Muscle glycogen is a vital component of energy storage in the human body. Its formation and accumulation are closely linked to carbohydrate consumption, making it essential for restoring and maintaining our energy stores.
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 the athlete's recovery , rest and nutrition stand out (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 Recovery Glycogen , rather than 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: What it is
Glycogen is a branched polymer of glucose (up to 55,000 units) linked by α 1:4 and α 1:6 glycocid bonds around a central 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 the muscle biopsy technique in 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).
Where is glycogen stored in the human body?
Skeletal Muscle:
Muscle is one of the human body's main glycogen stores (up to 600g). The amount stored, however, depends on several variables, such as, of course, the individual'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.
The 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.
Liver:
The approximate amount of liver glycogen is estimated to be around 80-100 g, although this varies between individuals. The importance of this store lies in its ability to "send glucose" into the blood and thus regulate blood glucose levels.
This is thanks to the enzyme glucose-6-phosphatase, which is absent, for example, in skeletal muscle. Furthermore, as we discussed in this guide, hepatic glycogen resynthesis is related to the availability of fructose, something to keep in mind when restoring 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:
Recent discoveries indicate 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.
There are studies that link this glycogen content with the potential exercise-induced central fatigue we already mentioned in this guide. This undoubtedly opens a new field of research 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 needs to 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. (14)
As we know, muscle glycogen not only has to be recovered at the muscle level, but it also has to be recovered at the liver level for optimal 100% recovery, and this is where our Recovery comes into play with a 2:1 carbohydrate ratio (maltodextrin: fructose) and a 3:1 total (carbohydrates: proteins).
Subsarcolemmal
On a relative and general level, it represents 5-15% of total muscle glycogen . However, the percentage varies depending on the type of muscle fibers.
- In human type I (slow) fibers it represents 9-12%.
- In type II (fast) fibers, 7-9%.
It is located precisely in the outermost part of the cell, just beneath the cell membrane and between the contractile filaments. Its function appears to be primarily related to local regulatory and energy functions , something that is not difficult to understand given the multitude of biological processes that occur around the cell membrane that are necessary to supply energy. (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 fibers and 60% in type II fibers, while in the legs (vastus lateralis), it is 60% in type I fibers and an almost insignificant decrease in type II fibers.
Intermyofibrillar
Relatively, it accounts for 75% of muscle glycogen , representing the highest 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.
After 1 hour of arm and leg exercise, depletion also varies between muscle groups, although a significantly different decrease does not appear to be observed depending on the type of muscle fibers:
- In the arms (triceps brachii) there is 75% depletion in type I fibers and 70% in type II fibers.
- In the legs (vastus lateralis), however, type I fibers showed a decrease of 55% and type II fibers by around 10%.
Intramyofibrillar
It represents a relatively low percentage of the total (5-15%). In type I fibers, it accounts for 12% of the total glycogen, while in type II fibers, it accounts for a smaller amount (8%).
Its location is crucial. It is located within the myofibrils, within the contractile myofilaments, and specifically around the first band of the sarcomere. It is therefore distributed very closely to the myofibrillar structures involved in the contraction process.
After 1 h 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.
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 the athlete, after exercise, is the replacement of muscle and liver glycogen through the ingestion of CHO. (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 that leads to increased glycogen synthesis is glycogen depletion. (22)
However, the major determinant of muscle and liver glycogen resynthesis is found in a high intake of CHO around 1-1.5 grams/kg of body weight immediately post-effort 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, most notably, the amount, timing, and type of CHO (2:1 ratio) ingested during recovery (24).
Now, how is glycogen stored?
When we talk about how glycogen is stored, we're talking about the timing of carbohydrate intake, which is essential for restoring our energy stores.
To understand the importance of timing CHO intake , it is necessary to understand the two phases of CHO resynthesis. Several studies have indicated that post-exercise glycogen resynthesis occurs in a biphasic pattern (25).
