| A. Severe deficiency |
| 1. Severe malabsorption (affecting the physiological intrinsic factor cubam receptor axis) |
| a. Pernicious anemia (autoimmune gastritis) |
| b. Total or partial gastrectomy |
| c. Gastric bypass or other bariatric surgery |
| d. Ileal resection or organ reconstructive surgery (ileal conduit diversion & ileocystoplasty) |
| e. Inherited disorders affecting B12 absorption (affecting either intrinsic factor or the cubam receptor) |
| 2. Abuse of nitrous oxide |
| 3. Inherited metabolic |
| a. Impaired ability to transport B12 (TC deficiency) |
| b. Impaired ability to process B12 (8 distinct inborn errors of cobalamin metabolism resulting in homocystinuria and/or methylmalonic acidemia) with varying clinical spectra involving the nervous system and blood |
| B. Mild to moderate deficiency |
| 1. Mild to moderate malabsorption (impaired ability to render food B12 bioavailable) |
| a. Protein-bound vitamin B12 malabsorption |
| b. Mild, nonimmune, chronic atrophic gastritis |
| c. Use of metformin |
| d. Use of drugs that block stomach acid |
| e. Chronic pancreatic disease |
| 2. Dietary deficiency |
| a. Adults: vegans/vegetarian diet, or diet low in meat and dairy products |
| b. Infants: breastfeeding in infants with vitamin B12-deficient mothers |
| A. Severe deficiency |
| 1. Severe malabsorption (affecting the physiological intrinsic factor cubam receptor axis) |
| a. Pernicious anemia (autoimmune gastritis) |
| b. Total or partial gastrectomy |
| c. Gastric bypass or other bariatric surgery |
| d. Ileal resection or organ reconstructive surgery (ileal conduit diversion & ileocystoplasty) |
| e. Inherited disorders affecting B12 absorption (affecting either intrinsic factor or the cubam receptor) |
| 2. Abuse of nitrous oxide |
| 3. Inherited metabolic |
| a. Impaired ability to transport B12 (TC deficiency) |
| b. Impaired ability to process B12 (8 distinct inborn errors of cobalamin metabolism resulting in homocystinuria and/or methylmalonic acidemia) with varying clinical spectra involving the nervous system and blood |
| B. Mild to moderate deficiency |
| 1. Mild to moderate malabsorption (impaired ability to render food B12 bioavailable) |
| a. Protein-bound vitamin B12 malabsorption |
| b. Mild, nonimmune, chronic atrophic gastritis |
| c. Use of metformin |
| d. Use of drugs that block stomach acid |
| e. Chronic pancreatic disease |
| 2. Dietary deficiency |
| a. Adults: vegans/vegetarian diet, or diet low in meat and dairy products |
| b. Infants: breastfeeding in infants with vitamin B12-deficient mothers |