| Nutritional deficiency |
| Substance abuse |
| Alcoholism |
| Poor dietary intake |
| Overcooked foods |
| Depressed patients |
| Nursing homes |
| Malabsorption |
| Celiac disease (sprue) |
| Inflammatory bowel disease |
| Infiltrative bowel disease |
| Short bowel syndrome |
| Drugs (various mechanisms) |
| Methotrexate |
| Trimethoprim |
| Ethanol |
| Phenytoin |
| Increased requirements |
| Pregnancy, lactation |
| Chronic hemolysis |
| Exfoliative dermatitis |
