There was a thread on here recently about the problem with taking methylcobalamin alone. The Wiki article on methylcobalamin said that one should take adenosylcobalamin as well as methylcobalamin as the latter cannot be converted to the former. Or that one should take cyanocobalamin or hydroxocobalamin as both can be converted to both of the active forms.
The Wiki article refers to a paper as the source of this non-conversion of methyl to adenosyl. However, the abstract didn't explicitly state that the conversion wasn't possible - it just made the statement that one needed both methyl and adenosyl supplements.
I got hold of a copy of the whole paper and found that it doesn't mention the methyl-adenosyl conversion. So I'm a bit doubtful about their conclusion. Actually, I'm not keen on the paper as a whole, especially as there's one obvious error (the penultimate paragraph, where it says "The suggested regimen for the oral route is about 2000 μg of CoA
daily" when talking about oral doses of B12. CoA is not B12.
Thanks. The next step is to get the Wikipedia entry changed to eliminate the phrase "Methylcobalamin on the other hand is not converted into adenosylcobalamin".
Hi, I lifted this information from the web - it talks about B12 combinations....
Deficient in vitamin B12? Trying to decide on a B12 supplement?
Cyanocobalamin, hydroxocobalamin, methylcobalamin, adenosylcobalamin, what form is best? The better question is what combination is best.
With vitamin B12 playing a vital role in the formation of healthy blood cells, energy production, nervous system, cognitive function and homocysteine regulation, it is important to supplement with the right form and combination for bioavailability and absorption.
One of the forms of B12 that is readily available and inexpensive is cyanocobalamin. However, it is synthetic and requires methylation to be utilized and eliminated from the body. The irony is that many people supplement with B12 for the express purpose of supporting methylation.
So what about hydroxocobalamin?
Hydroxocobalamin is a natural form of B12, attached to a hydroxyl group and if your body easily converts hydroxocobalamin to the active forms of B12 then this may be all you need to supplement a B12 deficiency. However, for those who need help in the conversion process or have methylation or neurological dysfunction then methylcobalamin and adenosylcobalamin will likely be required.
The European Journal of Clinical Nutrition concluded that the best way to address a B12 deficiency is the combination of the active forms MeCbl and AdCbl.
Why?
The active coenzyme forms of B12, methylcobalamin and adenosylcobalamin bypass usual mechanisms of absorption that rely on intrinsic factor.
The combination is effective in addressing both the neurological and haematopoietic pathways.
Methylcobalamin is found in the cytosol of cells and interacts with an enzyme called methionine synthase; a critical enzyme involved in DNA synthesis and may be particularly useful for individuals with impaired methylation capacity.
Adenosylcobalamin is the mitochondrial form of the B12 vitamin found in cellular tissues and interacts with an enzyme called methylmalonyl CoA mutase, a metabolic enzyme, and may be useful for those with impaired energy production.
Methylcobalamin and adenosylcobalamin have different functions thus providing a total solution for those with B12 deficiency.
Seeking Health provides this winning combination of methylcobalamin and adenosylcobalamin in the following supplements:
Unfortunately, the information from the web site isn't supported by the journal article they cite - which is the one that I linked to in the original post.
That article doesn't even imply that MeCbl and AdCbl "bypass usual mechanisms of absorption that rely on intrinsic factor." - because that is totally, 100% wrong.
The rest of it is correct, apart from the final conclusion. The article that they say they use as the source for their information actually recommends supplementing with AdCbl and MeCbl - OR - hydroxocobalamin or cyanocobalamin, because both HOCbl and CNCbbl are both converted to the two active forms.
This is a prime example why it can be erroneous to trust articles found on the Web. Sites like 'Seeking Health' exist solely for the purpose of making money. They will tell all sorts of lies and spread all sorts of misinformation to try and sell their 'special' supplements to people who would be served just as well by using much cheaper stuff.
They imply that they're using information from legitimate sources by supplying references that most people can't follow up. I'd be willing to bet that they've only read the abstract.
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Methyl Cobalamin Injections
Methyl or Hydro B12 injections - is it safe to switch between both each month? 
Adenosylcobalamin
cobalamin v methylcobalamin sublinguals
