Vitamin B12 (Cobalamin)

Vitamin B12 is a nutrient your body needs to make healthy red blood cells, keep your nervous system working properly, and support DNA production. It is mainly found in animal-based foods like meat, fish, dairy, and eggs, so people who follow a vegan or vegetarian diet are at higher risk of having low levels. Low B12 can cause fatigue, weakness, numbness or tingling in the hands and feet, memory problems, and anemia. High B12 levels from supplements are generally not harmful, but unexpectedly high levels without supplementation may sometimes signal an underlying health condition worth discussing with your doctor. Your healthcare provider may order additional tests to get a clearer picture of your B12 status.

When elevated: Persistently elevated serum B12 levels (>1000 pg/mL) without supplementation may suggest hepatocellular disease, myeloproliferative neoplasms, solid tumors (particularly hepatocellular carcinoma or metastatic liver disease), or renal insufficiency. Elevated B12 in this context warrants clinical investigation rather than reassurance, as it may reflect pathological release from damaged hepatocytes or increased haptocorrin binding proteins. When low: Low serum B12 is associated with megaloblastic anemia, subacute combined degeneration of the spinal cord (posterior and lateral column demyelination), peripheral neuropathy, glossitis, cognitive decline, and elevated homocysteine. Neurological manifestations can occur in the absence of anemia and may be irreversible if deficiency is prolonged.

B12 supports red blood cell production and mitochondrial energy metabolism, both critical for aerobic performance and recovery. Athletes on plant-based or restrictive diets warrant periodic B12 screening, as deficiency impairs oxygen delivery and can delay adaptation to training; adequate levels support endurance capacity and neurological function during high-volume training.