Glossary with definitions related to lactase deficiency, lactose malabsorption and lactose intolerance
| Concept | Definition | |
| Congenital lactase deficiency | CLD | Very rare genetic disorder (typically frameshift mutations) leading to lack of expression of lactase and severe symptoms immediately after birth |
| Lactase non-persistence | LNP | Decrease of intestinal lactase expression in the first two decades of life. Phenotype in most individuals worldwide (biological wildtype) |
| Lactase persistence | LP | Continued expression of intestinal lactase expression beyond infancy; dominant phenotype in Western countries. |
| Lactase deficiency | LD | Inability to digest large amounts of lactose due to low lactase expression in the small intestine |
| Lactose malabsorption | LM | Passage of lactose into the large intestine as a consequence of LD or other pathology (eg, rapid transit) |
| Primary lactose malabsorption | Lactose malabsorption due to lactase non-persistence (dominant phenotype worldwide). | |
| Secondary lactose malabsorption | Lactose malabsorption due to lower lactase expression, typically in the setting of intestinal inflammation (may be reversible). | |
| Lactose intolerance | LI | Appearance of typical intestinal symptoms such as abdominal pain, bloating, diarrhoea in individuals with LM after lactose ingestion determined by appropriate testing (ideally blinded testing). |
| Functional lactose intolerance | Symptoms of LI on lactose challenge in individuals without lactose malabsorption. | |
| Self-reported lactose intolerance | SLI | History of LI symptoms without formal testing of either LM or LI. |