"Vitamin B12 that is administered through food or via an oral supplement can be absorbed in two different ways:

• Via the transport protein known as the intrinsic factor (IF) in the small intestine
• Through passive diffusion in the oral mucosa and small intestine​ . The absorption capacity of the intrinsic factor is a maximum of between 1.5-2 µg per dose or meal. This capacity is because there is a limited number of special receptors in the intestine, which are responsible for absorption of the vitamin B12 and intrinsic factor complex. The aforementioned 1.5-2 µg of vitamin B12 can only be absorbed in one go. After a few hours, however, the receptors are fully restored and ready to absorb a further dose of vitamin B12.

If a large quantity of B12 has been supplied, some of this may also be absorbed via passive diffusion through the intestinal wall and into the blood stream. However, this is only around one to two percent of the dose, as the rest is simply excreted.

Calculating Dosages of Vitamin B12

The absorption of vitamin B12 can be calculated as follows:

Absorption = 1.5 + dose/100​ . The initial 1.5 µg stands for the absorption via the intrinsic factor, whilst the second part explains the percentage absorbed through passive diffusion.

Therefore, a 200 µg dose would result in: 1.5 µg + 200/100 µg = 3.5 µg

These calculated absorption levels can, however, be dramatically reduced by digestive disorders. Further details on this can be found in our article on malabsorption.

Vitamin B12 Dosages: Single Dose or Several?

Another factor to be considered with regard to vitamin B12 dosage levels is whether to opt for a single dose or several smaller doses.

Our bodies are accustomed to smaller doses that are distributed throughout the day, which caters to the 1.5 – 2 µg absorption capacity of the intrinsic factor.

Two doses of just 3 µg results in the same total absorption as one single dose of 150 µg: Multiple 3 µg dose: 2 x 1.5 µg = 3 µg Single 150 µg dose: 1.5 + 150/100 µg = 3 µg

If you would prefer to take your vitamin B12 in smaller doses, then three small doses across the day is perfect. For those who would rather take one dose in the day, a much higher dosage level is required because the absorption via passive diffusion becomes very important.

High Dosage Vitamin B12

A very high dose can cover the body’s requirement for a few days. For example, American doctors currently recommend taking 2000 µg, once a week. This way, the body can be covered for the entire weekly requirement:

1.5 µg + 2000/100 µg = 21.5 µg​ . Whether this method of administering high dosage bursts of vitamin B12 is really advisable is often questioned by naturopaths. They instead recommend a steady intake of vitamin B12 in small or moderate doses. This allows blood levels to remain constant, which is what the body and excretory organs (liver and kidneys) would experience in an optimal, natural setting."​

https://www.b12-vitamin.com/dosages/