Carbohydrate Metabolism

Posted by Scott Christensen

Understanding and following proper nutritional protocol is crucial to the success of any athlete.  Human cells require nutrients ingested from outside of the body to provide an energy platform for muscle contractions, amino acids for synthesizing a wide variety of protein-based components, and micro-nutrients that provide a sort of co-enzyme function in many cellular functions. Carbohydrates are very important.

 

Eating is a very personal human activity.  The role of the high school cross country coach is to provide the athlete with a basic understanding of scientific principles regarding nutrition, not to plan and dictate every meal and snack for the team members.  Perhaps the most important concept to be taught is the role of carbohydrates in a cross country runner’s diet.  Armed with that information a runner can make sensible choices about diet selection and gain an understanding about the timing of carbohydrate ingestion before their races.

 

Think of carbohydrates as the body’s jet fuel supply.  Carbohydrates are the preferred energy-rich molecules for striated muscle contraction and the only fuel source the brain can operate with.  For that reason, storage of carbohydrate is essential in the body.  Carbohydrate is stored in the three separate ways: Muscle and liver glycogen and blood glucose.  Consuming adequate carbohydrate on a daily basis is necessary to replenish both types of glycogen between daily training sessions and competitive events.

 

Stored muscle glycogen dwarfs the glycogen amount stored in the liver, but both sources are important in the training regime of the cross country runner.  For example, a 12 mile long run for a runner (training at 60 miles per week) will come close to exhausting both supplies of glycogen if done too fast.  Glucose saturated in the blood only contains enough energy to run one mile, so it needs to be restocked by glycogen stores continuously while running longer distance.

 

The energy demands of running dictate that carbohydrate is the preferred fuel for exercise intensities above the aerobic threshold.  Lipid oxidation cannot supply ATP fast enough to support such high-intensity running, although it contributes a fraction of the amount needed.  Lipids supply more calories per gram of substrate than carbohydrate, 9 KCAL/gram compared to 4 KCAL/gram but the process of oxidation of lipids is much slower.

 

There is a statistically significant correlation between the pre-exercise muscle glycogen stores and the length of time running can be performed at 75% of VO2 max.  The more embedded the quantity of muscle glycogen, then the further one can run.

 

Studies have shown that a mixed diet of 50%/50% carbohydrate produced a muscle glycogen content of 106 mmoL/kg wet muscle weight and enabled the subjects to run for 115 minutes at 75% VO2 max before the glycogen supply was compromised.  A low carbohydrate diet (less than 10% of KCAL from carbohydrates) produced a muscle glycogen content of 38 mmoL/kg and supported one hour of running at the same intensity.  However, a high carbohydrate diet (more than 80% KCAL from carbohydrate) provided 204mmoL/kg of muscle glycogen and enabled the subjects to exercise for 170 minutes at the same intensity.

 

Liver (hepatic) glycogen stores are also important for distance runners.  Hepatic glycogen stores can be emptied by a 15 hour fast of carbohydrate.  One study showed a drop from 490 mmoL/kg on a 50/50 mixed diet to 60 mmoL/kg on a low carbohydrate diet.  A high carbohydrate diet can increase liver glycogen content to about 900 mmoL/kg.

 

A high carbohydrate intake acutely enhances a cross country runner’s recovery and improves endurance performance over 24 to 72 hours.  One study reported that a high carbohydrate diet (80% KCAL carbohydrate or 8.8 gm/kg/day) restored endurance capacity within 22.5 hours of recovery between training sessions.  Cross country runners in season should consume 7 to 12 gm/kg/day of carbohydrate.  For a 140 pound distance runner (62 kg) that converts to about 620 grams of carbohydrate per day.  That would be the equivalent of two foot-long veggie submarine sandwiches per day.

 

The Glycemic Index (GI) provides a scientific way to rank carbohydrate rich foods according to the blood glucose response following intake.  Generally foods are divided into those with a high GI (glucose, bread, potatoes, breakfast cereals, sport drinks), a moderate GI (sucrose, soft drinks, oats, tropical fruits [bananas]), or a low GI (fructose, milk, yogurt, lentils, pasta, nuts, non-tropical fruits [apples, oranges]).  GI tables are readily available on the internet.

 

Some research suggests that manipulating the GI of snacks and meals may enhance carbohydrate availability and improve athletic performance.  For example, low GI carbohydrates may be recommended four to six hours before exercise to promote sustained substrate availability.  Moderate GI carbohydrates may be recommended during breaks between events at track meets to promote rapid carbohydrate oxidation and after exercise to promote glycogen repletion.  High GI carbohydrates should be consumed during the two to four hour window following a cross country meet to enhance long term glycogen repletion.

 

It should be emphasized to your athletes that the total amount of carbohydrate consumed is the most important consideration for replenishing glycogen stores following daily training sessions and competitive events like time-trials and races.  In the current era of diet books that run the range of recommended carbohydrate intake it should also be emphasized that any runner on a low-carbohydrate diet is asking for chronic exhaustion and decreased performance.

 



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The Athlete Profile-Part 2

The Healthy Runner Project

Workout Themes

How to Establish Cross Country Training Groups

Fatigue, Part 5 – The Central Governor Theory

  • Olivier

    Great stuff, as ever !
    This is true not only for cross country runners : carbohydrate supply is of tremendous importance for all endurance athletes, whatever their sport or event. This is perfectly outlined in your article.
    And even for “shorter” athletes, it shall be one important focus. As explained in your text, adequate (time + amount) intake of carbohydrates is a key factor of recovery.
    For our sprinters (or mid-distance athletes), who may not rely as heavily on carbs as their primary fuel, we still shall stress that regular and well proportioned carbs consumption is one of the secrets to be able to sustain training volume, and not run to complete exhaustion. True between reps, true between sets, true from an exercice to the next… And still true from one workout to the next.

    Would you mind sharing the reference studies you are talking about in the post ?

    By the way… I’m not in track & field. I’m a speedskating coach. But the great work you are doing at CTF translates very well to other sports. As coaches, we all have to find the right knowledge where it is :-D !

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