| Primary |
| Race and ethnicity |
| Developmental enzyme deficiency |
| Congenital lactase deficiency |
| Secondary |
| Small intestinal bacterial overgrowth |
| Infectious enteritis |
| Giardiasis |
| Mucosal injury |
| Celiac disease |
| Inflammatory bowel disease (especially Crohn disease) |
| Drug- or radiation-induced enteritis |
