Vitamin B12 Blood Tests: What They Show and Why They Matter for Athletes

Vitamin B12 is essential for red blood cell production and nerve function — both critical for anyone who trains. Yet B12 results are among the most misunderstood items on a blood report. Our editors explain what the different B12 tests measure, how to read them in UK units and why athletes, particularly those eating plant-based diets, should pay attention.

Why B12 matters for athletes

Vitamin B12 (cobalamin) is a cofactor for two enzymes: methionine synthase, which recycles homocysteine and supports DNA synthesis, and methylmalonyl-CoA mutase, involved in energy metabolism. When B12 is lacking, rapidly dividing cells — including red blood cell precursors in the bone marrow — are affected first.

The classic result is megaloblastic anaemia: fewer, larger red blood cells that carry oxygen less efficiently. For an endurance athlete, this can mean breathlessness, early fatigue and reduced capacity, very similar to iron deficiency.

B12 deficiency also affects the nervous system, causing pins and needles, numbness, balance problems, memory issues and low mood. Neurological damage can occur even before anaemia appears, and it may become permanent if diagnosis is delayed.

A review of micronutrient status and performance by Lukaski stressed that deficiencies of B vitamins can impair exercise capacity, although supplementation above adequate levels does not boost performance.

The tests and what they measure

Serum total B12 is the most common first-line test in the UK. It measures all B12 bound to transport proteins in the blood, including the part that is not available to cells. Results are reported in pmol/L or ng/L depending on the laboratory.

Active B12 (holotranscobalamin) measures only the fraction bound to transcobalamin, the protein that delivers B12 to tissues. Some UK laboratories use it as a first-line or second-line test because it may reflect early depletion better.

When results are borderline, functional markers help. Methylmalonic acid (MMA) and homocysteine both rise when cells lack B12. MMA is more specific; homocysteine also rises with folate deficiency, kidney problems and other factors.

The NICE guideline on vitamin B12 deficiency in over-16s, published in 2024, and British haematology guidance by Devalia and colleagues both stress that results must be interpreted together with symptoms, because no single test is perfect.

Interpreting results and causes of deficiency

Reference ranges vary by laboratory, so the result should always be read against the range printed on your report. As a rough guide, many UK laboratories regard serum B12 below about 150–180 pmol/L as deficient and a band above that as indeterminate, where further testing may be needed.

Timing and context also affect the picture. A result taken soon after starting a supplement or receiving an injection may look reassuring while tissue stores are still low, and high values can occasionally reflect liver or blood disorders rather than good intake, so unexpected results deserve a conversation with the doctor.

Low dietary intake is a leading cause in athletes who avoid animal products. A review by Pawlak and colleagues found high rates of deficiency among vegetarians and especially vegans without reliable B12 sources. Fortified foods or supplements are essential in a vegan diet.

Medicines matter too. Long-term metformin use was linked to lower B12 levels in a randomised trial by de Jager and colleagues, and proton pump inhibitors can reduce B12 absorption from food. Autoimmune pernicious anaemia, coeliac disease and previous stomach or bowel surgery are other important causes. The most frequent causes are summarised below.

• vegan or strict vegetarian diet without fortified foods or supplements;
• pernicious anaemia (autoimmune loss of intrinsic factor);
• long-term metformin or acid-suppressing medicines;
• coeliac disease, Crohn's disease or gut surgery;
• older age and reduced stomach acid.

Treatment, supplements and monitoring

Treatment depends on the cause. People with dietary deficiency usually respond to oral B12, and supplements are widely available in the UK as cyanocobalamin or methylcobalamin. When absorption is impaired, as in pernicious anaemia, treatment is typically with intramuscular hydroxocobalamin injections given under NHS care.

For healthy vegans, the Vegan Society and dietitians commonly recommend either regular fortified foods, a daily supplement or a larger weekly dose. B12 has a very low risk of toxicity because excess is excreted.

Athletes should be aware that a "normal" B12 result does not mean that extra B12 will improve performance. Injections are sometimes promoted as energy boosters, but there is no evidence of benefit in people who are not deficient.

Monitoring makes sense for at-risk groups: plant-based athletes, those on long-term medication and anyone with unexplained fatigue, anaemia or neurological symptoms. Discuss the frequency of testing with your GP.

Editorial conclusions

Vitamin B12 is vital for oxygen transport and nerve health, so deficiency can quietly erode training capacity and wellbeing. Serum B12 is a good starting point, but borderline results may need active B12, MMA or homocysteine to clarify.

Plant-based athletes, people taking metformin or acid-reducing drugs and those with gut conditions are at higher risk and should consider regular testing.

Supplementation corrects dietary deficiency effectively, but extra B12 offers no performance boost for people with adequate levels.

Related topics on our site include iron and ferritin testing for athletes, a guide to the full blood count and an overview of nutrition for vegan athletes.

References

1. National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). London: NICE; 2024.
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. Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160.
4. Green R, Allen LH, Bjørke-Monsen AL, et al. Vitamin B12 deficiency. Nat Rev Dis Primers. 2017;3:17040.
5. de Jager J, Kooy A, Lehert P, et al. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial. BMJ. 2010;340:c2181.
6. Pawlak R, Parrott SJ, Raj S, et al. How prevalent is vitamin B12 deficiency among vegetarians? Nutr Rev. 2013;71(2):110–117.
7. Lukaski HC. Vitamin and mineral status: effects on physical performance. Nutrition. 2004;20(7–8):632–644.