Testing for Vitamin B12
29 March 2021
Tests most commonly used to assess Vitamin B12 status measure Total B12. However, not all B12 in the blood is available to be used by the body - say what?!!
The majority of vitamin B12 that is measured by a Total B12 test is bound to a transporter (think of it as a taxi) called haptocorrin. B12 attached to haptocorrin cannot be used by the vast majority of the cells of the body.
But, a magic piece of biochemistry occurs - when transcobalamin (another taxi) and haptocorrin bind Vitamin B12, the result are complexes known as holo-transcobalamin (HoloTC) and holo-haptocorrin (HoloHC).
Holo-transcobalamin represents only 10-30% of the Vitamin B12 circulating in the blood but is the ONLY form of Vitamin B12 that is taken up and used by cells of the body, hence it’s other name… ACTIVE-B12.
Think of it this way: You take your car to fill up with petrol, as the gauge on the dashboard is showing half empty. However, you top up the tank to full with diesel, rather than unleaded. The gauge shows “full”, but it’s not the correct fuel in the tank to drive the car! I’ve made that mistake before!
So why do we need B12?
Vitamin B12 has an effect on multiple areas of our health - as an important coenzyme.
Stop right there ✋ what is a coenzyme? It is a compound that binds with an enzyme to allow a reaction to happen. Think about it this way - when making a cake, we need a raising agent that causes a reaction; facilitating the cake to rise. Think of these bioactives facilitating that magic to happen ✨ in different systems & cells in the body.
And in differing forms - there are 4 which we’ll come to in good time. B12 is required by every cell in the body and influences:
So signs & symptoms of low B12 are far reaching:
Different forms of B12 and where can we find them
Chemically speaking B12 is “cobalamin” but is always bound to other molecules therefore has 4 different forms & names: Cyanocobalamin, Hydroxocobalamin, Methylcobalamin, Adenosylcobalamin
Methylcobalamin + adenosylcobalamin are the two bioactive coenzyme forms of B12:
B12 in roughly descending order of 1) hydroxocobalamin, 2) adenosylcobalamin, 3) methylcobalamin can be found in almost all meat and animal products. The largest concentration in offal (liver in particular) and then continues to decrease through lean meat to dairy products.
Plants cannot produce vitamin B12; nonetheless sometimes it is detected in very small amounts in different species. Plants are not a reliable source of vitamin B12.
Check out my next blog where I talk about what inhibits absorption & transport B12, methylation and how genetic variants - MUT, TCN2, MTHFR, impacts all the above….