Active B12 (Holotranscobalamin, HoloTC)
Other marker
Active B12
Active B12 (Holotranscobalamin, HoloTC)
The biologically active fraction of vitamin B12, bound to transcobalamin and available for uptake by cells. It differs from the standard total Vitamin B12 test, which also measures B12 bound to haptocorrin that cells cannot use. Active B12 is considered a more sensitive early indicator of true B12 status.
PED Notes
Useful when total B12 is borderline or gives a confusing picture. Bodybuilders commonly supplement or inject B12 for energy and recovery, which raises total B12 and can mask an underlying functional deficiency; measuring active B12 (holotranscobalamin) alongside functional markers like homocysteine and methylmalonic acid gives a truer read of whether tissues are actually getting enough. Vegan or plant-heavy athletes, and those on metformin or long-term acid suppression, are at higher risk of true deficiency.
When low
When low (<35 pmol/L suggests deficiency; 35-50 pmol/L is borderline):
Confirm functional deficiency: Pair with homocysteine and methylmalonic acid (MMA); both rise in genuine B12 deficiency and help distinguish a true deficit from a low-normal number.
Supplements:
- Methylcobalamin (active B12) -- 1000mcg/day sublingual for maintenance and mild deficiency
- B12 injections (hydroxocobalamin or cyanocobalamin) -- 1000mcg IM, commonly used in the bodybuilding community and preferred where absorption is impaired (pernicious anaemia, post-bariatric, metformin use)
- Ensure adequate folate and B6 as well, since they work together in homocysteine metabolism
Lifestyle / cause-finding:
- Review diet (vegan/vegetarian), metformin use, proton pump inhibitors, and any history suggesting malabsorption
- Persistent deficiency despite oral supplementation points to an absorption problem and favours injections plus a workup for pernicious anaemia (intrinsic factor antibodies)
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range