yet an authorised physiotherapist.