Over the past 50 years, a significant amount of research has shown the important role of glycogen for delaying fatigue in athletes competing in endurance and ultra endurance events. Glycogen is the storage form of carbohydrates. In humans, small amounts of glycogen are also found in tissues and cells, like the kidneys, red blood cells and brain. Glycogen is primarily stored primarily in the cells of the liver (~100g) and skeletal muscle (~500g). Skeletal muscles transform chemical energy to mechanical work. Glycogen is the body's predominant source of energy during moderate to high intensity activity. The depletion of muscle glycogen during endurance activity causes early fatigue.
Dietary carbohydrates are converted to glucose - the main source of energy for the body and brain. For the non-diabetic population, when you eat a carb-rich meal, your blood glucose will rise. Increased blood glucose signals the pancreas to produce insulin - a very important hormone that helps the body cells take up glucose from the bloodstream to be used for immediate energy or for storage. Insulin tells the liver and muscle cells to produce the enzyme glycogen synthase that links chains of glucose together to form a glycogen molecule to be delivered to the liver and muscles.
Because glycogen has a limited storage capacity, the intake of carbohydrates is extremely important for endurance athletes. High muscle glycogen content improves high intensity and endurance performance by increasing the time to fatigue. Due to the high demand for glycogen as an energy source, many athletes stay in a state of low glycogen depletion. Trying to complete high intensity or high volume workouts in a state of very low glycogen can increase the risk for damaged muscle tissue, immune system suppression and injury. Thus the importance of proper fueling, nutrient timing and understanding daily energy needs to support the current training load. And for all this to happen, the athlete must have a good relationship with food and the body.
The heavily researched topic of carb-loading focuses on increasing the content of stored glycogen in an anticipation for the upcoming endurance event. Maximizing glycogen stores can help reduce fatigue in events lasting longer than 90-minutes.
There are a lot of misconceptions around carbo-loading and a few key considerations to help you get the most out of glycogen supercompensation.
Dietary carbohydrates are converted to glucose - the main source of energy for the body and brain. For the non-diabetic population, when you eat a carb-rich meal, your blood glucose will rise. Increased blood glucose signals the pancreas to produce insulin - a very important hormone that helps the body cells take up glucose from the bloodstream to be used for immediate energy or for storage. Insulin tells the liver and muscle cells to produce the enzyme glycogen synthase that links chains of glucose together to form a glycogen molecule to be delivered to the liver and muscles.
Because glycogen has a limited storage capacity, the intake of carbohydrates is extremely important for endurance athletes. High muscle glycogen content improves high intensity and endurance performance by increasing the time to fatigue. Due to the high demand for glycogen as an energy source, many athletes stay in a state of low glycogen depletion. Trying to complete high intensity or high volume workouts in a state of very low glycogen can increase the risk for damaged muscle tissue, immune system suppression and injury. Thus the importance of proper fueling, nutrient timing and understanding daily energy needs to support the current training load. And for all this to happen, the athlete must have a good relationship with food and the body.
The heavily researched topic of carb-loading focuses on increasing the content of stored glycogen in an anticipation for the upcoming endurance event. Maximizing glycogen stores can help reduce fatigue in events lasting longer than 90-minutes.
There are a lot of misconceptions around carbo-loading and a few key considerations to help you get the most out of glycogen supercompensation.
- Muscle damage limits the capacity of the muscle to store glycogen, even while consuming a high carbohydrate diet. If you go into a race with damaged muscles (ex. downhill running, back-to-back racing), don't assume that a high carbohydrate diet will magically make your muscles perform/feel better.
- Most of the research on carb-loading is on men. But there is a little research showing that females may require a slightly different pre-race fueling strategy due to metabolic differences from different hormonal profiles (specifically estradiol). Whereas men are advised to increase carbohydrate intake before a race, women are advised to increase carbs and calories. In other words, women may need to "energy load."
- Muscle glycogen concentration is dependent on the diet. The higher carb intake in your diet, the higher the glycogen stores.
- Training increases the amount of glycogen that can be stored in the muscle. Endurance training increases muscle glycogen stores and reduces the reliance on glycogen as a result of increased use of free fatty acids by active muscle cells. This metabolic adaptation allows your body to become more efficient at storing glycogen while also becoming more metabolically efficient.
- After prolonged intense exercise, muscle glycogen levels may fall from 150-200 mmol/kg wet weight to <50 mmol/kg wet weight. When muscle glycogen levels fall to less than 70 mmol/kg wet weight, calcium release from the sarcoplasmic reticulum is impaired - which impacts muscle function, power output and performance.
- Muscle glycogen resynthesis is a very slow process. It may take several days of rest or very light training (along with a higher carb diet) for muscle glycogen levels to reach adequate levels after several days of hard training. Consuming a high carbohydrate diet is critical to meet the demands of high volume/intensity training.
- Carbo-loading should not be confused with overeating. Effective glycogen loading should the the result of emphasizing more energy-dense, low fiber carbohydrate sources and reducing fat intake alongside reducing (not stopping) energy expenditure (tapering).
- One to two days of carbohydrate loading is sufficient to prepare your muscles for the upcoming event.
- Glycogen depletion is considered an important limiting factor to performance. But eating more carbohydrates will not make you a better trained athlete. However, just because you didn't complete your training as planned, this doesn't mean that you need to punish your body by underfueling.
Carb-Loading Tips
Sources:
- A carbohydrate intake of 5-7g/kg/day in the 1-2 days before your endurance event is recommended.
- If you have a sensitive stomach or are prone to GI issues, be strategic with the foods you consume. Opt for a low-residue diet.
- Males should aim for ~70% of the daily diet from carbs (without making a major change to caloric intake). A female athlete should aim to eat ~70% from carbs with an additional 30% increase in normally consumed calories.
- Carbo-loading does not remove the need to stay well-fueled and hydrated throughout your event. Carb-loading is designed to help delay fatigue - not avoid fatigue.
- Consume small meals, regularly throughout the day. Create a schedule (and plan) for eating so that you don't forget to eat or go long hours without eating.
- If you feel "heavy" when you carb-load, this is good. This reflects the water weight that is added from glycogen storage. For every gram of glycogen stored in your body, you store ~3 ounces of water. This is ok! It doesn't have to do with a gain in body fat but water weight. This is also why people immediately lose weight on a low-carb diet.
- Aim to eat your largest carb meals in the morning hours (breakfast - lunch) to allow more time for digestion.
- Stay well-hydrated throughout the day to help with digestion.
- Practice with similar pre-race foods in training to build familiarity and confidence for race day.
Sources:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6566225/
https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280221
https://physoc.onlinelibrary.wiley.com/doi/10.1113/JP280221
https://www.220triathlon.com/training/nutrition-training/do-female-athletes-need-to-carb-load-more-or-less-than-male-ones-before-a-race/
https://www.mysportscience.com/post/carb-loading
https://www.scienceforsport.com/carbohydrate-loading-for-endurance-still-a-good-practice/
https://www.mysportscience.com/post/carb-loading
https://www.scienceforsport.com/carbohydrate-loading-for-endurance-still-a-good-practice/