Energy Pathways

Energy for Running

Our body converts the food we consume in to energy to be used for all training, Marathon races, and other activities. As a marathon runner preparing for the next marathon race or other endurance distance it is crucial to have at least a basic understanding of how the system of energy conversion works.

Nutrition or energy is retained from converting carbohydrates, fat, and protein to adenosine triphosphate or ATP. When ATP is broken down energy is released which allows the muscles to contract. Different nutrients have their unique properties which determine how they get converted to ATP.

For any training of moderate to high intensity carbohydrate is the main source of energy. Low intensity exercise for longer periods of time uses fat as its main source of energy. Body tissue and especially the damaged muscles are repaired by proteins which are normally not used as fuel for muscle activity.

It is not easy for our body to store ATP (whatever stored ATP there is would be used up in a matter of seconds), so it is critical to continuously create ATP during exercise. There are two major energy conversion ways which are:

  • Aerobic metabolism (with oxygen)
  • Anaerobic metabolism (without oxygen)

These two metabolism pathways can be split up into combinations of energy systems that supply the fuel needed depending on the intensity and duration of the exercise:

  • The ATP-CP (adenosine triphosphate - creatine phosphate) is an energy pathway that supplies about 8 to 10 seconds of energy for short extremely high running bursts such as a 100 meter sprint. This anaerobic energy pathway doesn't need any oxygen to create the ATP. First all the ATP is stored in the muscle is used up after that the creatine phosphate is used to re-synthesize ATP until there is no more CP. After this process the body will change its energy supply to either aerobic or anaerobic metabolism (energy from glycogen) to produce ATP to fuel exercise.
  • The anaerobic glycolysis energy pathway produces ATP from carbohydrates only. This energy pathway produces lactic acid as a by product. Anaerobic glycolysis doesn't require oxygen for the (partial) breakdown of glucose. For short high- intensity running periods which last no longer than several minutes before the lactic acid build-up reaches a threshold know as the lactate threshold this pathway is employed. After reaching the lactate threshold, burning muscle pain and fatigue make it difficult to continue running at the same intensity.
  • Aerobic metabolism is the pathway which provides energy for long distance running. Aerobic metabolism converts carbohydrates, fats, and protein to ATM using oxygen. Relying on the circulatory system to supply oxygen to the working muscles before ATP can be created, this pathway is slower than anaerobic energy systems. Longer endurance runs can be maintained at a 50 to 70 percent of maximum heart rate.

While running for more than 5 minutes you will move through these metabolic pathways. In the first stages when you start running, ATP is produced by anaerobic metabolism until breathing and heart rate increase. With increased breathing and heart rate the endurance runners' body switches to aerobic metabolism as more oxygen becomes available. Your running can then continue until you reach the lactic threshold. At this stage running intensity needs to be reduced to lower lactic acid levels.

Nutrients fuel the energy systems when converted to ATP depending on the duration and intensity of the run. Of all nutrients it is carbohydrate which is the main energy provider for moderate to high intensity runs. Fat is a great energy provider for low intensity runs like LSD or a recovery run. Humans have enough fat stored to continue running at an intensity below 50 percent of their maximum heart rate for hours and even days provided there is a steady supply of oxygen for fat metabolism to occur.

When your body has warmed up during running, carbohydrate metabolism takes over from fat metabolism. Using carbohydrate for fuel is more efficient than fat but has limited availability. Stored carbohydrate reserves will be able to provide energy for moderate to high intensity runs for about 2 hours. When the stored carbohydrates are used up glycogen depletion occurs and unless they are not replaced runners may hit the wall or bonk. To avoid this runners need to refuel during their run or race. Especially during a full marathon it is important to eat easily digestible carbohydrates if you are running for more than two hours (no one has run a marathon under two hours yet). Failing to consume carbohydrates will force you to reduce your running intensity while shifting back to fat metabolism.

When running intensity increases, there is a dramatic drop off in carbohydrate metabolism efficiency due to a lack of oxygen and anaerobic metabolism takes over.

carbohydrates can produce about twenty times more energy (ATP) per gram if it is metabolized in the presence of adequate oxygen than when generated in the oxygen-starved, anaerobic environment that occurs during intense running or sprinting.

With appropriate training, these energy systems adapt and become more efficient and allow greater running duration at higher intensity.


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