Vitamin B12 deficiency is sneaky. The symptoms — fatigue, brain fog, low mood, tingling fingers, weakness — can creep in over months and look like a hundred other things. Vegetarians, vegans, anyone over 50, and people on certain common medications are at much higher risk than they realise. The blood test is simple, the fix is usually straightforward, and the difference can be remarkable.
What is Vitamin B12?
Vitamin B12 (also called cobalamin) is a water-soluble vitamin essential for:
- Red blood cell formation — without enough B12, your body produces fewer, larger, less effective red cells
- Nervous system function — B12 is needed to maintain the myelin sheath around your nerves
- Energy production — B12 plays a key role in cellular metabolism
- DNA synthesis — every dividing cell in your body needs it
- Brain function and mood regulation
Your body can't make B12 — you have to get it from diet or supplementation. And uniquely among vitamins, it's only naturally present in animal foods.
Why is B12 deficiency so common?
According to the NHS, B12 deficiency affects around 1 in 10 people over 75 — and the rate is rising in younger adults too. Common causes:
- Plant-based diets — vegans must supplement; vegetarians often need to
- Age over 50 — stomach acid declines with age, reducing absorption
- Long-term acid-reducer use (omeprazole, lansoprazole, ranitidine) — these block stomach acid needed for B12 absorption
- Metformin for diabetes — interferes with B12 absorption
- Pernicious anaemia — an autoimmune condition that prevents B12 absorption
- Crohn's, coeliac, or gut surgery — anything that affects the small intestine
- Heavy alcohol use
Symptoms of low B12
The tricky thing about B12 deficiency is that symptoms develop slowly and overlap with stress, ageing, or low iron:
- Persistent tiredness despite adequate sleep
- Brain fog, difficulty concentrating
- Low mood or apathy
- Tingling, numbness, or "pins and needles" in hands or feet
- Pale or yellow-tinged skin
- Sore, smooth, or red tongue ("glossitis")
- Mouth ulcers
- Muscle weakness or balance issues
- In severe cases: memory problems, depression, irreversible nerve damage
Importantly, nerve damage from prolonged B12 deficiency can become permanent if not corrected — which is why catching it early matters.
What do the numbers mean?
B12 is measured in nanograms per litre (ng/L) or picomoles per litre (pmol/L) depending on the lab. UK reference ranges vary slightly:
- Below 200 ng/L — deficient
- 200-300 ng/L — borderline / "grey zone" (often symptomatic)
- 300-900 ng/L — typical
- Above 900 ng/L — high (often from supplements; not usually concerning)
The "grey zone" between 200 and 300 ng/L is interesting. Many people in this range have classic B12 deficiency symptoms, even though their result technically falls within "normal". That's why some labs also offer active B12 (holotranscobalamin) or methylmalonic acid (MMA) as more sensitive markers when the standard result is borderline.
Three things you might not know about Vitamin B12
1. The body stores enough for years — until it doesn't
The liver stores 1-5 years' worth of B12. That's why deficiency can take years to show up after a person stops getting enough — and why someone who turns vegan in their 30s might not feel symptoms until their 40s. By then, stores are exhausted.
2. Folate can mask B12 deficiency
High folate intake (from leafy greens, fortified cereals, supplements) can correct the blood picture of B12 deficiency without correcting the underlying nerve damage. This is why testing both B12 and folate together gives a clearer picture than either alone.
3. Sublingual and injectable B12 work better than tablets for some people
If absorption through the stomach is the problem (older adults, people on omeprazole, post-gastric-surgery), then standard oral tablets often don't work well. Sublingual (under-the-tongue) tablets and B12 injections bypass the stomach and tend to be much more effective.
Best food sources of B12
- Red meat, especially liver (highest source)
- Fish, particularly salmon and sardines
- Eggs and dairy
- Fortified plant milks and breakfast cereals (essential for vegans)
- Nutritional yeast (popular in vegan cooking)
How to test your Vitamin B12
B12 is included in our Vitamin and Minerals panel and our Anaemia Profile. Both tests typically also include folate — important because the two work together. For a wider energy and nutrition picture, see Tired All The Time.
Visit our County Durham clinic or a UK partner clinic — results in 3-5 working days as a clear PDF report. If your B12 is low, the fix is usually 4-12 weeks of high-dose supplementation followed by a retest. Many people feel a noticeable difference within 2-3 weeks.
This article is informational. Chxhealth is a biomarker and genetic data provider — we do not diagnose, treat or prescribe. If your results are outside typical ranges, talk to your GP or a healthcare professional about next steps.
