Vitamin B12: who is at risk and why active B12 is tested

Vitamin B12 helps your body make red blood cells and keeps your nerves and brain working properly. Your liver stores several years' worth, so a deficiency usually develops slowly. By the time it causes symptoms, levels may have been falling for a long time.

That slow build-up is why B12 is worth testing if you are in a higher-risk group, even if you feel well.

What low B12 can cause

• Tiredness, weakness and breathlessness, from anaemia with large red cells.
• Pins and needles or numbness in the hands and feet.
• Unsteadiness or problems with balance.
• Poor memory, low mood or confusion.
• A sore, red tongue or mouth ulcers.

Nerve symptoms can appear without any anaemia, and nerve damage from long-standing deficiency may not fully recover. If you have numbness, tingling or balance problems, see a doctor rather than waiting.

Who is at risk

B12 is found naturally only in animal foods such as meat, fish, eggs and dairy. It is absorbed in a complex way: stomach acid releases it from food, a protein made in the stomach called intrinsic factor binds it, and the pair is absorbed at the end of the small bowel. A problem at any step can lead to deficiency.

• Plant-based diets. Vegans, and to a lesser degree vegetarians, get little or no B12 from food unless they use fortified foods or a supplement.
• Metformin. This widely used diabetes medicine lowers B12 absorption over time, and the risk grows with dose and years of use.
• Acid-reducing medication. Long-term use of proton pump inhibitors such as omeprazole or pantoprazole, and similar medicines, reduces the stomach acid needed to release B12 from food.
• Older adults. Thinning of the stomach lining becomes more common with age, reducing acid and intrinsic factor.
• Pernicious anaemia. An autoimmune condition where the body attacks the cells that make intrinsic factor. It is more common in people with other autoimmune conditions, such as thyroid disease.
• Stomach or bowel surgery, Crohn's disease and coeliac disease.
• Recreational nitrous oxide (laughing gas), which inactivates B12 and can cause sudden, serious nerve damage.

Why test active B12

Most B12 in the blood is attached to a carrier protein that cells cannot use. Only a smaller part, bound to a protein called transcobalamin, is delivered to your tissues. This is active B12, also known as holotranscobalamin.

The older total B12 test measures both forms together. Changes in the carrier proteins can make total B12 look low when the body has enough, which happens in pregnancy and with the contraceptive pill, or look normal when the usable part is short. Active B12 reflects what your cells can actually use, which makes it a more useful first test for many people.

When the result is unclear

If active B12 falls in the grey zone, a doctor may ask for methylmalonic acid or homocysteine. Both build up when cells are short of B12, so raised levels support a true deficiency. Homocysteine is part of LifePrint's Complete panel. Methylmalonic acid is a specialist test your doctor can arrange.

B12 and folate work together

B12 and folate are partners in making red cells, and low levels of either cause the same kind of anaemia. This is why both are on the Energy and Nutrients panel. It also matters for treatment: taking folic acid when you are short of B12 can correct the anaemia while the nerve damage carries on unnoticed. B12 status should always be known before folate is given on its own.

Before you test

High-dose biotin, found in many hair, skin and nail products, can interfere with the lab methods used for vitamin levels. Pause it for 48 hours before your draw. If you already take a B12 supplement or have B12 injections, tell us, because it changes how the result should be read.

What happens next

Your LifePrint report explains your active B12 alongside your blood count and folate, and flags how urgently to follow up. A low result needs a registered healthcare practitioner to find the cause, because the right treatment depends on it. Someone on a vegan diet and someone with pernicious anaemia need very different approaches, and the second usually needs lifelong treatment.

If you are at ongoing risk, such as years on metformin or a long-term plant-based diet, ask your doctor how often B12 should be rechecked.