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Vitamin B12 is stored over a very long time such that vegans, for example, only get any signs or symptoms of B12 deficiency after a span of years, even though they have hardly any sources of the vitamin in their diets.

The dose required daily to prevent disease seems to vary dramatically, or maybe it is very hard to determine accurately.

For example:

  • This site says pregnant women need less than 3 micrograms (µg)/day (where pregnant women need more than non-pregnant women).
  • A friend of mine got her blood values in order by ingesting 7 µg/day.
  • My apothecary person told me 500-1000 µg per day are needed to even start absorbing meaningful quantities by ingestion.
  • My doctor prescribed 200 µg/day for me.

Another factor is that apparently your intestinal bacteria determine the rate of absorption.

So what is the amount of B12 needed per day? Please note that this is not about deficient absorption, where the amount would be above what healthy people need.

  • 1
    Don't be so quick to dismiss deficient absorption. It is a very common ailment, although the full-blown version that puts people in the hospital is now very rare in the United States (unless you are an alcoholic). Also be aware that blood measurements of B-12 are NOT the best way to diagnose deficiency. Sally Stabler's recent review article in the New England Journal discussed this. – Iron Pillow Jul 3 '15 at 7:34
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The estimated daily requirement needed to maintain body stores of B12 varies, an estimate being from 2µg to 5µg, more if stores have been depleted in any way. It is estimated that the average person stores about 1 mg (1000 µg) of B12 in their liver, and other smaller amounts elsewhere. The recommended daily allowance assumes a 50% absorption rate of ingested B12.

This is a good question to discuss the limits of medical science. How is the requirement for B12 determined?

Longitudinal studies are those that follow people over many years (even decades.) Humans are not subjected to longitudinal studies involving, say, B12 because:

  • it would be unethical to withhold from some subjects a vitamin necessary for health and well-being while providing it to others just to get an exact number (for the sake of scientific curiosity)
  • there are ethical issues in involvement of the young (parental consent should not extend to potential harm)
  • if participants are paid volunteers, the study introduces a bias in recruitment (more poor people?) This affects the ability to generalize to the total population, because there may be inherent risks of confounding variables
  • the cost of such a study would be prohibitive (who will collect the data, control the diets, pay for the food, and determine outcomes, etc.)
  • it is impossible to regulate someone's diet for years or decades (one person sneaking out to eat a dozen oysters could ruin the experiment)
  • it would be unethical to control someone's diet for decades (what if someone who signed up for the study later became a vegan on moral grounds? They would either be forced to eat meat or drop out of the study)
  • longitudinal studies suffer from cumulative attrition - people die of unrelated causes, move to another area, decide to drop out for other reasons, etc.
  • (many more problems)

Therefore different study models must be used, which give us less accurate information, such as retrospective studies, animal studies, studies of treatment of pernicious anemia (a result of B12 deficiency), pregnant and lactating vegans, people who have undergone certain bypass procedures, etc. By studying those patients, it can be determined how much B12 is necessary to the first signs of B12 deficiency away (usually apparent in blood).

B12 is a particularly difficult vitamin to pin down because of its complexity, the fact that it is synthesized by intestinal flora, and the various steps involved in its absorption which might be influenced by age and other factors.

Again, the estimated daily requirement of B12 varies from ~ 2µg to 5µg.

Since there are no known adverse effects of excess B12 intake, it's not unreasonable to take more than the minimum if warranted. However, a recommendation of 500 - 1000 µg/day seems quite unnecessary.

Tietz Fundamentals of Clinical Chemistry and Molecular Diagnostics (Carl A. Burtis, David E. Bruns, 2014, p 474

  • B12 intake well in excess of 5 µg/day would be required for persons with pernicious anaemia because their lack of intrinsic factor means they only absorb B12 through passive diffusion (about 1%). Prevalence of pernicious anaemia is low in the general population, though some people will develop it as they move into old age. – Nic Apr 12 '18 at 22:33
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    @Nic - Per the OP: "Please note that this is not about deficient absorption...". They are asking how much B12 a vegan diet should contain to ward off pernicious anemia. If you read (the question and) my answer, I covered the problems in estimating B12, including in the elderly. In addition, please note: "Since there are no known adverse effects of excess B12 intake, it's not unreasonable to take more than the minimum if warranted." The treatment of pernicious anemia is indeed different, including IM injections of B12. – anongoodnurse Apr 13 '18 at 0:57
  • @Nic megaloblastic anemia becomes more common due to B12 malabsorption as the population ages. Is there a significant increase in IF mediated pernicious anaemia as well? – Graham Chiu Apr 13 '18 at 1:00
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Let me address this point mentioned in the question: "My apothecary person told me 500-1000 µg per day are needed to even start absorbing meaningful quantities by ingestion."

This addresses a problem with the way we absorb vitamin B12. As explained here, when vitamin B12 is ingested, certain so-called transport proteins are needed to move it to the bloodstream. Now, at any given time you only have an amount of enzymes capable of transporting about 1.5 micrograms. When you take 5 to 50 micrograms, you saturate the capacity of the transport proteins, and you'll get close to the maximum of 1.5 micrograms of vitamin B12 into your body.

However, a small fraction of the vitamin B12, about 1% will pass through the stomach wall without the help of transport proteins. This means that you can evade the 1.5 micrograms per meal absorption limit by taking huge dosages of the order of many hundreds of micrograms. This is useful for people who are deficient, they'll typically have a problem causing vitamin B12 to not be absorbed using the transport proteins. Even if there is no problem here (e.g. in case of malnutrition) with only 1.5 micrograms per day, a deficiency cannot be corrected in a timely manner. High dose supplements or vitamin B12 injections must then be used.

The 200 micrograms prescribed by your doctor thus amounts to an effective dose of 1.5 micrograms plus 1% of 200 micrograms = 3.5 micrograms of vitamin B12 which is within the range of the RDA.

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