Vitamin B12 and energy: deficiency, symptoms and your blood values

Vitamin B12 is needed to make red blood cells, and those carry oxygen to your muscles. A deficiency can therefore cause anaemia and stubborn fatigue, even when you train well. You check your B12 status with a blood test.

I notice many athletes look for energy in their training first, while a simple blood value sometimes gives the real answer. Before you overhaul your schedule, it pays to check your B12 and your iron first.

Why is B12 important for your energy?

B12 plays a key role in making red blood cells and in your nerve function. Without enough B12 your body makes poorer red blood cells, so your blood carries less oxygen and you feel flat (Stabler, 2013).

That explains why a deficiency stands out during effort in particular. You notice it as breathlessness and a heavier feeling during your sessions.

What are the symptoms of a B12 deficiency?

The symptoms are vague and develop slowly, so a deficiency stays unnoticed for a long time. Commonly mentioned complaints are fatigue, tingling in hands and feet, concentration problems and breathlessness.

• Persistent fatigue and low energy
• Tingling or numbness in hands and feet
• Concentration and memory problems
• Breathlessness during effort
• Pale skin

Neurological complaints can appear before your blood count clearly deviates. So do not underrate vague signals.

How do you test B12: total, active and MMA

Most labs first measure your total B12, but that number does not tell the whole story. Active B12 (holoTC) and methylmalonic acid (MMA) help to read a doubtful result.

Have your B12 value measured if you are tired for a long time without a clear reason. This article belongs to our guide vitamins and minerals for athletes.

Who is at risk: vegans and athletes

B12 sits mainly in animal products, so anyone eating vegan or strictly vegetarian runs a higher risk of a deficiency. The Netherlands Nutrition Centre advises people who eat no animal products to supplement B12.

Stomach complaints or certain medication can also reduce uptake. If you feel tired despite good training, read tired despite training.

What do you do about a deficiency?

A proven deficiency you top up with food or, after advice from your doctor, with supplements or injections. With a low iron and a low B12 at the same time, a broader story often plays out, so combine your results. Also read iron deficiency in athletes.

Retest after a few months to see whether your value recovers. Treat the result as a starting point for the conversation with your doctor, not as a final verdict.

The difference between a B12 deficiency and ordinary fatigue

Fatigue has many causes, from too little sleep to overtraining, and that makes a B12 deficiency hard to recognise. The difference often sits in the accompanying complaints and in the course. A deficiency comes on slowly and regularly goes together with tingling, pale skin or concentration problems.

Ordinary fatigue varies with your load and recovers after a calm week. A deficiency does not, because the underlying cause stays until you address it. That is exactly why a blood test is valuable: it separates an adjustable deficiency from a lifestyle issue.

If you feel flat structurally while your sleep and training are in order, measuring is wiser than puzzling on. Combine your B12 with your iron status, because those two together explain a large part of fatigue complaints in athletes.

That way you avoid adjusting your training for months while a simple value held the answer. Spotting a deficiency costs a single draw, while guessing on can cost you a whole season. That trade-off makes measuring a logical first step for most athletes.

References

1. Stabler SP. Clinical practice. Vitamin B12 deficiency. New England Journal of Medicine. 2013;368(2):149-160. PMID: 23301732.
2. Voedingscentrum. Vitamin B12. Accessed 2026.
3. RIVM. Intake of vitamins and minerals in the Netherlands. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.