The 3 energy pathways

Not much here on diabetes particularly, but just a background on energy pathways used during exercise which can be explored further in other topics.

Adenosine Triphosphate (ATP) is the immediate source of chemical energy for muscle contraction. Very little ATP is stored in the muscle cells. It can only power muscle contractions for a few seconds. It needs to constantly be replaced.

When your body uses ATP, it loses a phosphate molecule, and it becomes Adenosine Diphosphate (ADP). In order for it to be able to keep fueling cellular energy, ADP has to be restored back to ATP.

ATP loses a phosphate molecule, the energy that was stored in the bond between the phosphates is then used for muscular contraction. For further muscular contraction to be able to happen, the ADP must be converted back to ATP (the phosphate must be “put back on”). This also requires energy.

There are 3 ways the body has of replacing the phosphate, which are the 3 energy systems or energy pathways. Which of the 3 ways our body will use for the ATP to be rebuilt from ADP depends on the type of activity you are performing, the intensity, and the duration. But all 3 systems work by causing increased production of ATP, which is the only direct source of energy your muscles can use.

1)The first pathway is called the ATP-CP system, or the phosphagen system. It uses the breakdown of Creatine Phosphate (also called phosphocreatine) to obtain the phosphate molecule to use for ATP production. Creatine Phosphate (CP) is a chemical compound stored in the muscles. It aids in the re-manufacture of ATP. (ATP allows muscle contraction and results in ADP, which when coupled with CP regenerates ATP). But CP is in limited supply. So this pathway can only be used for short durations. The ATP-CP system is the quickest way to resynthesize ATP. No carbohydrate or fat is used in this process. The regeneration of ATP comes solely from stored CP. No oxygen is needed for this system. The ATP-CP system is the primary energy system used for all-out exercise that can last up to about 10 seconds, such as a 100 meter dash. Since there is a limited amount of stored CP and ATP in your skeletal muscles, fatigue happens very quickly with this system.

This type of energy system will not lower your blood sugar, because glucose is not used in this process.

The 2nd and 3rd energy pathways both use the metabolic pathway of glycolysis - the break-down of glucose molecules. Glycolysis has two possible routes:

Anaerobic glycolysis, the 2nd pathway, the lactic acid system, which happens without oxygen, forms 2 ATP molecules from each glucose molecule, and it also creates lactic acid and oxygen debt (out-of-breath after your activity).
Aerobic glycolysis, which gives us the 3rd pathway, aerobic cellular respiration, and forms 36 ATP molecules per glucose molecule.

2)The 2nd pathway, the lactic acid system, is used when the first system can no longer supply the energy needed. It is an anaerobic energy system (it does not use oxygen). It uses glucose molecules, a series of anaerobic chemical reactions (which we don’t want to get into here!), and ends up producing several things, including ATP molecules. The glucose molecules are used from available glycogen and blood glucose. In this system, ATP is made from the breakdown of glucose to pyruvic acid in the muscle cells. This process produces 2 ATP molecules for each glucose molecule that is metabolized.

This pathway happens much quicker than aerobic cellular respiration (the 3rd pathway, which I will discuss next) but not as fast as the first pathway (ATP-CP system).

Two Myths: A very common belief is that lactic acid or lactate causes muscle stiffness or soreness after an event, or the muscle pain or burn during an event. Neither of these is true. I can address these another day.

3)The third pathway is aerobic cellular respiration. It is the predominate energy source for longer duration activities. It is also the energy system used during rest. The fuel used for these activities is a combination of fat and carbohydrate. As the intensity increases, your body uses more carbohydrate than fat. As you approach maximum effort, your body will move toward 100% carbohydrate. Your muscle glycogen (the stored form of carbohydrate your muscles have) provides much more than your blood glucose. If you are in a glycogen depleted state, either from having used it all during long-duration exercise, or from being on a low-carbohydrate diet, your body will need more glucose from your blood.

During less intense activities, enough oxygen can be supplied to the cells. This allows for glucose to be broken down completely, hence avoiding the build-up of lactic acid. Although the aerobic cellular respiration system uses the same chemical reactions used in the 2nd pathway, since there is oxygen present, the process follows a different cycle (again, don’t want to get into the chemistry!). The result is more ATP molecules per glucose molecule. 36 ATP molecules per glucose molecule, instead of the 2 from the lactic acid system!

The aerobic system also relies on some fatty acids for energy production. More fat is used in lower intensity, but at higher levels of exercise intensity, fatty acids cannot be utilized quickly enough to supply the phosphates needed to create ATP. Only muscle glycogen can meet the need.

This system - while not as fast - gives much more fuel for longer duration events.

This opens the door to further discussions on energy systems and fueling them…

End of wiki ---------- comments start here


@Eric, what a GREAT write-up!

You ar a marathon runner. From the point of view of energy pathways, what happens to you when your body runs out of glycogen during a marathon?

isn’t that when you hit the wall?

The feeling is that you absolutely cannot keep going at the pace you want. Your legs just don’t move well, you have no energy to keep going, you have to slow down. It is like trying to run through quicksand. So hard to describe. Your body just does not respond.

Yes that is the common phrase that is used.