THE COMPLETE GUIDE TO GLYCOGEN STRUCTURE

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THE COMPLETE GUIDE TO GLYCOGEN SYNTHESIS FOR PERFORMANCE AND MUSCLE GAIN SUMMARY

• This article gives a comprehensive overview of muscle glycogen synthesis and breakdown for performance.
• Carbohydrate drinks/powders are unnecessary if you are training for less than an hour.
• Carbohydrate diets may be beneficial if you are training with high-volume training performing more than 10 sets per bodypart, or who train twice a day.
• Being hungry before exercise can result in a decrease in performance.
• Carbohydrate mouth rinsing may improve performance if training for less than an hour, but more research is needed.

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GLYCOGEN STRUCTURE AND GLYCOGEN SYNTHESIS

Sufficient carbohydrate intake will maximize stored carbohydrates in athletes’ muscles and supply fuel to the brain and central nervous system. The brain has a constant use of glucose, and under normal circumstances, glucose is the primary source of fuel to produce ATP in the brain. Glycogen is the storage form of glucose. The major storage sites for glycogen synthesis are the liver and muscle. The liver stores glycogen and is a reservoir to constantly supply the brain with glucose (i.e., 4 grams of glucose in the bloodstream).(1)

Most athletes will get excited when they hear about protein research and its effects on muscle growth, but if you switch the topic to the benefits of carbohydrates, their enthusiasm dwindles. Protein is critically important for athletes, but consuming more protein above a certain threshold will not have any performance-enhancing benefits and will usually come at the cost of sacrificing carbohydrates. In several studies, many athletes are not consuming the optimal number of carbohydrates for full glycogen synthesis and replenishment.(2-4)

Athletes also seem to share a disconnect between sports nutrition and its relation to body composition. In a study of college athletes, most underestimated their total calorie needs and carbohydrate requirements compared with their predictive needs for their sport. Interestingly, those with the lowest scores in sports nutrition had the highest levels of body fat.(5)

GLYCOGEN SYNTHESIS STUDIES

Similar studies have replicated these findings in which athletes consistently underestimate their daily energy requirements (-1284 kcals) and carbohydrates (-178 grams), compared to protein (-31.4 grams) and fat requirements (-27.9 grams).(6) Endurance athletes seem to be the worst for underestimating their carbohydrate needs. One study of 146 endurance athletes found that 80% of the athletes consumed the lower than recommended amounts of carbohydrates in their diet.(7)

Most athletes far exceed their protein requirements, but athletes typically fail to meet adequate, optimal carbohydrate requirements. We have all heard that there are no essential carbohydrates, which is an accurate statement, but carbohydrates are the rocket fuel to get your body moving.

WHAT HAPPENS WHEN GLYCOGEN STRUCTURE IS DEPLETED?

When glycogen stores are low, muscle cells cannot rapidly produce enough ATP to maintain high exercise intensity.(8) When muscle glycogen is depleted, it directly affects muscle function as calcium release from the sarcoplasmic reticulum is impaired, resulting in impaired force production.(9) Thus, intense muscle-damaging exercise can reduce force production after exercise regardless of high carbohydrates indicative of muscle tissue damage. (10, 11)

Compared to fats, there are relatively limited glycogen stores in the body. In the fasted state and during low-intensity exercise, a majority of the energy required by the muscle is provided by the oxidation of fats from adipose tissue. During high-intensity exercise, the contribution of fats as an energy source becomes less, and carbohydrates provide roughly two-thirds of the total energy needed.(12)

GLYCOGEN SYNTHESIS AND CARBOHYDRATE METABOLISM

Carbohydrate metabolism is the preferred source of fuel under these conditions because the rate of ATP production is two times higher than fatty acids. Carbohydrate produces more ATP per unit of oxygen than fat, so carbohydrates become an increasingly important fuel source with higher-intensity activities where more ATP is required per unit of time.

