Table 2.

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.