The author of this article [Solomon, 2005: (http://bloodjournal.hematologylibrary.org/cgi/reprint/105/3/978.pdf) (http://www.ncbi.nlm.nih.gov/pubmed/15466926)] discusses the way rigid standards for vitamin B12 (cobalamin) deficiency should not be used in the context of blood tests, given that many people have "normal" serum cobalamin or methylmalonic acid levels and nonetheless have severe neuropathy, for example, or other conditions that respond to cobalamin repletion. The author also discusses the advantages of methylcobalamin over cyanocobalamin, although the major advantage is that methylcobalamin is "not cyanocobalamin." Anything (any of the two commonly used forms of vitamin B12 that are not cyanocobalamin, namely hydroxocobalamin and methylcobalamin) is better than cyanocobalamin, in terms of transport into cells or mitochondria, in terms of the absence of an added cyanide burden from the cyanide moiety on cyanocobalamin, etc.
But the main point of that article is that a normal serum cobalamin level will not necessarily guarantee that the functional effects of cobalamin are "normal" in different tissues, and the use of "rigid" standards for deficiency vs. sufficiency would essentially mean that large numbers of people would go untreated for conditions, such as severe peripheral neuropathy, that would otherwise be ameliorated by cobalamin repletion. I collected some more dose-response "data" on the serum B12 responses to different doses of methylcobalamin, in those articles discussing the use of methylcobalamin to treat sleep disorders, and the serum B12 levels in response to dosages of methylcobalamin of 3 mg/d are really low and quite variable between individuals. I'll post the values from the articles, but I want to try to collect data from lots of different articles. It's really hard to come by that type of information, because the dosages are chosen arbitrarily. Many researchers refer to 5-mg dosages of oral forms of vitamin B12 as "massive," and those types of dosages may not even elevate a person's serum B12 much above the normal range. For example, one person might get a five-fold increase in serum B12 in response to 3 mg/d, and another person might just elevate his or her level to the upper limit of the normal range. I think there's a lot of room for different dosages, but using higher dosages of forms of vitamin B12 tends to not do much in the absence of concomitant supplementation with adequate dosages of L-methylfolate or L-leucovorin or folic acid. A certain percentage of a dose is absorbed by an intrinsic-factor-independent process (this is well-known now), probably by passive diffusion (paracellular diffusion) in the upper intestine. But there's probably interindividual variation in the extent to which this occurs, and the percentage that's absorbed may not remain constant over a range of different dosages. One really should, ideally, get serum B12 tests and work with one's doctor to adjust the dosage, etc.
But the serum B12 levels that people get from injectable forms of vitamin B12, from their doctors, tend to be much higher than most of these levels that one sees, in the literature, even from 3-6 mg/d of methylcobalamin. The normal range of serum B12 values does not really reflect anything except the types of values one sees in a population that gets vitamin B12 mainly from food. One can only get a certain amount of vitamin B12 from food, but that doesn't mean that the normal range defines a range of serum B12 values that are "physiological" or physiologically normal. The most common error in reasoning I see in journal articles is the idea that population norms reflect physiological norms, and it's just not the case. I've gone through the estimates of the intracellular cobalamin levels in relation to the Km's for the binding of cobalamin-derived coenzymes to their respective apoenzymes, and those data argue against the idea that the normal range of serum B12 levels represents some vague concept of "physiological" saturation or who-knows-what.
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