How the human body uses fuel for energy

The human body always has a need for energy. This is required to sustain life (organ function, breathing, brain activity, etc), perform exercises and for recovery. When you think of the term metabolism, this all refers to the behind the scenes where the chemical compounds are converted by the body. There are three (3) main sources of fuel that the body can use (Fat, Protein and Carbohydrates). All food ingested will be digested and broken down in to substrates. I will keep this really high level but break it down to make it easy to understand with regards to the NASM Certification exam.

There are three metabolic pathways in which cells can generate ATP (used for energy). Two of these pathways require no oxygen and the third does:

  1. The ATP-PC system (Anaerobic system) – When you perform an action the body will use ATP immediately for energy. When muscles work they first use the ATP-PC system which can become depleted in 10 to 15 seconds. ATP is a very limited resource in the body and it is for VERY short-term use. With regards to the gym think about when you first pick up a heavy weight and possibly do your first few repetitions or just getting on the treadmill and starting to walk. You can regenerate ATP through rest. The approximate time frames are
  • 20 to 30 seconds will allow approximately 50% recovery of ATP and PC.
  • 40 seconds will allow approximately 75% recovery of ATP and PC.
  • 60 seconds will allow approximately 85 to 90% recovery of ATP and PC.
  • 3 minutes will allow approximately 100% recovery of ATP and PC.

2. The Oxidative System – (Anaerobic and Aerobic system) – This is the last system to be used. This system is used when you are performing long duration exercises like walking on a treadmill or running on the track. You have already completed getting on the treadmill and started your walking. This is most complex of the three energy systems because the body needs oxygen to be used along with digested food (substrates – fat, protein and carbohydrates) to help produce more ATP. Think about when you are pushing hard during cardio. You start to breathe heavier which raises your heart rate (both items are linear). Your body starts to use broken down food. Throughout this process your body will generate heat. This is where you start to sweat, and this is a by-product of your body trying to cool itself as well as producing energy (like someone shoveling coal on a train). The more coal you shovel in the furnace the more power you get).

3. The Glycolytic System – (Anaerobic system) – This is the second system to be used after ATP has been depleted. After that initial first 10 – 15 seconds the body needs to find another energy source. Now the body will start to break down glucose or glycogen (which is stored in the muscle). Before glucose or glycogen can even be used to generate energy, it has to be converted to a something called Glucose-6-phosphate. Important thing here is that to convert glucose to Glucose_6-phosphate you still need to use ATP, glycogen does not. With regards to the exercise this process is used which will lasts up to the next 30 – 50 seconds.


There are three oxidative or aerobic systems include:

  1. Aerobic glycolysis – Pyruvic acid (without oxygen the end product is lactic acid), but in the presence of oxygen, pyruvic acid is converted into an important molecule in metabolism called acetyl coenzyme A (acetyl CoA).  This end result determines the outcome of building up of pyruvate or lactic acid). Lactic acid is when you start to feel that muscle burn.

See the source image

  1. The Krebs cycle – Acetyl CoA is an important molecule because it contributes substrates for use in the second process of oxidative production of ATP, called the The Krebs cycle

  1. The electron transport chain (ETC) is a process where chemicals are converted in the bodies cells and used for metabolic functions.


Here is a very cool video to watch on Aerobic Cellular Respiration, Glycolysis, Prep Steps


  2. Wikipedia
  3. Bogobiology
  4. NASM CPT – Fundamentals of Human Movement Science 
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