Fundamentals of Glycogen Metabolism for Coaches and Athletes
Bottom Line
Glycogen is the key fuel source for athletes who regularly train at high intensity. Understanding the metabolism of carbohydrates and glycogen is critical for coaches and athletes. Many athletes don’t eat enough for their goals.
Key Ideas
Adequate glycogen levels are vital for sustained intense training, necessitating appropriate carbohydrate intake. Many athletes under-eat for their training requirements.
The ideal carbohydrate consumption for athletes is influenced by training goals, overall diet, nutrient timing.
Decreased glycogen levels are associated with reduced force production. This is likely because glycogen supports calcium ion release. With decreased glycogen, calcium ion release decreases, lowering force production in muscle contraction.
Glycogen supercompensation is a state where glycogen stores are increased beyond the normal level.
Glycogen restoration involves two phases: a rapid phase shortly after exercise and a prolonged phase lasting hours to days, aided by carb supplementation.
Summary
Sustaining rigorous training requires maintaining sufficient glycogen levels, which necessitates an appropriate intake of carbohydrates. Many athletes under-consume, often barely meeting maintenance needs. Carbohydrate intake boosts performance and hastens recovery.
During extended or intense exercise, glycogen breaks down into glucose, serving as energy. As exercise intensity rises, so does the rate of glycogen degradation. At intensity levels above 60% of VO2Max, blood glucose and muscle glycogen emerge as primary energy sources, due to the engagement of fast-twitch muscle fibers. At lower intensities, free fatty acids are used, preserving glycogen. Glycogen depletion is slower at low intensities, but with increased volume, will occur.
Although nutrition should align with training objectives, around 8 to 12 grams of carbs per kilogram of body weight is commonly recommended for glycogen replenishment in athletes who regularly train at high intensity.
Note: This paper focuses mainly on muscle glycogen. While the liver maintains significant glycogen stores, and can produce glucose from amino acids and glycerol, its primary purpose is to stabilize blood glucose to support brain activity, not intense exercise. Sufficient stores of muscle glycogen also reduce dependence on blood glucose for muscle energy.
Roughly 75% of muscle glycogen resides between muscle fibers, while the remainder is distributed within muscle cells. Intramuscular glycogen facilitates calcium ion release for muscle contraction, so its depletion likely contributes to fatigue by weakening contractions. Fatigue attributed to hampered calcium ion cycling can persist for days without sufficient carb intake.
Several glycogen replenishment approaches include:
Consistent: Exceeding 8 g/kg carbs / body mass per day.
Loading: Three intense training days on low carbs, followed by three high-carb light days.
Modified Loading: A three-day taper with high carb intake.
Timing: Low carbs in the morning before training, then high carbs after.
Glycogen restoration post-exercise involves two phases: a swift phase, occurring around 30-40 minutes after exercise, and a prolonged phase lasting hours to days. Glycogen manifests in two particle forms: pro-glycogen (about 15% of total) and macro-glycogen (85%). This may explain the two-stage replenishment process after exercise.
After exercise, glucose transporters (GLUT4) reposition near cell membranes, enhancing membrane permeability to glucose and facilitating glycogen replenishment. If carbs are not eaten soon after intense exercise, GLUT4 will relocate away from cell membranes, reducing membrane permeability to glucose.
Low-carb training may improve oxidative capacity and endurance performance, while glycogen remains vital for resistance training. Intensity requires glycogen. For trained athletes, muscle glycogen levels average around 150 millimoles / kg, surging up to about 200 in a super-compensated state. This can drop to as low as 50 after high intensity training. Muscle contraction weakens when glycogen dips below 70. Therefore, a replenishment rate of 5-6 mm/ kg within 24 hours fully restores glycogen for most athletes. Following a 2-hour cycling session, a high-carb diet restored 93% of muscle glycogen in 24 hours, while a low-carb diet replenished only 13%.
Glycogen utilization during exercise activates glycogen synthase, the enzyme which assembles glucose into glycogen particles. Post-exercise carbs switch on insulin and increase glycogen synthase activity, elevating replenishment levels. With a 24-hour interval between sessions, 10 g / kg of body weight suffices for complete muscle glycogen restoration, and 1-1.2 g of carbs / kg per hour can restore 60% in six hours.
Exceeding 10 grams per kilogram yields no extra replenishment benefit. Additionally, energy balance must be sustained to fully replenish glycogen, requiring adequate calorie intake. In other words, athletes must eat enough to fuel basic needs. Carbs above basic energy needs are used for glycogen resynthesis. The target of 10 grams per kilogram is suitable after intense training or before significant competition. As a rule:
3-5 g/kg is light
5-7 g/kg is moderate
6-10 g/kg is high
8-12 g/kg is very high
Restoration duration varies based on degradation extent: 40 mmol/kg can be restored in 4-5 hours, 150 mmol/kg can be restored within 24 hours. The maximum restoration rate is about 10 mmol/kg per hour for 4-5 hours, gradually declining. Supercompensation is feasible with ample carb intake and 24 to 72 hours rest after intense sessions.
Extended recovery (over 24 hours) is influenced by total carb intake, not solely type or timing. Post-exercise, high glycemic index foods expedite glycogen restoration. The effectiveness of high vs. low glycemic carbs before is uncertain, but high carb meals post-exercise can fully restore glycogen if eaten in sufficient quantities. High glycemic meals are advantageous for multiple sessions or competitions.
Terms for Further Study
1. Glycogen: A complex carbohydrate stored in the liver and muscles, serving as a primary energy reserve for the body.
2. Carbohydrates: Organic compounds, including sugars and starches, that are a primary source of energy for the body.
3. VO2 Max: Maximum oxygen consumption, a measure of aerobic fitness.
4. Fast-twitch Muscle Fibers: Muscle fibers that contract rapidly and powerfully, typically used for short bursts of intense activity, and relying primarily on glycolytic power.
5. Insulin: Hormone produced by the pancreas that regulates blood sugar levels and facilitates the uptake of glucose into cells.
6. Oxidative Capacity: The ability of tissues to use oxygen to generate energy.
7. Supercompensation: The process of storing more of a substance than normal after a period of depletion.
8. Enzyme: A biological molecule that catalyzes chemical reactions in living organisms.
9. Glycemic Index: A measure of how quickly carbohydrates in food raise blood sugar levels.
Original Paper
Fundamentals of Glycogen Metabolism for Coaches and Athletes