Vitamin B12 Above or Below Normal: Causes in Athletes and What to Do
Vitamin B12 outside the reference interval is a reason to analyze the causes, not to automatically buy supplements or cancel them. The editorial team explains why the cobalamin level in athletes decreases or rises, which tests clarify the situation and how to proceed.
How to understand that a result is truly deviant
The vitamin B12 test has a wide "grey zone", so the very word "deviation" needs clarification. Clinical reviews usually consider a level below approximately 148 pmol/L (200 pg/mL) to be convincingly low, and the range 148–221 pmol/L to be borderline. The upper limit of the reference interval in most laboratories is in the range of several hundred pmol/L; the specific figures are given on the report.
A single number is not a diagnosis. Before looking for a cause, it is worth making sure the test conditions were standard: fasting, no intake of B12 supplements on the day of the test, reporting of biotin and medications.
If the result is borderline, the guideline of the British Committee for Standards in Haematology (2014) suggests clarifying tests: methylmalonic acid, homocysteine or holotranscobalamin. They help distinguish a true tissue deficiency from a laboratory artifact.
In athletes the picture is complicated by concomitant iron deficiency: it can mask the enlargement of red blood cells characteristic of B12 shortage. Therefore a whole complex needs to be assessed — B12, ferritin, complete blood count.
Causes of low B12
The causes of deficiency can be divided into three groups: insufficient intake, impaired absorption and increased destruction or blockade of the vitamin. In the athletic population all three occur.
| Group | Examples | Relevance for athletes |
|---|---|---|
| Low intake | Veganism, strict diets, low-calorie "cutting" | High in aesthetic and endurance sports |
| Poorly absorbed | Autoimmune gastritis (pernicious anemia), celiac disease, Crohn's disease, GI surgery | Independent of sport, but occurs |
| Medications | Proton pump inhibitors, metformin | PPIs are often used for reflux and while taking NSAIDs |
| Inactivation | Nitrous oxide | Recreational use — a real risk in young people |
The link between long-term use of PPIs and H2 blockers and B12 deficiency was shown in the large study by Lam and co-authors (JAMA, 2013). For metformin, a similar link was confirmed by the long-term DPPOS follow-up. These drugs are also common among people who are physically active.
Pernicious anemia is an autoimmune disease in which intrinsic factor is not produced. Here no diet will help, because the vitamin simply is not absorbed from food; treatment under a doctor's supervision is required.
Finally, it is worth remembering the large reserves of B12 in the liver. A person who has switched to a vegan diet may have normal tests for years and then face a deficiency that has developed "suddenly".
Causes of high B12
Elevated serum B12 in an athlete most often has a simple explanation: supplements, injections or energy drinks fortified with vitamins. After an injection the blood level can remain high for weeks.
However, if the person is not taking the vitamin, a persistently high B12 requires attention. The review by Arendt and Nexo (2013) lists among the causes liver diseases (release of reserves), renal failure, myeloproliferative blood diseases and some tumors that raise the level of carrier proteins.
- intake of supplements, injections, fortified products and drinks;
- liver damage, including toxic damage;
- reduced kidney function;
- blood diseases with elevated transcobalamins;
- "macro-B12" — binding of the vitamin to antibodies, which creates a laboratory artifact.
For athletes who use oral anabolic steroids or hepatotoxic supplements, high B12 together with elevated liver enzymes can be one of the signals of liver damage. Such a coincidence requires examination.
Paradoxically, a high total B12 does not rule out a functional deficiency: if the vitamin is bound to "inactive" proteins, cells may be under-supplied. In doubtful cases MMA helps.
What to do with the result
The algorithm of action depends on the direction of the deviation and the presence of symptoms. An unchanging rule: determine the cause, not just "fix the number".
In case of low B12 the doctor assesses the diet, medications, symptoms from the stomach and nervous system, and orders clarifying tests. If the deficiency is related to diet, regular supplements or fortified products are enough. In case of impaired absorption the doctor may choose the injectable form.
Treating B12 deficiency with folic acid is unacceptable: folate can improve blood counts but will not stop nerve damage. This is the classic warning of all hematological guidelines.
In case of high B12 against a background of supplements, it is enough to stop the intake and repeat the test in a few weeks. If there were no supplements — it is worth checking liver enzymes, creatinine and the complete blood count, and then acting on the recommendation of a physician.
Editorial conclusions
Low B12 in athletes is most often associated with a plant-based diet, the use of PPIs or metformin, or stomach diseases. High B12 is associated with supplements, but sometimes also with liver or kidney damage.
Borderline results are clarified with the help of MMA, homocysteine or holotranscobalamin.
The main thing is to find the cause, not just to raise or lower the blood level.
We also recommend reading our materials on what the vitamin B12 test shows, on folate above or below normal and on the homocysteine test.
References
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160.
- 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.
- Arendt JF, Nexo E. Unexpected high plasma cobalamin: proposal for a diagnostic strategy. Clin Chem Lab Med. 2013;51(3):489–496.
- 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.
- 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.
- Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
