Vitamin B12: What Your Blood Level Means

Vitamin B12 (cobalamin) is required to build red blood cells, maintain the myelin sheath around nerves, and recycle homocysteine. It is stored in the liver, so deficiency develops slowly.

The standard serum B12 test measures total B12, most of which is bound and unavailable to cells. That is why methylmalonic acid (MMA) or homocysteine is sometimes added to confirm borderline results.

Deficiency can cause fatigue, tingling in the hands and feet, memory changes, and a macrocytic anemia with large red blood cells (high MCV).

Nerve symptoms can appear before the blood count changes, so a low-normal B12 with symptoms is worth investigating rather than dismissing.

B12 occurs naturally only in animal foods: shellfish, beef, liver, fish, eggs, and dairy. Fortified nutritional yeast and plant milks are the reliable vegan sources, and most long-term vegans need a supplement.

Metformin, long-term acid-suppressing medication, gastric surgery, pernicious anemia, celiac and Crohn’s disease, aging, and heavy alcohol use all reduce absorption.