The causes of vitamin B12 deficiency are varied and often interrelated, ranging from dietary insufficiencies to genetic factors and medical conditions that interfere with absorption. The most straightforward cause is a low vitamin B12 diet, often seen in individuals who follow strict vegetarian or vegan diets, as the vitamin is naturally found in significant amounts only in animal products. While plant-based foods can be fortified with B12, individuals relying solely on unfortified sources may gradually develop deficiencies over years. In addition to diet, stomach inflammation or chronic gastritis can reduce hydrochloric acid production, which is necessary to separate B12 from protein sources in food, impairing absorption. Pernicious anemia, an autoimmune condition, prevents the production of intrinsic factor, a protein essential for the absorption of B12 in the small intestine. Digestive diseases such as Crohn’s disease, celiac disease, or inflammatory bowel disease also disrupt nutrient absorption, making it harder for the body to obtain sufficient B12 from food. Surgical interventions, including gastric bypass or other gastrointestinal surgeries, can remove portions of the stomach or small intestine crucial for B12 absorption, leaving patients vulnerable to deficiency. Excessive alcohol consumption further compounds the problem by damaging the digestive lining and impairing nutrient absorption. Finally, genetic transport problems, such as transcobalamin II deficiency, interfere with the body’s ability to transport B12 to tissues, leading to cellular deficiency even when dietary intake is adequate. Understanding these causes is essential for both prevention and treatment, as identifying the underlying factor can guide appropriate supplementation, dietary adjustments, or medical intervention.