The great fatigue, which is always part of M.E., is believed to be caused by a fault in the way that the body makes energy. This type of fatigue is called 'Post-Exertional Neuroimmune Exhaustion (PENE pen-e)'. PENE is an inability to produce sufficient energy on demand caused by physical problems within the body's systems.
Two body systems are believed to be main contributors to PENE.
System 1: the heart and the body's blood transport (cardiovascular) system
This system also relates to the transportation (movement) of oxygen around the body.
System 2: the mitochondria (which are found in large numbers in many cells of the body). This is why M.E. is sometimes referred to as a mitochondrial disorder.
The miochondria act as energy charging stations within the body. So, inside the mitochondria the biochemical processes of respiration and energy production occur - so creating the body's energy.
In M.E. the mitochondria can dysfunction so that the normal system used within the mitochondria to create the body's energy does not work properly. This can result in cells dying more quickly than normal which also reduces the amount of energy available to the body.
The mitochondria act like battery chargers. Usually they can frequently re-charge and so we have the energy 'on demand' that we need. This re-charging is done through ATP (Adenosine triphosphate). ATP is a high-energy molecule that works within the mitochondria to store the energy. Usually the ATP is able to be reused about 16 times. However, when M.E. is present the ATP molecules can only be reused a few times so the person with M.E. has less available energy than someone who does not have M.E.
When a torch battery is low in charge it only has a little bit of energy and so only a very small light can be made. Likewise, a person with M.E. has less energy than normal. So, the person has to store a lot of energy before, and after, doing activities by taking lots of rest. The more severe the M.E. the greater the mitochondrial dysfunction - resulting in the person needing much more rest than is normal.
If the person does not rest then a relapse occurs. The resting system used by people who have M.E. is called Pacing. If the person does not Pace properly then relapses will be frequent. (Go to M.E. Uncut 'Spotlight on M.E.' page for Pacing tips.)