In the past i had trialled a number of methylation protocols which are often recommended as an alternative treatment for difficult to treat conditions such as chronic fatigue syndrome, fibromyalgia etc.
Like many other individuals with CFS who try the methylation protocols, i would always encounter the dreaded "low potassium methylation" crisis or hell as i like to call it.
Symptoms would usually involve severe heart palpitations, muscle cramping, muscle weakness, being inable to stand for more than a few seconds, intense anxiety, personality changes and just an all round feeling like i was going to die.
Now this has happened to me a number of times when trying the methyl-cobalamin form of Vitamin B12 or even just standard cyanocobalamin. I would get a brief glimpse of energy and it would be followed by the horrible "low potassium" crisis, that could often last months or became my permanent state of health for a long period.
The consensus from many of the methylation experts is that these symptoms are caused by low potassium and the theory seems to be that the cells are demanding more uptake of potassium as the methylation process kicks in and new cells are created.
However i would like to offer a slightly different hypothesis as to what i believe is happening.
Whilst many of the above symptoms can be attributed to low potassium levels and hypokalemia, i believe they are actually induced by having low magnesium levels.
My theory is that when the methylation process kicks in, it demands significantly more energy(ATP) than the body is capable of providing. Energy metabolism tends to already be impaired in individuals with CFS as mitochondrial dysfunction is a hallmark characteristic of true chronic fatigue syndrome.
What is interesting anecdotally is that the individuals who tend to be
most affected by the low potassium/magnesium symptoms are those with
chronic fatigue syndrome.
Individuals with CFS/ME tend to already have low intracellular magnesium stores and when we start to upregulate other processes in the body such as methylation, there simply isn't enough magnesium as a co-factor to produce adequate ATP(energy).
Hypokalemia(low potassium) has been commonly associated with magnesium deficiency.
"Magnesium deficiency is frequently associated with hypokalemia.
Concomitant magnesium deficiency aggravates hypokalemia and
renders it refractory to treatment by potassium.
Herein is reviewed literature suggesting that magnesium deficiency
potassium wasting by increasing distal potassium
A decrease in intracellular magnesium, caused by magnesium
releases the magnesium-mediated inhibition of ROMK
channels and increases potassium secretion. Magnesium deficiency alone,
however, does not necessarily cause hypokalemia. An
increase in distal sodium delivery or elevated aldosterone levels may
be required for exacerbating potassium wasting in
magnesium deficiency." 
After reading others experiences with the "low potassium hell" on forums such as Phoenix Rising, when i first developed these symptoms on the methylation protocol, obviously my first thought was to supplement with some extra potassium, which did mildly reduce the symptoms, but not completely and as others have probably observed, you can keep supplying extra potassium and it do very little to stabilize these symptoms.
I have read of other individuals experiences of having to consume around 1-3g+ of extra potassium intake from supplements daily to try minimize these symptoms and even then they complain that their body is just eating the extra potassium up and it doing very little to help. What is interesting is that the study above gives a reason why this happens and how it relates to low magnesium levels:
"Concomitant magnesium deficiency aggravates hypokalemia and
renders it refractory to treatment by potassium."
This is a good explanation of why these individuals can consume very high intake of potassium from supplements and it still not stabalize the hypokalemia symptoms. Like i say i believe high potassium supplementation to be the wrong way to treat this issue and it is the low magnesium which actually needs addressed here.
Infact i believe that monitoring magnesium should probably be a fundamental basic part of any methylation protocol, especially for those with CFS/ME, as they tend to already have depleted intracellular stores of magnesium and seem to be especially prone to this "low potassium/magnesium" crisis.
Anecdotally i found that keeping ontop of my magnesium with extra supplementation was enough to not require any potassium supplementation. Simply topping up my magnesium levels when i felt symptoms such as cramping or palpitations was enough to keep my potassium levels stable.
Another compounding problem with the high potassium supplement recommmendation is that many or "most" individuals with chronic fatigue syndrome/ME tend to also suffer from hypothalamic-pituitary-adrenal axis dysregulation aka adrenal fatigue, hypocortisolism and adrenal insufficiency.
As such high potassium intake is actually contraindicated for individuals with severe adrenal fatigue as they tend to already have disturbed sodium/potassium ratio due to sub-optimal adrenal gland function. Personally whilst the extra potassium mildly reduced some of the symptoms, it actually "shut off" my adrenals and caused further problems in that area making me more "adrenal fatigued" and debilitated due to the even lower levels of cortisol.
Monitoring magnesium levels as described above, i believe to be the real key factor in minimizing these very serious and distressing symptoms.