Do we have any well-run research which has established the relative absorption of the various forms of B12 when taken orally?
methyl-
hydroxo-
cyano-
adenosyl-
(I'm hoping that we can avoid the sublingual vs. swallowed debates. That is, I'm hoping that any differences by form are independent of precise route of absorption. But accept that might not be possible.)
rummaged but did not find a whole lot. These two have some info at least.
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Increase in circulating holotranscobalamin after oral administration of cyanocobalamin or hydroxocobalamin in healthy adults with low and normal cobalamin status
Conclusions: Administration of cyanocobalamin resulted in a more than twofold increase in holoTC in comparison with hydroxocobalamin. The absorptive capacity was reached only by doses above 3 µg cobalamin. Our results underscore the importance of using the same form of cobalamin when comparing uptake under different conditions.
pubmed.ncbi.nlm.nih.gov/290...
Absorption and retention of free and milk protein-bound cyano- and hydroxocobalamins. An experimental study in rats
Conclusions: Cbl added to milk (spiked with rbTC) has high bioavailability matching that of free Cbl. OHCbl and CNCbl are absorbed equally well, but much more OHCbl accumulated in the liver. Benefits of oral supplementation with OHCbl compared to CNCbl should be investigated.
Scientist, not medic. Often in error, but never in doubt.
I think you're on a hiding to nothing on this one, because measurement of absorption nowadays would be done using 'surrogate tests' such as changes in transcobalamin saturation, and any such tests would require to be done in 'before' and 'after' modes, and thereby multiply up the errors.
I can think of a valid scientific method for doing it, although it would require endless issues; here goes:
You would need a supply of radioactive B12 doses, in the four forms. Then, once the consented subjects have been rendered B12-replete, you would 'simply' administer each form in turn, with a couple of weeks between them, and collect 24 hour urine samples following the administration. Count the radioactivity in the urine and express as a %age of the dose administered.
The technical, scientific and ethical problems are probably insurmountable. So, it could be done, but I'm sure it won't be. It would cost a fortune, and at the end of it, probably have very little utility.
Thanks for giving me the opportunity to contribute my two penn'orth.
The Schilling test was the standard when I was diagnosed in 1985. Sadly it's no longer used due to long half life of cobalt isotopes. However, it was totally accurate for absorption tests. Intrinsic factor and similar modern tests are not reliable enough, as I found after my injections were stopped in 2020.
When we had the Schilling Tests, they were as good as it got, although there were 'issues', to say the least. But it worked.
DiCoPac did at least cut it down to one visit and one 24 hr urine, but in the days of 'Mad Cow Disease' as well as radioisotopes, it's no wonder it went away.
One further possibility would be to obtain a supply of stable Cobalt isotope-labelled B12 in the four forms, do the test as I described, and then use mass spectrometry rather than radioactivity to get to the result. [No, that's not going to happen either, is it?]
The Cochrane review done in 2018 says the evidence is very poor:
cochranelibrary.com/cdsr/do...
There are many other (typically gastro) conditions and drugs (metformin and PPI) that lead to Vitamin B12 deficiency.
I've got Bile Acid Malabsorption (BAM) which is one of the gastro conditions that leads to low levels of Vit B12; my levels were 201 ng/L in a range from 197 - 711 ng/L. The main symptom of BAM is diarrhoea; apparently bile acid seeps in the colon and the body tries to flush it out, resulting in what is called Bile Acid Diarrhoea (BAD).
I started on a daily oral cyanocobalamin tablet (100mcg) and did a test after a year and my levels had gone up to 319 ng/L. I switched to methylcobalamin (1mg) and within 6 months my levels went up considerably to 503 ng/L.
I imagine that it was having 10x the dosage that made the difference, rather than the switch from cyanocobalamin to methylcobalamin tablets.
My dizziness and balance problems disappeared and curiously the episodes of BAD are far less frequent.
Over the last few years, I've had the weirdest conversations with doctors where they say that 201 ng/L is normal and I say it's low 
It was very much because of the different experiences of the various B12 vitamers that I asked. I was mentally trying think through how different absorption and different effects once absorbed could be separated out from each other.
I'm so glad I read this. I have AMAG with PA. You describe some of my symptoms. My doctor thinks I have bile reflux and prescribed sucralfate after each meal and before bedtime to coat my stomach. I haven't been compliant other than to take it after my supper. It does calm my stomach but I don't like to take meds in general.. and this one has aluminum in it. I will talk to doc about your experiences.
People who have PA typically have low stomach acid. BAM is typically associated with gallbladder removal.
This discussion about PA and low stomach acid might be helpful:
healthunlocked.com/pasoc/po...
Thank you, I will read it through. As an aside, I had my gallbladder removed about 15 years ago. Would something like this happen 13 years after the removal? I've been dealing with this about two and a half years now.
there’s a patient support group for Bile Acid Diarrhoea here in the UK:
Their forum uses Facebook; there are quite a few people from the USA in the group.
In the UK BAM is diagnosed using a SehCAT scan; you swallow a radio labelled bile acid capsule and have a first scan which shows where the bile acid is in the digestive system; a week later you have another scan to see how much of the radio labelled bile acid is still in the digestive system. The less bile acid, the more severe your BAM is.
I don’t think the SehCAT scan is available in the USA; instead a bile acid stool test and a blood test called the 7aC4 blood test is used as you will see in this Mayo Clinic article:
mayoclinic.org/medical-prof...
in answer to your question, BAM is common in people who have had their gallbladder removed; you could ask on the Facebook forum how long it takes for BAM to start. BAM is associated with fat in the liver (hepatic steatosis) and pancreas; I imagine it takes time for this to build up. People with BAM also have high triglycerides in their blood.
Thank you!!
Does anyone know the absorption rate of b12 injections?
PA and oral B12 supplements
Oral B12
