Vitamin B12, Active Unit Converter

Active vitamin B12, also called holotranscobalamin (holoTC), is the fraction of vitamin B12 in your blood that is bound to the transport protein transcobalamin. This is the only form of B12 that can enter your cells and be used by the body.

While active B12 makes up only 10-30% of the total vitamin B12 in your blood, it represents the biologically available portion. The remaining 70-90% is bound to haptocorrin (called holohaptocorrin) and cannot be taken up by most cells.

Because active B12 has a shorter half-life than inactive forms, it tends to decrease earlier when B12 stores become depleted. This makes it a sensitive marker for detecting vitamin B12 deficiency before total B12 levels drop.

Vitamin B12 is obtained from animal-based foods such as meat, fish, poultry, eggs, and dairy products, or from fortified foods and supplements. It is absorbed in the small intestine (ileum) with the help of intrinsic factor produced by the stomach, then bound to transcobalamin for transport to cells throughout the body.