Table 2

Glossary with definitions related to lactase deficiency, lactose malabsorption and lactose intolerance

Congenital lactase deficiencyCLDVery rare genetic disorder (typically frameshift mutations) leading to lack of expression of lactase and severe symptoms immediately after birth
Lactase non-persistenceLNPDecrease of intestinal lactase expression in the first two decades of life. Phenotype in most individuals worldwide (biological wildtype)
Lactase persistenceLPContinued expression of intestinal lactase expression beyond infancy; dominant phenotype in Western countries.
Lactase deficiencyLDInability to digest large amounts of lactose due to low lactase expression in the small intestine
Lactose malabsorptionLMPassage of lactose into the large intestine as a consequence of LD or other pathology (eg, rapid transit)
Primary lactose malabsorptionLactose malabsorption due to lactase non-persistence (dominant phenotype worldwide).
Secondary lactose malabsorptionLactose malabsorption due to lower lactase expression, typically in the setting of intestinal inflammation (may be reversible).
Lactose intoleranceLIAppearance 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 intoleranceSymptoms of LI on lactose challenge in individuals without lactose malabsorption.
Self-reported lactose intoleranceSLIHistory of LI symptoms without formal testing of either LM or LI.