Vitamin B12
Essential for nerve function, DNA synthesis, and red blood cell formation — deficiency is common, especially after age 50.
Optimal Range
> 500 pg/mL (optimal) · 200-900 pg/mL (lab range)
Risk-Stratified Targets
| Population / Context | Target |
|---|---|
| Optimal | > 500 pg/mL |
| Adequate | 300–500 pg/mL |
| Low-normal (possible functional deficiency)Check MMA and homocysteine | 200–300 pg/mL |
| DeficientRisk of irreversible neurologic damage | < 200 pg/mL |
Why It Matters
B12 deficiency causes irreversible nerve damage if left untreated and is associated with elevated homocysteine, cognitive decline, and anemia. Absorption decreases with age and with use of common medications like metformin and PPIs.
Understanding Vitamin B12
Vitamin B12 (cobalamin) is an essential water-soluble vitamin required for DNA synthesis, myelin maintenance (the insulating sheath around nerves), red blood cell formation, and the recycling of homocysteine to methionine. It is found exclusively in animal-derived foods — meat, fish, eggs, and dairy — making deficiency common in vegans and vegetarians.
B12 deficiency is far more prevalent than commonly recognized, especially in older adults. Absorption of B12 from food requires adequate stomach acid and intrinsic factor — both of which decline with age. By age 60, an estimated 10–30% of adults have difficulty absorbing food-bound B12. Common medications further impair absorption: proton pump inhibitors (PPIs like omeprazole) reduce stomach acid, and metformin (the most widely prescribed diabetes drug) reduces B12 absorption by up to 30%.
The consequences of B12 deficiency are serious and sometimes irreversible. Neurological symptoms — numbness, tingling, difficulty walking, memory loss, and cognitive decline — can occur even before anemia develops and may not fully reverse once nerve damage is established. Standard lab ranges (200–900 pg/mL) include levels where deficiency symptoms already occur. Functional deficiency can be present at levels below 400–500 pg/mL, and many clinicians target levels above 500 pg/mL. Methylmalonic acid (MMA) and homocysteine are useful confirmatory markers — both rise in B12 deficiency.