Table 3.

Definition of lactose- and lactase-related concepts

Term	Definition
Lactase persistence (LP)	Persistence of high lactase expression activity at the jejunal brush border into adulthood, resulting in individuals with the ability to digest lactose beyond weaning phase
Lactase nonpersistence (LNP)	The natural decline in lactase expression activity at the jejunal brush border post- weaning, resulting in some individuals experiencing symptoms due to minimal ability to digest lactose
Estimated lactase persistence (LP) and lactase nonpersistence (LNP) rates	Percentages of population according to lactose digestion status as determined by combined measurements of indirect (e.g., breath hydrogen, blood glucose, or urinary galactose excretion following lactose challenge) and direct genotyping methods. The latter evaluates the expression levels of lactase (LCT) polymorphic variants which can be used to differentiate among homozygous digesters, maldigesters, and heterozygous persons with intermediate LCT activity
Lactose malabsorption (LM)	Inefficient or incomplete digestion of lactose due to LNP (primary) or other intestinal pathologies (secondary)
Lactose intolerance (LI)	Gastrointestinal symptoms presented in individuals with LM, including flatus, gas, bloating, cramps, diarrhea, and vomiting (rarely)
Lactose sensitivity	Systemic symptoms (e.g., nausea, depression, headache, fatigue) with or without the presence of LI symptoms
Lactase deficiency (LD)	Reduction of intestinal LCT expression activity due to either genetic or secondary causes such as diseases or injury to the proximal small bowel mucosa