Our bodies store carbohydrate as glycogen, the critical fuel supply for endurance running.
While the human body has sufficient stores of fat to run vast distances, the supply of carbohydrate is quite limited. This carbohydrate store is in the form of Glycogen, a branching chain of glucose molecules.
- Burning glycogen for energy requires less oxygen than fat, making it more efficient. However, the store of glycogen is limited, and when the supply runs low we "hit the wall".
- Glycogen is stored primarily in the muscles, but that glycogen can only be used by the muscle it’s stored in and cannot flow through the blood to other places.
- Some glycogen is stored in the liver where it flows through the blood to all tissues. The human liver typically stores between 90 and 160 grams of Glycogen, or 350 to 650 Calories.
- Blood typically contains less than 20 calories of glucose. (This assumes 5 liters of blood and 100mg/dL of blood glucose, which is 5g of glucose.)
- Glycogen can also be created from Protein via a process called gluconeogenesis, but not from fat.
- Eccentric exercise, such as Downhill Running, can cause DOMS and impair glycogen replenishment.
- Glycogen stores may not be replenished between daily hard runs, such as 10 miles at 80% of V̇O2max.
2 Glycogen Usage
At low exercise intensity the majority of the energy comes from free fatty acids in the blood, with a little bit of blood glucose and a little bit of muscle triglyceride. As the exercise intensity increases the contribution of free fatty acids drops. The contribution of blood glucose increases with exercise intensity, but not as dramatically as the contribution of muscle glycogen. At higher intensity muscle glycogen is the major energy source and is critical for performance.
At 65% V̇O2max, the usage of different substrates changes over time. The reduced usage of muscle glycogen may be due to a reduction in the availability of the glycogen. Over the two hour period shown, the fat:carbohydrate ratio changes from around 55:45 to 65:35. This change would reduce power output (running speed) at the fixed percentage of V̇O2max (see ‘Glycogen Depletion and Breathing’ below).
3 Glycogen Depletion
The chart below shows that muscles do not become glycogen depleted at the same time. At all intensities shown, slow twitch fibers become depleted before fast twitch. The depletion within a fiber type is also not equivalent, with some fibers becoming depleted while others are fully loaded. This pattern implies a pattern of Muscle Recruitment, where a subset of muscle fibers are recruited until they become exhausted, at which point other fibers are then used. As the slow twitch fibers become exhausted, fast twitch fibers are used in turn.
4 Glycogen Depletion and Breathing Rate
It requires more oxygen to produce energy from fat than carbohydrate. This may be why higher intensity exercise harder shifts to burning more carbohydrate. When our muscles become depleted of glycogen, muscles are forced to burn more fat. At any given exercise intensity we will use more oxygen when we are glycogen depleted. This means our Heart Rate will be higher and out Breathing will be deeper and faster. It also means our perceived exertion is much higher for a given pace when glycogen depleted. This effect is most noticeable at the end of a long run or a marathon race, and it becomes much harder to stay on target pace. In fact, it can become up to 20% harder and this can be the difference between relaxed easy Breathing and panting for breath. This increased demand for oxygen can often be seen in the calculated running efficiency. In addition, the amount of O2 that is extracted from the air is lower with glycogen depletion, probably because breathing rate is driven by CO2 concentrations.
5 Glycogen Depletion and Muscle Damage
Muscle biopsies taken after a marathon show damage to muscle fibers, but this damage appears focused on a subset of the fibers. Some fibers show no damage, but adjacent fibers are badly affected. The damaged fibers are depleted of Glycogen and lipids (fat). It seems reasonable to me that this pattern of selective damage is due to the pattern of fibers recruitment, with the fibers that are recruited first becoming both glycogen depleted and damaged. Similar damage can be seen with Delayed Onset Muscle Soreness. The images below are taken from the gastrocnemius (calf), 24-48 hours after a marathon race,
6 Glycogen and other fuels
Glycogen is one of several types of fuel the human body can metabolize. Below is a table describing the characteristics of the primary types of fuel. Some important notes:
- Carbohydrate requires less oxygen than fat to produce a calorie of energy.
- Muscles generally burn BCAA.
- The ratio of O2 to CO2 is called the respiratory quotient or RQ.
- The figures for fat assume full metabolism rather than a ketogenic state. A ketogenic metabolism of fat can result in an RQ below 0.7.
- It is possible for RQ to be above 1.0 if the carbohydrate is converted to fat rather than metabolized.
|Oxygen needed per gram (l/g)||0.75||0.83||0.965||1.24||2.02|
|Energy per gram (Kcal/g)||3.75||4.17||4.3||3.76||9.3|
|Energy per Liter of Oxygen (Kcal/l O2)||5.03||5.03||4.46||3.03||4.61|
|Carbon Dioxide produced per gram (l/g)||0.75||0.83||0.781||0.92||1.43|
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