Article Summary: This is part three of a debate on long run/marathon fueling strategies. In part one, Mark Iocchelli put forward the case for a minimalist approach to fueling (less is more). In part two, Steve Runner takes the opposite view – that fueling on the run is the advisable strategy. In part three, Lee Miller discusses how the body metabolizes food for energy.
Over the last week CRN hosted a lively debate over whether or not to supplement during long races or not. This discussion raised a number of questions as to how the body metabolizes food for energy. This article hopes to inform the audience about the basic concepts of energy production and use in the human body.
The Energy Currency
Adenosine triphosphate (ATP). This word may conjure up nightmares of high school biology class, but it is the substance used for energy in all cells. It is by breaking down ATP that energy is released to power all forms of biological work which includes muscle contraction. The energy in the food we eat is extracted by various biochemical reactions to rebuild more ATP.
The problem with ATP is that the body doesn’t store very much, only approximately 3 ounces. This allows a full out sprint capability for only a few seconds. So where does the energy to run longer distances come from?
The Energy Supply Systems
There are 3 primary energy systems in the body:
- Phosphocreatine- Adenosine triphosphate (PC-ATP)
- Glycogen-Lactic acid
Let’s examine each one briefly:
- PC-ATP: As mentioned above ATP provides energy for short intense bursts of activity. The upper time limit for depletion of this system is about 8 to 15 seconds. So 100 meter sprints, weight lifting, high jump, tennis serves are types of activity powered by this system. The storage of this energy supply is in the muscles, no oxygen is required to run this system and no lactic acid is produced as a by-product. We also call this type of energy production anaerobic.
- Glycogen-Lactic acid: This system provides energy quickly but not instantly. Depletion of this system occurs after approximately 30-40 seconds. Glycogen, which is stored in the muscles and liver, is the primary fuel source. Brief, moderate exertions, surges or racing the 400-800 meter distances tend to activate this system. A significant amount of lactic acid is formed as a by-product of this energy system and no oxygen is required to run this system. This system is also known as anaerobic glycolysis.
- Aerobic: This system does not provide energy very quickly. Depletion of this system takes quite some time and is related to glycogen depletion or what is called “bonking” or “hitting the wall”. The fuel for this system consists of carbohydrates, fats and proteins that combine with oxygen to produce enormous amounts of energy. Endurance events rely almost exclusively on this system.
So now that we have a basic understanding of the energy systems in the body, we can look more closely at how we can make the system work for us.
Stay tuned for the next article on the aerobic system where we look at how this system works and how we can try to optimize it.