Glycogen synthesis is rapidly initiated at the onset of exercise and increases exponentially with exercise intensity.(13) Muscle contraction is a process dependent on ATP being replenished. During high-intensity exercise lasting 30 – 180 seconds, the ATP required for muscle contractions is mainly powered through glycolysis (i.e., breakdown of glycogen).(14)

For sprinting events, the last part of the race is won by the athlete who can most efficiently utilize anaerobic glycolysis. From an evolutionary perspective, carbohydrate utilization and storage promote human survival by enhancing shorts burst of high-intensity contractions to facilitate chasing prey or evading predators. However, in today’s sedentary society, excess carbohydrates have led to an increase in type II diabetes and other metabolic conditions with excess calories.

GLYCOGEN SYNTHESIS AFTER EXERCISE

Carbohydrates are recommended to facilitate enhanced performance and optimize recovery. Glycogen synthesis occurs in two phases:

a.) a rapid phase glycogen synthesis phase that does not require insulin and occurs 30- 60 minutes after exercise.

b.) slow glycogen synthesis phase is in which glycogen is synthesized over a period of hours.(15)

MUSCLE GLYCOGEN DEPLETION

Athletes involved in multi-day events and twice-a-day training are encouraged to consume carbohydrates to facilitate the rapid restoration of glycogen stores. It is recommended that carbohydrates be consumed in the first two hours after exercise to maximize glycogen re-synthesis. Delaying carbohydrate ingestion by three hours after exercise resulted in a 26% lower muscle glycogen synthesis rate than ingesting carbohydrates immediately after exercise.

A 2-hour delay in intake of a carbohydrate-rich meal had no effect when glycogen content at 8 h post-exercise.(16) Thus, it is recommended that athletes prioritize carbohydrates after exercise in the first few hours after exercise. The ISSN recommends regular ingestion of snacks or meals; providing carbohydrates and protein in a 3:1 ratio helps promote recovery and replenishment of muscle glycogen.(17)

FOODS TO REPLENISH GLYCOGEN AFTER WORKOUT

It was once thought that a majority of the carbohydrates needed to be consumed immediately after exercise, but the feeding frequency of carbohydrates is also irrelevant with extended recovery (i.e., 24 hours). Consumption of carbohydrates in four large meals or 16 small snacks had comparable effects on muscle glycogen storage by 24 hours.(18) If sessions are less than eight hours apart, it is recommended that for optimal glycogen synthesis, 1.0-1.2 g/kg of carbohydrates be consumed for the first four hours, followed by the resumption of the daily carbohydrate requirements.(16) Importantly, no additional benefit to muscle glycogen rates occurs with carbohydrates beyond 1.0-1.2 g/kg.(19)

DO YOU NEED TO HAVE FULL GLYCOGEN STORES EACH TIME YOU EXERCISE?

So if you want to replenish glycogen stores fully, how much carbohydrates do you need to consume? It has been found that close to 10 g/kg/bw per day (i.e., 4.5 grams per pound of body weight) can fully replenish carbohydrates within 24 hours.(20, 21)  These two studies used 600-750 grams of carbohydrates in 24 hours to restore glycogen in 24 hours! This is only if you want to maximize glycogen stores before an exhaustive race or a prolonged competition.

However, research has shown that an athlete does not need to have full glycogen capacity to train. A review of the literature found that over a period of a week, a high-carbohydrate diet can maintain higher levels of glycogen stores, but consuming a moderate carbohydrate diet resulted in lower muscle glycogen but was still able to meet the demands of hard training.(22) In a study that compared a high (10g/kg/bw) vs. moderate (5g/kg/bw) carbohydrate diet over seven days, the moderate carbohydrate group had 30-36% lower muscle glycogen but still had similar performance to the high carbohydrate group.(23)

