I discovered there are 2 types of vitamin B12 being sold, cyanocobalamin and methylcobalamin. Cyanocobalamin needs to be converted to methylcobalamin in the body in order for it to be utilized, so it is better to take B12 in the form of methylcobalamin to better ensure the body gets what it needs. I read that the liver only converts 1% of ingested cyanocobalamin into methylcobalamin.
The recommended daily allowance(RDA) for B12 may be a small amount like 4mcg, but one needs to ensure that the body would get at least that amount after the process of absorption, so one should buy B12 supplements that should have much more than the RDA amount of 4mcg. The RDA for various vitamins is for preventing deficiency, to ensure good health, one has to consume more than the RDA.
Research has found that the small intestine absorbs B12 in varying percentages due to the limitations of the Intrinsic Factor. If B12 is taken in small amounts such as 5-50mcgs, then the absorption amount is limited to about 1.5 mcgs, falling below the RDA, so it has to be taken at least twice during the day to fulfill the RDA, while giving the Intrinsic Factor a few hours' time to recover. But if B12 is taken in a high dosage such as 1000 mcg each time, the vitamin will be absorbed in 10-15 mcgs, fulfilling the RDA in one dosage.
There are some people on the internet saying that one does not need to take too much B12, and that B12 that comes with over 100s of mcgs are due to companies wanting to make money. But they are actually stupid to not realize that B12 also needs to be undergo the process of absorption, and not all the amounts would eventually get used by the body(I think this would also apply to other vitamins and minerals).
In dietary supplements, vitamin B12 is usually present as cyanocobalamin , a form that the body readily converts to the active forms methylcobalamin and 5-deoxyadenosylcobalamin. Dietary supplements can also contain methylcobalamin and other forms of vitamin B12.
Existing evidence does not suggest any differences among forms with respect to absorption or bioavailability. However the body’s ability to absorb vitamin B12 from dietary supplements is largely limited by the capacity of intrinsic factor. For example, only about 10 mcg of a 500 mcg oral supplement is actually absorbed in healthy people ."
"Some people—particularly older adults, those with pernicious anemia, and those with reduced levels of stomach acidity (achlorhydria) or intestinal disorders—have difficulty absorbing vitamin B12 from food and, in some cases, oral supplements [22,23]. As a result, vitamin B12 deficiency is common, affecting between 1.5% and 15% of the general population [24,25]. In many of these cases, the cause of the vitamin B12 deficiency is unknown"
Perhaps the ignorance about the proper intake amount of B12 has led to severe brain and nervous system damage in lots of vegans. And those crazy and emotionally extreme animal rights activists could be one of those suffering from extreme B12 deficiency.
More links to information about vitamin B12: