Much to say about glycogen

Different strategies such as cold water immersion, active recovery, compression garments, massage and electrical stimulation are currently being used in order to improve the recovery of the athlete, depending on the type of activity performed, the time until the next training session or competition, as well as the equipment and medical personnel available (1)

Among the different factors that can improve an athlete's recovery , the most important are rest and nutrition (2) , the latter being one of the most popular and accessible methods to facilitate the restoration of performance and physiological disturbances after exercise. In one of our guides we discussed how you can improve your recovery with a 3:1 ratio like our Glycogen Recovery Drink and not a 2:1 or 2:2 ratio.

The main objectives of nutritional strategies during the recovery phase are: replacement of muscle glycogen (3) , restoration of the body's hydroelectrolytic balance, repair of damaged muscle tissue and adaptations to exercise (4) , and restoration of those physiological systems altered during training/competition such as the hormonal (5) and/or immune (6) system.

Glycogen: A mere energy store?

Glycogen is a branched polymer of glucose (up to 55,000 units) linked by α1:4 and α1:6 glycoside bonds around a core protein, glycogenin (7) . The importance of muscle glycogen as a determinant of exercise capacity was first recognized as early as the late 1960s with the introduction of muscle biopsy into exercise physiology (8) .

Glycogen is much more than an energy store (9) , acting as a regulator of different signaling pathways related to the oxidative phenotype, insulin sensitivity, contractile processes, protein degradation and autophagic processes (10).

Skeletal Muscle:

Muscle is one of the human body's main glycogen reservoirs (up to 600g). The amount stored, however, depends on several variables, such as, of course, the subject's muscle mass, physical fitness, diet, etc. It has been documented that trained endurance athletes have a greater glycogen storage capacity in skeletal muscle, this being one of the main adaptations to such exercise. Glucose stored in muscle glycogen is a bioavailable source exclusive to the muscle itself and plays a fundamental role in both regulation and signaling, as well as in the metabolic control of muscle cells. In this regard, it is worth noting that glycogen is stored in different cellular compartments (pouches) (subsarcolemmal, intramyofibrillar, and intermyofibrillar) and, depending on its location, performs different functions, such as providing energy to the cell or generating ATP for the proper functioning of the calcium and/or Na+, K+ ATPase pump. Finally, it should be added that its resynthesis is mainly stimulated by Glucose.

Liver:

Approximately, it is estimated that the amount of Liver Glycogen is around 80-100 g, although this differs between subjects. The importance of this store lies in its ability to "send glucose" to the blood and therefore regulate blood glucose. This happens thanks to the Enzyme Glucose-6-Phosphatase, which does not exist, for example, in skeletal muscle. In addition, it should be added that, as we discussed in this guide, the resynthesis of Liver Glycogen is related to the availability of Fructose, something to take into account when recovering Glycogen levels after or before exercise. Therefore, both maltodextrin and fructose are necessary in an optimal recovery product, such as our Recovery GLYCOGEN (11)

Brain:

Here's the news. Recent discoveries speak of a significant amount, although 100 times smaller than other stores, located in the brain. Specifically, in astrocytes, glial cells that perform a wide range of functions related to supporting neurons and the nervous system. What's interesting about this is the ability these cells offer to nourish neurons and act as energy support through the glycogen available in them. Recent studies link this glycogen content with the possible exercise-induced central fatigue already mentioned in this guide. Without a doubt, a new field of research is opening up that can help us determine the true limiting factors of performance. (12)

Kidneys:

In the kidneys, as well as in smooth and cardiac muscle, the amount of glycogen is minimal, so the significance is also very low.

Blood and white blood cells:

Likewise, we also find small amounts of glycogen in red and white blood cells, and the classic amount of glucose (not glycogen) available in the blood (glycemia), which is approximately 5g, an amount that will differ depending on many factors (diet being the first). (13)

And knowing where it is stored, which compartment should be filled more and better?