Initially, there is a rapid increase in the rate of resynthesis, independent of insulin concentrations and lasting approximately 30–60 minutes after exercise; this supports the high glycogen synthesis in the immediate 60 minutes following exercise completion. (26)
For this reason, FANTÉ recommends innovative usage modes compared to current recovery programs, differentiating time and dosage based on the athlete's weight, something never seen before in any other usage mode to date.
In this phase, an increase in the translocation of the glucose transporter protein (GLUT-4) can be observed, due to an increase in calcium concentrations at the level of the rhabdomyocyte sarcoplasm (a consequence, in turn, of the multiple action potentials that take place during the effort) (27), up to two times, gradually decreasing until reaching pre-exercise levels 2 hours after its completion. (28)
As for liver glycogen, it is rapidly restored during post-exercise food intake with a content of 0.2 to 0.5 grams fructose/kg weight, helping to maintain normoglycemia or, when CHO intake is not performed post-exercise, via gluconeogenic action from lactate. (29)
In light of the above, there appears to be a potential post-exercise window of opportunity that athletes should take advantage of for muscle glycogen recovery (30).
In fact, when immediate CHO ingestion is compared to an ingestion up to 2 hours after exercise, it results in 45% lower concentrations of muscle glycogen (31).
In the context of recovery from exhaustive exercise, it is known that an intake of 6-12 g/kg is sufficient to restore endogenous glycogen stores when the recovery time is ≥ 24 h. (32, 33)
However, when recovery time is limited (< 8 h), specific strategies aimed at accelerating glycogen resynthesis become necessary (26).
Similar to the effects of the glycemic index of foods over longer periods (i.e., 24 h), the frequency of CHO ingestion does not appear to influence muscle glycogen resynthesis, however, when recovery time is limited, the frequency at which CHO is ingested may have an influence.
This has been demonstrated in studies that have shown that with CHO intake occurring at intervals of 15-30 minutes, the rate of muscle glycogen resynthesis is approximately 40% higher than when it is supplied every two hours (34, 35, 36).
Amount of carbohydrate intake
Regarding the recommended amount of CHO for glycogen replenishment , van Loon et al. (12) showed that the intake of 1.2 g/kg/hour of CHO resulted in a 150% higher glycogen resynthesis (from 17 to 45 mmol/kg dm/h) in relation to a lower dose of 0.8 g/kg/hour (12).
Searching for the optimal amount in this regard, Howarth et al. (2009), (37) showed how the ingestion of 1.6 g/kg/hour did not stimulate glycogen resynthesis further, considering that the recommended amount of post-exercise CHO will be around 1.0-1.5 g/kg/hour maximum within the first hour after cessation of exercise and will continue with an intake of 1.0-1.5 g/kg/h every 4-6 hours or until resuming regular meals (38).
Type of carbohydrates
An important factor determining muscle glycogen resynthesis is insulin-mediated glucose uptake into muscle cells.
The intake of CHO with a moderate or high glycemic index (GI) is a good option to achieve glycogen restoration, in part, by providing rapid glucose availability and insulin response (39).
When fructose is compared with glucose or sucrose, it is observed that the insulinemic response is lower in the former, which is attributed to a greater use of this monosaccharide in the resynthesis of hepatic glycogen (26, 40).
On the other hand, glucose and sucrose seem to have a similar effect on muscle glycogen resynthesis, as was recently demonstrated in a study, where it was shown that the intake of 1.2 g/kg/h of glucose, glucose + fructose or glucose + sucrose during recovery caused similar rates of muscle glycogen resynthesis (41).
In this regard, it is recommended to ingest a mixture of glucose + fructose in a 2:1 ratio that provides an optimal dose of CHO for the effective restoration of liver and muscle glycogen. (26)
The ingestion of liquid or solid forms of CHO appears to be equally effective in restoring muscle glycogen , so the individual preference of the athlete should prevail (42). However, as Ranchordas (2017) points out (30), from a practical perspective it would be interesting for athletes to have access to mixtures of both solid and liquid foods, in order to avoid such problems.