HOW LONG DOES IT TAKE TO DEPLETE GLYCOGEN STORES

In an excellent review of the literature titled Muscle Glycogen Metabolism and High-Intensity Exercise Performance: A Narrative Review, it was concluded:

a.) If muscle glycogen is only moderately depleted, no effect on high-intensity performance occurs; however, low-carbohydrate diets impair exercise performance

b.) no effects on performance or fatigue are observed with brief single-effort exercises

c.) continuous and repeated bouts of high-intensity exercise results in impaired performance when muscle glycogen is lowered

d.) low-carbohydrate diets can reduce exercise performance, but elevated muscle glycogen with high carbohydrate dies provides no additional performance benefits.(24) Thus, every time you train, you don’t need to have maximal glycogen stores; you just need to have sufficient glycogen stores dependent on the intensity and duration of exercise. The research clearly shows the importance of carbohydrates for performance and the detrimental effects of low-carbohydrate diets, but you can meet the demands with adequate carbohydrates in the diet.

GLYCOGEN AND MUSCLE DAMAGE

If an exercise damages muscle, such as eccentric exercise or downhill running, more carbohydrates are needed to restore muscle glycogen due to decreased ability to take up glucose. (25) After intense muscle-damaging exercise, insulin resistance occurs, and cellular carriers of glucose transport (i.e., GLUT-4) are reduced by damaging exercise.(26, 27) It has been found that muscle glycogen synthesis can be impaired for up to 72 hours following damaging muscle eccentric exercise.(28) Sports that involve a high number of breaking movements or changes in directions, such as soccer or football, involve a high number of eccentric contractions have also been found to have delayed glycogen restoration 24 hours after matches.(29)

There are no differences in glycogen synthesis between males and females as long as sufficient carbohydrates are met. There have been some studies that suggest that glycogen storage is affected by the menstrual cycle.(30) Athletes must consider the optimal glycogen benefits for their sports. 1 gram of glycogen holds 3 grams of water. This could be beneficial for athletes concerned with hydration (i.e., athletes exercising in a hot environment) and problematic for athletes that need to make weight (i.e., wrestlers, MMA athletes, etc.). Other supplements that can enhance glycogen synthesis are creatine monohydrate and whey protein hydrolysates, whereas caffeine, glutamine, and flavonoids are less well documented.(31, 32)

CARBOHYDRATE IS DEPENDENT ON YOUR EXERCISE INTENSITY

The relative contribution of carbohydrates as a fuel source during exercise depends on the intensity and duration, with a larger contribution of carbohydrates needed as exercise intensity increases. There is inconclusive evidence on whether low or high glycemic index carbohydrates have any performance-enhancing effects.(33, 34) Carbohydrates are stored as muscle glycogen (~350-700 g) and liver glycogen (~100 grams) depending on the athlete’s training status and diet.(35)

High carbohydrates are especially warranted during prolonged aerobic exercise (>90 minutes) and intermittent high-intensity team sports (i.e., soccer, basketball, hockey, etc.) lasting greater than an hour to an hour and a half. If you perform back-to-back anaerobic exhaustive exercise (i.e., tournament settings), carbohydrates can enhance exercise performance. In a study of trained cyclists in which they performed a time trial (i.e., 60-90 minutes of exercise), four days of carbohydrate super-compensation between exhaustive exercise increased exercise intensity and resulted in superior endurance (36)

Carbohydrate supplementation

Carbohydrate supplementation has been shown to increase the amount of work and the duration of the exercise performed during aerobic exercise.(37) It is well documented that during low-to-moderate exercise intensity (~30-65% of aerobic capacity), fat oxidation (i.e., fat burning) is the predominant fuel source. Low-carbohydrate diets may be beneficial for ultra-marathons and other sports that require exercise for several hours. Low carbohydrates may be beneficial in these situations, as low-carbohydrate diets can promote cellular adaptations to improve fat metabolism and enhance mitochondrial biogenesis (i.e., the creation of more mitochondria).(38, 39)

Although fat oxidation increases during high fat, low-carbohydrate diets, chronic adaptation to a high fat, low-carbohydrate diet results in impaired exercise economy and negative performance, resulting in impaired aerobic capacity.(40) This suggests that real-life endurance is impaired despite burning more fat as an energy source during keto diets. Despite the abundance of research for aerobic-based exercise, the research regarding carbohydrates and resistance exercise is less well documented.

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