During exercise at 50% VO2max, the approximate rate of glycogen utilization is 0.6 mmol of glucosyl units/kg of dry muscle/minute, while if the intensity rises to 100%, this ratio rises to 3.6 mmol/kg.dw/min. Likewise, during maximum effort, its utilization can reach 30-50 mmol/kg.dw/min. It seems that we know all this well today, but, nevertheless, we still have lines to draw in order to fully understand many other things. (14)

As we know, muscle glycogen must not only be recovered at the muscle level but also at the liver level for optimal recovery. This is where our recovery comes into play, with a 2:1 carbohydrate ratio (maltodextrin: fructose) and a 3:1 total ratio (carbohydrate: protein). Once ingested, where does this glycogen go (compartments) and what function does each of them have?

Subsarcolemmal:

On a relative and general level, glycogen represents 5-15% of total muscle glycogen. However, depending on the type of muscle fiber, its percentage varies. In human type I (slow) fibers, it represents 9-12%, while in type II (fast) fibers, it represents 7-9%. It is located exactly in the outermost part of the cell, just below the cell membrane, and between the contractile filaments. Its function seems to be mainly related to local regulatory and energy functions, something that is not difficult to understand if we consider the multitude of biological processes necessary to supply energy that occur around the cell membrane. But at the same time, it has a direct relationship with mitochondria, due to their relative proximity. ( 15 , 16)

Its utilization during exercise varies between fibers and muscles. In the arms (triceps brachii), its depletion after 1 hour of maximum exercise is close to 80% in type I and 60% in type II, while in the legs (vastus lateralis), it is 60% in type I fibers and an almost negligible decrease in type II.

Intermyofibrillar:

Relatively, it accounts for 75% of muscle glycogen, representing the largest quantitative amount among the three locations. Depending on the muscle fiber type, it is stored in greater quantities in type II (84%) than in type I (77%). Its location, between the myofibrils, makes its energy bioavailability very high. In fact, it is located very close to the mitochondria and the sarcoplasmic reticulum. In this sense, it appears to fulfill a priority energy function that is "constant" and efficiently regulated. In practical terms, it is the quantitative storage we commonly "imagine" when we talk about muscle glycogen.

After 1 hour of arm and leg exercise, depletion also varies between muscle groups, although a significant decrease does not appear to be observed depending on the type of muscle fiber. In the arms (triceps brachii), depletion was 75% in type I fibers and 70% in type II fibers. In the legs (vastus lateralis), however, type I fibers showed a 55% decrease, and type II fibers showed a decrease of around 10%.

Intramyofibrillar:

It represents a relatively low percentage of the total (5-15%). In type I fibers, it accounts for 12% of total glycogen, while in type II fibers it accounts for a smaller amount (8%). Its location is key. It is located within the myofibrils, within the contractile myofilaments and, specifically, around the first band of the sarcomere. Its distribution is therefore very close to the myofibrillar structures related to the contraction process, as you will see below. Before beginning to detail the approximate percentages of its use, we want to make it clear that this subcellular storage is primarily used during moderate-to-high-intensity exercise, compared to the other two compartments (intermyofibrillar and subsarcolemmal).

After 1 hour of all-out cross-country skiing (a 20-km time trial) in professional skiers, intramyofibrillar glycogen in the arms (triceps brachii) was depleted by 90% in type I fibers and 17% in type II fibers. In the legs (vastus lateralis), however, it was depleted by 70% in type I fibers and showed a curious increase in type II fibers.

Summary

If we go back to the most basic aspects of human physiology, it is evident that muscle contraction and relaxation (muscle function) is the first requirement for human movement and, therefore, for physical exercise.

Calcium is a fundamental ion for muscle contraction, especially for the first "triggering" step of the process: Excitation-Contraction Coupling. This step represents the electrical activation of muscle cells to trigger the contractile process. Within this process, as we mentioned, calcium is essential. A low availability of this ion results in inadequate action potentials and in contraction not occurring under the appropriate conditions. The calcium available in the cell's internal environment, beyond entering through ion channels across the cell membrane, is regulated by the sarcoplasmic reticulum (SR). In other words, the SR is the calcium store that regulates its intracellular concentration and responds to different depolarizing stimuli by "sending" more or less calcium. For this reason, at Fanté, we include calcium in recovery products and not in gels or single-dose supplements , since adding an additional solute to the ingredients would increase osmolarity and reduce gastric emptying, thereby impairing performance. Calcium should be included in your regular and post-workout diet, not during training.