Protein YES or NO in recovery
Various nutritional factors are being studied to enhance glycogen resynthesis in conjunction with CHO intake. Several studies have shown that the simultaneous intake of CHO and protein may be beneficial for glycogen resynthesis. (37)
This is because protein intake increases insulin secretion by the pancreas, stimulating glycogen resynthesis.
The type of protein appears to influence insulin secretion. Thus, hydrolyzed protein (isolate) has been shown to have a greater effect on insulin secretion than intact protein, which is related to its accelerated digestion and absorption rate (43, 44).
Furthermore, whey protein appears to be a greater insulin stimulator than casein, possibly due to its higher leucine content (45). Therefore, we use whey protein isolate rather than other types in our GLYCOGEN recovery .
So carbs and protein 3:1?
How do the following factors interfere with glycogen repletion?
Glutamine
Glutamine is a conditionally essential amino acid widely used in sports nutrition, especially for its immunomodulatory role. However, glutamine performs several other biological functions, such as cell proliferation, energy production, glycogenesis, and ammonia buffering, among others.
Another potential anti-fatigue property of glutamine is its ability to prevent dehydration . Glutamine is transported across the intestinal brush border by a sodium-dependent system, promoting faster absorption of fluids and electrolytes from the intestine.
Therefore, the inclusion of glutamine in rehydration solutions could increase sodium absorption and water flux.
The amount of sodium
When we eat a recovery diet, it must have a good supply of carbohydrates appropriate for our weight, the 2:1 carbohydrate type, protein in an adequate amount, and an adequate amount of specific minerals with the goal of replenishing what we have expended.
The quantity is essential not only to replace salts, but having a high quantity and being a hypertonic drink favors the entry of solute (cho) into the cell faster than an isotonic (single-dose) or hypotonic drink.
Therefore, at FANTÉ we opted for hypertonic recovery based on current scientific evidence, adding 0.8g of sodium per dose for weights up to 50kg and 1.5g of sodium for those over 90kg per dose.
Remember! Being hypotonic, you resynthesize glycogen very quickly, but you're not hydrated, so we recommend preparing another bottle of water and drinking from that as well, not just Fanté GLYCOGEN.
Creatine
Creatine has also been studied for its synergistic effect on glycogen resynthesis. Studies have shown that creatine monohydrate intake increases the expression of genes involved in various activities , including glycogen resynthesis, which is suggested to be mediated by the osmotic effect of this ergogenic aid (46).
Robert et al. (2016) (47) observed an increase in post-exercise glycogen storage following creatine supplementation (20 g/day) along with a high-CHO diet.
This was most evident in the 24 h after exercise and was maintained during 6 days of post-exercise recovery with a high-CHO diet.
It is important to consider the 1-2% body weight gains that may be due to creatine use, which may interfere with some sports, where weight gain can impair performance (e.g. high jump) (38).
Caffeine
Another nutrient studied in this regard is caffeine. One study found that ingesting 8 mg/kg of caffeine along with CHO (1 g/kg/h) resulted in a substantial increase in glycogen content during 4 h of post-exercise recovery (48).
However, the potential interference of such high caffeine intake with the athlete's sleep should be taken into account. Furthermore, other similar studies have found no difference in glycogen content (49).
A recent systematic review has analyzed how different compounds contained in coffee can affect muscle glycogen resynthesis, showing how some of these compounds can activate different molecular pathways leading to an increase in muscle glycogen synthesis. This leads the authors to conclude that coffee is a potential option for athlete recovery.
More study is still needed.
Alcohol
Finally, it should be noted that alcohol can interfere with glycogen replenishment.
In relation to this, Burke et al., (2003) (50) showed how alcohol intake (approximately 120 g) could indirectly interfere with glycogen storage during recovery, displacing CHO intake.
However, the direct effects have not yet been clarified.
