Lactose intolerance results from lactose malabsorption and is primarily due to alactasia (total absence of lactase activity) or hypolactasia (low lactase activity).1 Lactose intolerance is diagnosed when there is a symptomatic response produced in response to substantial lactose malabsorption e.g. diarrhea, abdominal discomfort, bloating.5

It is important to distinguish between primary lactase deficiency related to genetics and secondary causes of lactase deficiency, including coeliac disease, infectious enteritis, or Crohn’s disease, which have distinct pathogenic and therapeutic implications.5 Lactose intolerance can be further subdivided into four classifications depending on its origin.1

Congenital Lactase Deficiency
Congenital lactase deficiency is a very rare genetic disorder and is characterised by a total absence or significant reduction of the lactase enzyme.1 This condition is present at birth and persists throughout the individual’s life.1 There have only been a few dozen cases reported, mainly in Finland.1

Developmental (Neonatal) Lactase Deficiency
Developmental lactase deficiency is defined as the low level of lactase activity observed in preterm infants (28-32 weeks).1 Lactase and other disaccharide enzymes are deficient until at least 34 weeks’ gestation and reach maximum activity at birth.1

Primary lactase deficiency or adult hypolactasia
Primary lactase deficiency results from a progressive and permanent decrease in lactase activity.1 Approximately 70% of the world’s population have primary lactase deficiency.2 The prevalence of primary lactase deficiency varies according to geographical region, ethnicity and is also related to the consumption of dairy products in the diet, resulting in genetic selection of individuals with varied capacities to digest lactose.2 Although primary lactase deficiency may present with a relatively acute onset of milk intolerance, its onset typically is subtle and progressive over many years. Most lactase deficient individuals experience maximum onset of symptoms in late adolescence and adulthood.3

Secondary lactase deficiency
Secondary lactase deficiency is a temporary condition that occurs when the production of the enzyme lactase is interrupted due to different mechanisms such as chronic enteropathy, atrophy of villi or other gastrointestinal diseases that can damage the brush border e.g. infections.1 Lactase is the first disaccharidase enzyme to be compromised and the last to regenerate following damage to the gut.1 This condition is usually reversible once the underlying pathology has been resolved resulting in restoration of normal lactase activity.6