Vitamin B12 Deficiency Causes and Diagnosis

Primary Mechanisms of Deficiency

Atrophic gastritis affecting the gastric body is the leading cause of B12 deficiency in older adults, impairing the release of food-bound cobalamin through reduced gastric acid production 1, 2, 3, 4
Pernicious anemia causes intrinsic factor deficiency, though this now accounts for a minority of cases compared to food-bound malabsorption 4
Ileal resection or ileal Crohn's disease prevents B12 absorption in the terminal ileum, requiring lifelong intramuscular supplementation 4, 5
Post-bariatric surgery reduces both intrinsic factor and gastric acid production 4, 5
Proton pump inhibitors or H2 receptor antagonists used for >12 months impair B12 absorption by reducing gastric acid needed to cleave food-bound cobalamin 4
Metformin use for >4 months increases deficiency risk, particularly important given widespread diabetes treatment 4, 5
Vegan or strict vegetarian diets lacking animal products provide inadequate B12, as the vitamin is not present in plant-based foods 4

Adults >75 years have 18.1% prevalence of metabolic B12 deficiency, increasing to 25% in those ≥85 years, even with "normal" serum B12 levels 5
The combination of atrophic gastritis prevalence and medication use makes older adults particularly vulnerable 1, 2, 3
Celiac disease impairs B12 absorption and frequently coexists with autoimmune conditions 4
Inflammatory bowel disease, particularly Crohn's disease affecting the terminal ileum, increases risk 4
Autoimmune thyroid disease has 28-68% prevalence of B12 deficiency due to associated autoimmune gastritis 5
Type 1 diabetes and other autoimmune conditions increase risk through autoimmune gastritis 4, 5

Any patient with risk factors (age >75, PPI use >12 months, metformin >4 months, vegan diet, gastric surgery, ileal disease) PLUS clinical features (fatigue, cognitive difficulties, neuropathy, macrocytosis, anemia unresponsive to iron) should be tested 4, 5
All patients with autoimmune thyroid disease at diagnosis and annually thereafter should be tested 5
Post-bariatric surgery patients require routine monitoring 4, 5
Initial test: Total serum B12, with <180 pg/mL indicating confirmed deficiency, 180-350 pg/mL indicating indeterminate results, and >350 pg/mL indicating deficiency unlikely 5
Confirmatory test for indeterminate results: Methylmalonic acid (MMA), with >271 nmol/L confirming functional B12 deficiency 5

Metabolic B12 deficiency increases stroke risk through hyperhomocysteinemia; B vitamin supplementation reduced ischemic stroke by 43% in meta-analyses 5
Lower B12 biomarker status is associated with increased risk of cognitive dysfunction and dementia in older adults 1, 2, 3
Deficient B12 status is linked to increased depression risk; deficient B6 status is associated with increased anxiety 1, 2
B vitamin deficiencies are associated with higher risk of bone disease 1, 2, 3
Subacute combined degeneration causes permanent spinal cord damage if untreated 4, 6