As early as the 1970s, the existence of a direct relationship between the glycogenolytic complex and the SR was discovered. In some way, this complex is related to the regulation of glycogen and proteins regulating glycogenolysis (a pathway for obtaining energy from glycogen), glycogenesis (the creation of glycogen from glucose), and glycolysis itself (the production of energy from glucose). At that time, the existence of a glycogen-regulated feedback loop was hypothesized to play a fundamental role in the release of calcium from the SR and, therefore, in muscle contraction. At the same time, it was documented that the drop in calcium, induced by decreased SR function, was directly related to muscle fatigue, along with multiple other factors and intracellular metabolites. Thus, it was not surprising to think that the relationship between glycogen and muscle function is more than close.

Along these lines, various authors advanced our knowledge through complex quantification, isolation, and correlation methods, both in animals and humans, concluding that calcium and muscle strength are directly related to glycogen content. Therefore, part of the explanation for muscle fatigue could be related to the exercise-induced decrease in muscle glycogen. In fact, studies in which adequate cellular phosphocreatine and ATP availability were maintained, but with low glycogen availability, showed that muscle function was similarly reduced (even with sufficient energy in the cell), suggesting that glycogen is much more than a mere energy store.

Only the intramyofibrillar glycogen content is directly related to the rate of calcium output from the SR, with the excitation-contraction coupling system and, therefore, with muscle function.

Probably due to its location and the glycogen-SR complex, intramyofibrillar glycogen is the main regulator of muscle function, as a source of glycogenolytic energy. Along these lines, novel (2019) studies have demonstrated this same loss of muscle function by limiting glycogenolytic flux while maintaining an adequate energy environment in the cell. This highlights the importance of intramyofibrillar glycogen in this process.

And what does it mean?

It's clear that glycogen, beyond its bioenergetic function, plays a crucial role in internal regulatory processes. But its relationship with muscle function has also become clear. All of this leads us to believe that its availability is more than directly related to muscle fatigue through a narrow mechanism. What does this mean? Specifically, its depletion (intramyofibrillar) will limit the muscle's contractile capacity and, therefore, force us to stop.

To give you an idea, a 10% decrease in calcium output has been documented with a 20% depletion of glycogen in the Vastus Lateralis. Similarly, a 25% decrease in calcium has been found in the arms (triceps brachii) along with a 60% depletion of intramyofibrillar glycogen. These data, however, were found after 1 hour of exercise (cross-country skiing) with an intake of 1 g/kg of body weight/h of carbohydrates. Can you imagine the loss without carbohydrate intake? Add to this the fact that the athletes analyzed were elite endurance athletes, trained for more than 11 years with around 700 hours per year? Now imagine that in a person of a lower level.

So how do I recover and store muscle glycogen after training? In this guide , I explain what you should do and which Fanté product you can use to optimize it .

Replenishing lost glycogen: Refueling

The restoration of endogenous CHO stores is crucial in determining the time required for recovery (19) , therefore, one of the main nutritional approaches in athletes after exercise is the replenishment of muscle and liver glycogen by CHO ingestion (20) . The process of muscle glycogen resynthesis begins immediately after exercise, being much faster during the first 5-6 h of recovery (21) . One of the main stimuli leading to increased glycogen synthesis is glycogen depletion (22) . However, the greatest determinant of muscle and liver glycogen resynthesis is a high CHO intake of around 1-1.5 grams/kg of body weight immediately after exercise and during recovery, increasing resynthesis to 5-10 mmol/kg of dry weight/h (23) . The optimal CHO ingestion strategy to maximize glycogen stores varies greatly and depends on a number of factors including, primarily, the amount, timing, and type of CHO (2:1 ratio) ingested during recovery (24) .

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