How much B12 does a person need?

An important vitamin for the human metabolism

What actually does B12 do in the human body? The B12 forms methylcobalamin and 5′-desoxyadenosyl cobalamin are co-enzymes for two enzymatic reactions in the human metabolism: The enzyme methylmalonyl-CoA-mutase uses 5′-desoxyadenosylcobalamin to convert methylmalonyl-CoA into succinyl-CoA (SucCoA), which is necessary for the breakdown of odd-numbered fatty acids and certain amino acids. In case of a B12 deficiency, the concentration of methylmalonic acids (MMA) in the blood and in the urine increases. The enzyme methionin synthase (MS) uses methylcobalamin to convert homocysteine into methionine. In case of a B12 deficiency, both the concentration of homocysteine in the blood and the concentration of homocystine (two homocysteine molecules joined by a disulfide bond) in the urine increase.

"The daily vitamin B12 requirement
for adults without special needs is 4 µg"

European Food Safety Authority

It is estimated that, on average, adults ingest 4.2 µg to 8.6 µg daily, through the food they consume. This, of course, is not the case for vegetarians and vegans. One assumes that 40 to 50 percent of the B12 that is consumed is absorbed through the body's active absorption system. The maximum capacity of the absorption system in the ileum is 1.5 to 2 µg of B12 per meal. 

Furthermore, B12 can also be absorbed passively by the body. That is the case for 1 to 2 percent of the B12, regardless of how much B12 was consumed. Commercially available supplements with a very large quantity of B12 are aimed at this passive absorption.

Depending on the country and the organisation, the daily-recommended B12 dose is different. In Switzerland, according to the EDI (Eidgenössisches Departement des Innerns, the Federal Department of Home Affairs) regulations concerning the information on food stuff (LIV) (SR 817.022.16), the recommended daily dose for adults is 2.5 µg. In 2015, the EFSA-NDA (the Nutrition, Dietetic Products, and Allergies panel for the European Food Safety Authority) published a comprehensive scientific review with an opinion on the reference quantity for B12: for adults without special needs, 4 µg of B12 a day.

These recommendations apply to healthy individuals whose bodies can absorb B12 properly. If there is an absorption problem, or if a B12 deficiency is suspected, one should immediately see a doctor. In addition, above all, vegetarians and vegans should have their B12 status checked regularly by their primary-care physician.