The Important Take-home Message Ensure you are optimizing glycogen stores before exercise, maintaining it during exercise, and replenishing it after exercise. References Goodman, MN. Amino acid and protein metabolism. In Exercise, nutrition and energy metabolism, Eds. Horton, R. So all muscle cells contain a high-energy compound called creatine phosphate which is broken down to make more ATP quickly.
Creatine phosphate can supply the energy needs of a working muscle at a very high rate, but only for about 8—10 seconds. Fortunately, muscles also have large stores of a carbohydrate, called glycogen, which can be used to make ATP from glucose. But this takes about 12 chemical reactions so it supplies energy more slowly than from creatine phosphate. Oxygen is not needed — this is great, because it takes the heart and lungs some time to get increased oxygen supply to the muscles. A by-product of making ATP without using oxygen is lactic acid.
You know when your muscles are building up lactic acid because it causes tiredness and soreness — the stitch. Within two minutes of exercise, the body starts to supply working muscles with oxygen.
When oxygen is present, aerobic respiration can take place to break down the glucose for ATP. This glucose can come from several places:. Both glycogen and glucose need to be broken down before they can deliver energy to the muscle. The breakdown of glycogen is easy. That is because glycogen is a chain of glucose molecules, that has multiple places to start the breakdown.
Also, glycogen is already located in the muscle. The breakdown of glucose however, costs a little bit of energy. It needs to be transported from the blood into the muscle. Contrary to fat combustion, carbohydrate combustion increases exponentially with intensity. The faster you swim, run, ski, bike, … the more carbohydrates you burn. The exact amount of carbohydrates that an athlete burns at a certain intensity, depends among others on his metabolic profile.
INSCYD does not only accurately provide you those metabolic parameters, it also shows you exactly how much fat and carbohydrates you burn at any intensity e. Learn more about fat and carbohydrate utilization via this blog.
The carbohydrates that will be combusted come from two sources: carbohydrate stored in the muscle glycogen and carbohydrates located in the blood, as a result of carbohydrate food intake blood glucose.
In conclusion: the higher the intensity the more glycogen is needed. By consuming additional carbohydrates during exercise, you can decrease the amount of glycogen needed. However, since glycogen is preferred over blood glucose as a fuel, and because the amount of exogenous carbohydrate intake is limited, you can never exercise at a high intensity and not burn any glycogen.
We know glycogen storage can be depleted rapidly. We also know this will cause fatigue to develop quickly. But how long does it take before glycogen stores are empty? To give you a rule of thumb: after approximately 80 minutes of exercise at a maximum lactate steady state, glycogen stores are depleted.
Although this rule of thumb gives you an idea, a ballpark number, it does not help the individual athlete to train and perform better. It takes into account all the variables that affect glycogen availability and lets you know exactly how much glycogen is stored in your active muscles.
Combine this knowledge with the carbohydrate combustion rate we showed in the previous graph, and you know how long glycogen stores will last. Of course you can extent the time glycogen stores last. Read along to learn how to maintain glycogen stores during exercise. Knowing the importance of glycogen, it should come as no surprise that running out of glycogen will seriously hamper exercise performance.
As the carbohydrate combustion graph clarifies, it is impossible to exercise at higher intensities when there are no carbohydrates available. In short: running out of glycogen is the end of every high performance effort.
That is why you want to know exactly how much glycogen is available in an individual athlete, instead of having some rough estimates. Now you know the disastrous effects of running out of glycogen, you probably wonder how you can maintain glycogen stores during exercise.
The most obvious one is to decrease exercise intensity. This will decrease carbohydrate combustion, increase fat combustion, and as a result: maintain glycogen stores for a longer period of time. Examples are energy drinks, bars and gels.
Long-term, you can also maintain glycogen stores longer by increasing fitness level. As mentioned, a higher fitness level will increase the maximal amount of glycogen stored per kilo muscle mass.
When an increase in fitness level comes from an increase in aerobic power, you will also rely less on carb combustion and more on fat combustion.
By playing around with the INSCYD glycogen availability calculator, you can see how changes in fitness level and aerobic power have an effect on how long an individual can maintain glycogen stores during exercise.
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