Vitamin B12: What the Test Shows and Why It Matters for an Athlete

Vitamin B12 is often mentioned in connection with energy and endurance, yet the test for it has clear clinical logic. The editorial team explains what role cobalamin plays in an athlete's body, what exactly the laboratory measures, why a "grey zone" exists and which athletes should monitor this marker.

Why the body needs vitamin B12

Vitamin B12 (cobalamin) is a water-soluble vitamin with a complex molecule containing cobalt. In the human body it works as a cofactor for only two enzymes, but both are critically important. Methionine synthase, with the participation of B12 and folate, converts homocysteine into methionine, while methylmalonyl-CoA mutase participates in the metabolism of fatty acids and some amino acids.

Through the first reaction B12 is linked to DNA synthesis and cell division, primarily of blood-forming cells. That is why deficiency manifests as megaloblastic anemia — red blood cells become large and few in number. Through the second reaction B12 is necessary to maintain the myelin sheath of nerves.

For an athlete this means two key things: the ability of the blood to carry oxygen and the normal functioning of the nervous system. Deficiency may manifest as fatigue, shortness of breath during exertion, a drop in endurance, and later also tingling in the limbs or coordination disorders.

The sources of B12 are only animal products: meat, fish, eggs, dairy. Absorption depends on stomach acidity and Castle's intrinsic factor. The liver stores reserves of the vitamin for several years, so deficiency develops slowly and often imperceptibly.

What exactly the test shows

The standard test measures the total concentration of cobalamin in the serum. Most of it is bound to the protein haptocorrin and is not available to cells; only the fraction bound to transcobalamin — holotranscobalamin — is considered "active".

Therefore total B12 is a useful but imperfect screening marker. Stabler's review in NEJM (2013) and the guideline of the British Committee for Standards in Haematology (2014) emphasize: a normal result does not always rule out deficiency, and a reduced one does not always confirm it.

For clarification, functional markers are used: methylmalonic acid (MMA) and homocysteine. Both accumulate when there is not enough B12 for the corresponding reactions. MMA is more specific for B12, while homocysteine also rises with a lack of folate.

A complete blood count with the MCV index (mean corpuscular volume) complements the picture, but macrocytosis appears late and can be masked by concomitant iron deficiency, which is not uncommon in endurance athletes.

Reference points of the norm and the "grey zone"

Reference intervals depend on the laboratory, but clinical reviews often cite a reference point: a level below approximately 148 pmol/L (200 pg/mL) argues in favor of deficiency, while the range 148–221 pmol/L (200–300 pg/mL) is considered borderline, when it is advisable to determine MMA or homocysteine.

Units may differ: pg/mL and pmol/L. To convert pg/mL to pmol/L, the value is multiplied by approximately 0.74. Compare results only in identical units.

The test is usually taken fasting in the morning. It is important to report the intake of supplements with B12 and biotin: the FDA has warned that high doses of biotin can distort immunoassays, and a just-taken supplement raises serum B12 without reflecting tissue reserves.

The result also depends on pregnancy, the use of oral contraceptives and some diseases that change the level of carrier proteins. The doctor takes these factors into account during assessment.

Which athletes need the test most

B12 deficiency is not a typical "sports" problem, but in certain groups of athletes the risk is genuinely higher. For them the test should be included in regular medical monitoring.

• Vegans and strict vegetarians— without fortified products or supplements, deficiency develops almost inevitably.
• Athletes who take proton pump inhibitors for a long time— reduced acidity worsens the release of B12 from food.
• Individuals on metformin— the link between long-term use and deficiency was confirmed in the DPPOS study.
• Athletes with gastrointestinal diseasesor after surgery on the stomach and intestine.
• Athletes with unexplained fatigue, anemia or neurological symptoms.

Separately, the editorial team reminds readers about nitrous oxide ("laughing gas"), which inactivates B12. Its recreational use has caused severe neurological damage in young people even with normal nutrition.

B12 injections as an "energy booster" before competitions are a common myth. If there is no deficiency, additional vitamin does not increase endurance or improve recovery.

Editorial conclusions

Vitamin B12 is necessary for blood formation and the nervous system, so its deficiency can noticeably reduce an athlete's performance. However, it develops slowly, and the symptoms are nonspecific.

Serum B12 is only the first step. In the "grey zone" it is advisable to add MMA, homocysteine or holotranscobalamin, as well as a complete blood count.

The greatest benefit from the test is gained by vegans, people on PPIs or metformin and athletes with unexplained fatigue.

We also recommend reading our articles on the causes of B12 deviations from the norm, on the folate test and on preparing for the homocysteine test.

References

1. Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160.
2. Devalia V, Hamilton MS, Molloy AM; British Committee for Standards in Haematology. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol. 2014;166(4):496–513.
3. Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040.
4. Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016;101(4):1754–1761.
5. Lam JR, Schneider JL, Zhao W, Corley DA. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013;310(22):2435–2442.
6. Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine Joint Position Statement. Nutrition and athletic performance. Med Sci Sports Exerc. 2016;48(3):543–568.
7. U.S. Food and Drug Administration. The FDA warns that biotin may interfere with lab tests: FDA Safety Communication. 2017 (updated 2019).