Vitamin B12 Above or Below the Normal Range: Causes in Athletes and What to Do
A vitamin B12 result flagged as low or high on a routine blood test often worries athletes, and both directions can be misread. A low value may mean genuine deficiency or a borderline grey zone; a high value is usually harmless after supplementation but occasionally points to an underlying condition. Our editorial team explains how to interpret the numbers and what to discuss with your GP.
How B12 is measured
Most UK laboratories measure total serum B12, reported in nanograms per litre (ng/L) or picomoles per litre (pmol/L). Reference ranges vary between laboratories, but values below roughly 180–200 ng/L are commonly treated as low, while upper limits often sit somewhere around 700–900 ng/L.
Total B12 has well-known limitations. It measures vitamin bound to transport proteins, most of which is not available to cells. As a result, some people with "normal" results have functional deficiency, while others with low-normal results are perfectly well.
To clarify uncertain cases, clinicians may order active B12 (holotranscobalamin), methylmalonic acid (MMA) or homocysteine. Elevated MMA in particular suggests that cells genuinely lack B12. The British Society for Haematology guideline by Devalia and colleagues emphasises interpreting results together with symptoms rather than relying on a single cut-off.
The 2024 NICE guideline on vitamin B12 deficiency in over-16s similarly encourages clinicians to consider symptoms and risk factors and to use additional tests when results are indeterminate.
Why B12 may be low in athletes
Athletes are not inherently more prone to B12 deficiency than the general population, but several common situations in sport raise the risk.
- Vegan or largely plant-based diets without fortified foods or supplements. A review by Pawlak and colleagues found high rates of deficiency among vegetarians and especially vegans.
- Low energy availability in weight-category, aesthetic and endurance sports, where total food intake is restricted.
- Long-term medicines such as proton pump inhibitors or metformin.
- Pernicious anaemia, an autoimmune condition destroying intrinsic factor production, which can occur at any age.
- Gut conditions such as coeliac or Crohn’s disease affecting absorption.
- Recreational nitrous oxide use, which inactivates B12.
Symptoms may include fatigue, breathlessness on exertion, pale skin, sore tongue, pins and needles, poor concentration and mood changes. Athletes may notice a drop in performance before anything else, which can easily be misattributed to overtraining.
Neurological symptoms such as numbness, balance problems or memory difficulties need prompt medical attention, as nerve damage may become permanent if treatment is delayed.
Why B12 may be high
The most common reason for a raised B12 in active people is supplementation: multivitamins, energy drinks, fortified cereals, B-complex products or B12 injections. Levels can stay elevated for weeks or months after injections or high-dose oral products.
When there is no obvious supplement explanation, a persistently high B12 deserves attention. A review by Andrès and colleagues described associations with liver disease, kidney impairment, certain blood disorders and some solid tumours. Elevated B12 in these situations reflects release from damaged tissue or increased transport proteins, not too much intake.
A Danish population-based cohort study by Arendt and colleagues found that high plasma B12 was associated with an increased short-term cancer diagnosis rate. This is an association, not a cause, and the vast majority of people with high B12 do not have cancer, but it explains why doctors may investigate unexplained values.
In athletes, intense training can temporarily alter liver enzymes and other blood markers, but there is no robust evidence that exercise itself significantly raises B12.
Interpreting results: a practical overview
The table below summarises common scenarios. It is a guide for conversation with a clinician, not a self-diagnosis tool.
| Result pattern | Possible meaning | Typical next step |
|---|---|---|
| Low B12, symptoms present | Probable deficiency | GP assessment, check cause, start treatment |
| Borderline B12, no symptoms | Indeterminate | Repeat test, consider MMA or active B12 |
| Low B12 with anaemia and large red cells | Deficiency affecting blood production | Prompt treatment; test folate, intrinsic factor antibodies |
| High B12 on supplements | Expected | Review need for supplement |
| High B12 without supplements | Possible underlying condition | Liver, kidney and blood tests as advised |
Folate should usually be measured at the same time, as the two vitamins share pathways and both deficiencies cause similar blood changes.
Iron status is also worth reviewing in athletes, since coexisting iron deficiency can mask the large red cells typical of B12 deficiency.
Timing matters: if you have recently started supplements, a test may reflect the supplement rather than your underlying status.
What to do next
If your B12 is low, the first step is identifying the cause. Dietary deficiency, typical in vegans, often responds well to oral supplements. Absorption problems such as pernicious anaemia usually require hydroxocobalamin injections in the UK, at a schedule decided by your doctor.
Do not start high-dose folic acid alone before B12 has been assessed, as it can correct anaemia while neurological damage continues.
If your B12 is high and you take supplements, consider whether you actually need them. For most omnivores eating a varied diet, an extra B12 product brings no benefit. If the value remains high without supplements, follow your doctor’s advice on further investigation rather than ignoring it.
Athletes on a vegan diet should treat B12 as non-negotiable: fortified plant milks, yeast extract and cereals help, but a regular supplement is the most reliable route.
The editorial team's conclusions
A low B12 in athletes most often reflects diet, medicines or absorption problems, and nerve symptoms should never be ignored.
A high B12 is usually the result of supplements, but persistently raised values without an obvious cause warrant medical review.
Single numbers can mislead; additional markers and clinical judgement are often needed.
Related topics on our site include the best partners for vitamin B12, ferritin testing for athletes, and nutrition planning for vegan sportspeople.
References
- 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.
- National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). London: NICE; 2024.
- Green R, Allen LH, Bjørke-Monsen AL, Brito A, Guéant JL, Miller JW, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040.
- Andrès E, Serraj K, Zhu J, Vermorken AJ. The pathophysiology of elevated vitamin B12 in clinical practice. QJM. 2013;106(6):505–515.
- Arendt JF, Pedersen L, Nexo E, Sørensen HT. Elevated plasma vitamin B12 levels as a marker for cancer: a population-based cohort study. J Natl Cancer Inst. 2013;105(23):1799–1805.
- Pawlak R, Parrott SJ, Raj S, Cullum-Dugan D, Lucus D. How prevalent is vitamin B12 deficiency among vegetarians? Nutr Rev. 2013;71(2):110–117.
