I've never seen a single dietician, scientist or doctor recommend as much as 1000 mcg B12 supplementation per day on a regular basis (with one exception: a dietician who also happened to have an online store selling bottles of 1000 mcg B12 supplements). Such high amounts are only needed in special situations - like when someone has an acute B12 deficiency, or have an absorption problem meaning that the only way to absorb B12 orally is through so called 'passive diffusion' - which has a very low absorption rate, meaning that you need a very high B12 intake to achieve normal B12 absorption.
Meat eaters often consume more B12 than they need, and just to illustrate how much 1000 mcg B12 actually is:
In order to consume 1000 mcg B12 from sources that are common for non-vegans, one would have to eat eg.:
Pork: 8 lb (3,8 kg)
Ham: 525 lb (238,1 kg)
Mutton: 50 lb (22,5 kg)
Sardines: 25 lb (11,2 kg)
Shrimps: 118 lb (53,5 kg)
Cow’s milk (not boiled): 170 lb (76,9 kg), or 200-250 glasses of milk
Beef: 52 lb/23,6 kg
.... or hundreds (if not thousands) of chicken eggs.
There are a few more aspects to consider here.
"The bioavailability of physiological levels of vitamin B12 from food is
therefore limited by the capacity of the IF-mediated mechanism to ~1.5-2 µg/meal. This mechanism takes a few hours to recover and can then mediate the absorption of a similar amount as a subsequent event. Link) (Also discussed here.) So - whether someone gets B12 from food or from supplements, they won't see any linear increase in B12 absorption from consuming more B12. Eating twice as much 'something' which contain B12 does not mean getting twice as much B12. According to the food.gov.uk source, absorption per meal flattens out at around 2 mcg. This means that the few meat eaters who consume, say, 3-4 times as much B12 from meat etc., as the average omnivore don't absorb 3-4 times as much B12.
So - if an omnivore spreads his need for 1000 mcg B12 from his diet over multiple meals, he will absorb more than if he consumes all this food in one meal. According to the UK government source, he may get as much as 2 mcg pr meal, or eg. 8 mcg/day if he for instance eats 500 eggs for breakfast, 2 lb pork for lunch, 12,5 lb mutton for dinner and 29,5 lb shrimps just before going to bed.
But only circa 30% of the B12 he got from his 500 eggs for breakfast is absorbed, due to the known B12 absorption limitation in eggs. Too bad. He needs to set the alarm clock to wake him up in the middle of the night (he needs a few hours between the meals to recover his ability to absorb B12, right...), and have, say, 7.5 lb sardines.
Even if he actually would be able to consume 1000 mcg B12 in a day, he could absorb only circa 8 mcg, following the above suggested diet. But it is generally thought that 5-30% of the B12 found in the omnivorous diet are inactive molecules. An average of 5 and 30 is 17.5%. It is also often mentioned that those inactive B12 analogues may block the absorption of real, active B12 (this is a huge topic, but let's keep it simple for now....). So if he consumes 8 mcg B12, circa 1.4 mcg of these 8 mcg are probably inactive B12 analogues. That's 6.6 mcg left. But the 1.4 mcg inactive B12 analogues may also, in worst case, block for 1.4 mcg of the active B12. That's 5.2 mcg active B12 left.
1000 mcg B12 is a lot of B12. But people with B12 deficiency do need a lot of B12. However, it's often easier to absorb B12 from supplements than from food (this has to do with how B12 in food needs to be 'released' from it's source in order to be absorbed), so 1000 mcg B12 from a supplement should be more efficient that living on the shrimp/sardines/pork/eggs/mutton diet, and clearly be more enjoyable.
One well known theory is that when consuming more than circa 10 mcg in one meal/tablet, we can absorb only circa 0.5% of the B12 we consume. That would mean around 5 mcg per 1000 mcg table, or circa 10 mcg B12 from a 2000mcg tablet.
Here's what Vegan Society says, on this page:
And on this page:Vitamin B12: Readily available in fortified foods such as yeast extract, soya milk, breakfast cereal and margarine. Alternatively a supplement can be provided. Daily amount: 3 micrograms.
These numbers are based on the theory that we need more than what WHO, most governments etc think we do.Most vegans consume enough B12 to avoid anaemia and nervous system damage, but many do not get enough to minimise potential risk of heart disease or pregnancy complications.
To get the full benefit of a vegan diet, vegans should do one of the following:
•eat fortified foods two or three times a day to get at least three micrograms (μg or mcg) of B12 a day or
•take one B12 supplement daily providing at least 10 micrograms or
•take a weekly B12 supplement providing at least 2000 micrograms.
If relying on fortified foods check the labels carefully to make sure you are getting enough B12. For example, if a fortified plant milk contains 1 microgram of B12 per serving then consuming three servings a day will provide adequate vitamin B12. Others may find the use of B12 supplements more convenient and economical.
But whether we need 2.4, 6 or 10 mcg per day.... there are nobody out there who claims to have scientific evidence for saying that humans - vegan or not - need to consume 1000 mcg B12 daily.
The unfortunate existence of the many 'B12 killers' out there may suggest that most humans need more B12 than they think they do, but until there's at least some scientific evidence that we need a lot more B12 than current, general recommendations, it's IMHO not a good idea to start to take 150-200 times the recommended B12 amounts on a daily basis. The idea that vegans regularly should need to consume B12 amounts equivalent to eg. 53 kgs shrimps or 22.5 kgs mutton sticks out as a rather bizarre idea to me.
If you come across anyone who claims that we need this much B12 daily, please ask why - and ask where they have this information from, and - most of all - ask why they think we need to consume a lot more B12 than it's possible to get from omnivorous food.