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