Vitamin B12 Deficiency — Symptoms Most People Ignore and What to Do About It
Vitamin B12 Deficiency: Symptoms People Ignore and What to Do About It
It's sneaky, it's common, and the nerve damage it causes can be permanent if you wait too long.
B12 deficiency is one of those diagnoses that gets missed for years. Patients come in tired, foggy, a bit numb in their feet. They've been told they're stressed, or maybe anxious, or just not sleeping well enough. Then a blood test happens and their B12 is 80 pmol/L. The normal range starts at 148.
It's one of the more correctable problems in medicine. The problem is catching it before the nervous system damage sets in.
What B12 Actually Does
Vitamin B12 (cobalamin) does 3 critical things in your body. It helps make red blood cells. It maintains the myelin sheath around your nerves (the insulation that lets nerve signals travel properly). And it's essential for DNA synthesis.
When B12 drops, all 3 break down. You get anaemia because your red cells grow abnormally large and don't work properly (megaloblastic anaemia). You get nerve symptoms because demyelination starts. And your DNA repair processes slow down.
The Symptoms Worth Paying Attention To
Neurological
Haematological
Other
Who's Actually at Risk
| Risk Group | Reason |
|---|---|
| Vegans and strict vegetarians | B12 is found almost exclusively in animal products. Without supplementation, deficiency is near-inevitable over years. |
| Adults over 60 | Stomach acid production drops with age. Less acid means less intrinsic factor, which means less B12 absorption from food. |
| People on long-term metformin | Metformin (common diabetes drug) interferes with B12 absorption. Guidelines recommend monitoring B12 annually. |
| Long-term PPI users | Proton pump inhibitors (omeprazole, lansoprazole) reduce stomach acid and impair B12 absorption. |
| Pernicious anaemia patients | Autoimmune condition where intrinsic factor is absent. Oral supplementation alone doesn't work; injections required. |
| Crohn's disease / post-ileal surgery | The terminal ileum is where B12 is absorbed. Damage or removal of this segment means oral absorption is impaired. |
Diagnosing It
The first-line test is serum B12. Normal is generally considered above 148 pmol/L (or 200 pg/mL, depending on your lab's units). Deficiency is below 148 pmol/L.
But here's the problem: serum B12 isn't always accurate. Around 50% of patients with deficiency symptoms and low-normal B12 (148 to 300 pmol/L) will have elevated methylmalonic acid (MMA) or homocysteine, which are more sensitive markers of functional B12 deficiency.
If your B12 is in the grey zone and you have symptoms, ask for MMA and homocysteine. This is basic haematology, not an exotic request.
A normal full blood count doesn't rule out B12 deficiency. You can have significant nerve damage from B12 deficiency with a normal haemoglobin, particularly if you're also iron deficient (which can mask the macrocytic picture).
Treatment: What Actually Works
Intramuscular Injections
For pernicious anaemia or malabsorption (Crohn's, post-ileal surgery), oral B12 doesn't work. You need intramuscular hydroxocobalamin or cyanocobalamin injections. In the UK, the standard protocol is 1mg every other day for 2 weeks, then every 3 months for life.
High-Dose Oral Supplements
For dietary deficiency (vegans, vegetarians), high-dose oral B12 works. Doses of 1,000 to 2,000 mcg daily are typically used. At these doses, a small percentage absorbs passively through the gut mucosa, bypassing the need for intrinsic factor.
Dietary Sources
Liver (chicken or beef) is by far the richest source: around 70 mcg per 100g. Clams, mussels, mackerel, and salmon are good sources. Dairy and eggs provide modest amounts. Fortified foods (cereals, plant milks) vary widely in their B12 content.
"Nerve damage from B12 deficiency can be permanent. Checking a B12 level is one of the cheapest, most actionable tests in medicine. Don't wait."
The Timeline for Recovery
Anaemia usually responds within 4 to 6 weeks of treatment. Neurological symptoms are slower. Some improve fully; some don't, particularly if deficiency has been present for over 12 months. Early diagnosis genuinely changes the outcome.
Cognitive symptoms (brain fog, memory) typically start improving within 2 to 3 months of adequate treatment. Energy levels often improve within weeks.
The Bottom Line
B12 deficiency is common, causes serious harm over time, and is entirely treatable if caught. If you're vegan, over 60, on metformin, or on long-term PPIs, get your B12 checked. If you've been told you have depression, anxiety, or early dementia and your B12 hasn't been checked, ask for it.
It's one of those situations where a simple blood test genuinely changes what happens next.
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