Vitamin B12 deficiency is usually caused by either not getting enough B12 in the diet or, more commonly, by problems absorbing it in the stomach or intestines.

Key causes at a glance

  • Poor intake : Long‑term vegan or very low–animal‑product diets without B12‑fortified foods or supplements can lead to deficiency because B12 is found mainly in meat, fish, eggs, and dairy.
  • Pernicious anaemia (autoimmune) : The immune system attacks stomach cells that make intrinsic factor, a protein needed to absorb B12, so even a good diet will not prevent deficiency.
  • Stomach problems or surgery : Conditions like chronic atrophic gastritis or surgery such as gastrectomy or bariatric surgery reduce acid and intrinsic factor, making B12 absorption much harder.
  • Intestinal diseases : Crohn’s disease, celiac disease, and other small‑bowel disorders (especially involving the terminal ileum) can block B12 absorption.
  • Medications : Long‑term use of proton‑pump inhibitors, H2‑blockers (for acid), and metformin (for diabetes) is linked with lower B12 levels because they affect stomach acid or absorption.
  • Ageing : Older adults often have reduced stomach acid and more chronic gastritis, which decreases B12 absorption even if intake is adequate.
  • Pancreatic and other systemic disease : Chronic pancreatitis and some advanced systemic illnesses (including advanced HIV) can interfere with the normal absorption process.
  • Parasitic or bacterial overgrowth : Intestinal parasites like fish tapeworm or bacterial overgrowth can “consume” B12 before the body absorbs it.
  • Genetic causes : Rare inherited disorders affect intrinsic factor or the transport and cellular use of B12, leading to deficiency even in children with good diets.
  • Pregnancy and breastfeeding issues : Infants exclusively breastfed by B12‑deficient mothers (especially vegan mothers without supplementation) are at risk because their only source is maternal stores and milk.

How these causes lead to symptoms

  • B12 is needed for DNA synthesis in red blood cells and for maintaining the myelin sheath around nerves, so deficiency leads to megaloblastic anaemia and neurological problems like numbness, imbalance, and cognitive changes.
  • Because the liver stores several years’ worth of B12, deficiency often develops slowly and may not be obvious until stores are significantly depleted.

When to get checked

  • People at higher risk include vegans, older adults, those with autoimmune thyroid or adrenal disease, people with Crohn’s or celiac disease, those after gastric/intestinal surgery, and long‑term users of acid‑lowering drugs or metformin.
  • Evaluation typically includes a full blood count and serum B12 level, sometimes with additional tests (like folate, methylmalonic acid, or intrinsic factor antibodies) to clarify the cause.

If you suspect symptoms such as persistent fatigue, numbness or tingling, balance problems, or memory issues, medical evaluation is important because B12 deficiency is usually treatable but can cause lasting nerve damage if missed.

Information gathered from public forums or data available on the internet and portrayed here.