Vitamin B12 deficiency is usually caused by poor absorption in the gut, lack of intrinsic factor (pernicious anemia), or not getting enough B12 from food over time. Certain medications, gut surgeries, and digestive diseases also commonly contribute.

How B12 Is Normally Absorbed

Vitamin B12 comes mainly from animal foods like meat, fish, eggs, and dairy, and needs stomach acid and a protein called intrinsic factor to be absorbed in the last part of the small intestine (terminal ileum). Anything that disrupts stomach acid, intrinsic factor, or the ileum can eventually cause deficiency because body stores slowly run down over years.

Main Medical Causes

  • Autoimmune pernicious anemia, where the immune system attacks intrinsic-factor–producing stomach cells or intrinsic factor itself, so B12 cannot be absorbed.
  • Stomach problems or surgery (e.g., gastrectomy, bariatric surgery) that remove or damage acid-producing and intrinsic-factor–producing cells.
  • Small-intestine diseases (Crohn’s disease, celiac disease, other malabsorption syndromes, surgical removal of the ileum) that prevent uptake of the B12–intrinsic factor complex.
  • Chronic pancreatic disease or significant pancreatic insufficiency, which interferes with freeing B12 from food proteins.

Diet, Age, and Medications

  • Strict vegan or very low–animal-product diets without fortified foods or supplements, and breast-fed infants of untreated vegan mothers, can develop deficiency from low intake.
  • Older adults often produce less stomach acid, which decreases B12 release from food and raises deficiency risk over time.
  • Long-term use of acid-reducing drugs (proton pump inhibitors, H2 blockers) and metformin can impair B12 absorption and are now well-recognized contributors.

Other and Less Common Triggers

  • Nitrous oxide exposure (for example, repeated recreational use or prolonged anesthesia) can inactivate B12 and precipitate deficiency symptoms quickly.
  • Rare inherited disorders affecting intrinsic factor or ileal transport (such as Imerslund–Gräsbeck syndrome) and competition from parasites like fish tapeworm can also cause deficiency.
  • Obesity, restrictive dieting, and chronically low–micronutrient diets are increasingly reported in recent literature as associated with lower B12 levels.

When To Seek Help

  • Persistent fatigue, pale skin, tingling or numbness, balance problems, or trouble concentrating can be signs of B12 deficiency and should be evaluated with blood tests.
  • People at higher risk (vegans, older adults, those on metformin or acid-suppressing drugs, or with gut surgery/IBD) are often advised to have levels checked and consider preventive supplementation as guided by a clinician.

TL;DR: Most cases of vitamin B12 deficiency come down to absorption problems (autoimmune, gut disease, surgery, or medications) plus, less often, long-term low intake in vegan or highly restrictive diets. Information gathered from public forums or data available on the internet and portrayed here.