Metabolic Adaptation and the Endurance Athlete

Metabolic Adaptation of Endurance Athletes

Some athletes know all about the inner workings and mechanics of their bicycles.  They can discuss ceramic bearings and how many watts it saves an athlete at length, but they take the human body for granted and forget that it is the most scientific machine of all.  Afterall, it is the human body that is pushing that bicycle forward.  Despite approximately 20 years of furthering technology the finishing times of the top competitors at the Ironman World Championship held in Hawaii are not drastically different.  In 1996 the top pro male Luc Van Lierde swam 2.4 miles in 51:36, biked 140.6 miles in 4:30:44, and ran 26.2 miles in 2:41:48 for a total time of 8:04:08.  Nineteen years later in 2015 the top pro male Jan Frodeno completed the same distances in 50:50, 4:27:27, and 2:52:21 for a total time of 8:14:40.  You see the human body is the ultimate limiter in how fast we perform in a triathlon, no matter what technological advances are made in swimsuit fabric, bike technology, and running sneakers.  

It is important therefore to understand just what is happening to our bodies as we train. Only by understanding how our bodies adapt to aerobic training can we devise training plans to improve upon one’s endurance.  

The following are metabolic adaptations to muscle as it pertains to increases in endurance training:

  • Endurance training, specifically at low to moderate intensities such as those training in recovery to aerobic zones, which translates to approximately 74- 86% of one’s threshold heart rate requires the recruitment of slow twitch muscle fibers first, also known as Type I muscle fibers and then the recruitment of fast twitch (a) fibers or Type IIa.  Endurance training does not increase the percentage of slow twitch fibers, it does however recruit the use of fast twitch (b) fibers (Type IIb) thereby asking them to perform more like fast twitch (a) fibers which do have a moderately high oxidative capacity although not quite as high as slow twitch fibers.
  • A significant known adaptation to endurance training is the increase in muscle capillaries. An increase in capillary production allows more blood to get to a working muscle, this in turn allows for a greater exchange of both nutrients and waste between the blood and a working muscle.
  • Myoglobin is an iron and oxygen binding protein found in muscle fibers.  When oxygen is delivered to the muscle it binds to myoglobin and the myoglobin releases it to the mitochondria when oxygen is limited during muscle action.  Endurance training has been shown to increase muscle myoglobin by as much as 80%.
  • Mitochondria are organelles that produce a cell’s energy.  The ability for a cell to use oxygen and produce energy relies directly on the number and size of its mitochondria and how efficiently its mitochondria are working.  We know that increases in the volume of endurance training leads to both an increase in the size and number of mitochondria in a cell.
  • The production of energy relies on the efficiency of mitochondria to use certain types of oxidative enzymes.  The activities of these enzymes are increased with increases in endurance training, thus contributing to a muscle’s aerobic capacity.

It is here where one can begin to understand what happens to an athlete’s body as it is adapting to an aerobic training stimulus.   In layman’s terms the take home points are, as we train and increase our training volume our muscles become able to endure longer workloads and utilize oxygen for energy more efficiently.  Blood flow to our working muscles is increased allowing them to get much needed nutrients and get rid of unwanted waste.  Finally, muscle’s powerhouse cells (mitochondria) are increased allowing our muscles to work longer, harder and more efficiently.  It is true that genetics limits a good portion of these changes.  However, we can maximize on our own abilities by developing proper nutrition plans to maximize energy and developing proper individualized training plans that take into account the appropriate volume, frequency, and intensity for a given athlete at certain times of their training cycles.  

That leads us to the second component of metabolic adaptation. Similar to how our muscles adapt at the cellular level and how they utilize oxygen, our muscles also adapt to how it uses its fuel sources.  Particularly carbohydrate and fat.  The more efficient our muscles utilize its fuel sources the more resistant our muscles are to fatigue.  As carbohydrate and fat are the two primary fuel sources for a working muscle.  

Our bodies store glycogen in our muscle and liver and we use this glycogen to fuel our training sessions.  As we rest and provide our bodies with sufficient carbohydrate we will store this glycogen in our muscles.  It has been shown that trained muscle stores significantly more glycogen than untrained muscles.  This allows us as athletes to tolerate increased training as we have a greater amount of glycogen stored that we can then utilize for energy.  However, we can only store this glycogen if we make a conservative effort to.  Therefore, we can not as athletes simply eat according to hunger, but we must incorporate nutrient timing to make sure we are restocking our body with the fuels it needs at the right times.  Chronic fatigue and an overall feeling of tired is not simply a result of increased training, as much as it is a result of an imbalance between glycogen use and carbohydrate intake.  It is important that we ingest carbohydrate both during training sessions and at the right times after training sessions to prevent an imbalance. Especially as we are training on back to back days and even more importantly for those of us who train twice a day.

Trained muscles have also been shown to increase the ability to oxidize free fatty acids.  This allows us to burn fat more efficiently, and decrease the demands on our supply of muscle glycogen.  Many at this point will start saying to themselves “I knew I should start trying a low carbohydrate, high fat diet to increase my performance.” However, this is easier said than actually done by the body.  You see, simply eating fat does not allow us to burn fat. This is because as we eat fat we tend to elevate our blood’s triglyceride levels.  This then has to be broken down to free fatty acids in order for our body to utilize it as fuel.  For the most part we have been unsuccessful in dietary attempts to elevate free fatty acids.  

It is important to be open to new ideas and modern practices and see what advantages and/or disadvantages they might provide.  In my mind it is clear that by knowing how our muscles adapt to utilizing its fuel sources we should still be fueling our training sessions as well as recovering by ingesting carbohydrate as our primary fuel source.


Wilmore, Jack H. (1994).  Physiology of Sport and Exercise. Illinois: Human Kinetics.
Burke, Louise M. (2015). Re-Examining High Fat Diets for Sports Performance: Did We Call the “Nail in the Coffin” Too Soon?. Sports Med, 45(Suppl 1): 33-49.
Hermansen,L., & Wachtlova, M. (1971). Capillary density of skeletal muscle in well-trained and untrained men. Journal of Applied Physiology, 30. 860-863.


Christopher Breen, PA-C, ACSM EP-C is a Certified Physician Assistant specializing in sports medicine and orthopaedics, a Certified Exercise Physiologist by The American College of Sports Medicine, and a USAT Level 1 Certified Triathlon Coach.  He is the founder and head coach of ARIA Endurance Coaching, LLC and also works at Winthrop Orthopaedic Assoc., PC in Long Island, NY.  He can be reached at and


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