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Table 1 Clinical classification of lactose intolerance

From: Lactose intolerance and gastrointestinal cow’s milk allergy in infants and children – common misconceptions revisited

Developmental lactase deficiency Observed in premature infants (less than 34 weeks of gestation) due to temporary lactase deficiency which improves with time. The peak lactase expression is reached at term when an infant typically tolerates up to 60-70 g of lactose per day, corresponding with one liter of breast milk.
Congenital lactase deficiency (alactasia) Rare and severe autosomal recessive disorder presenting in newborn infants with severe osmotic diarrhea at commencement of breast feeding. Case reports are mainly from Finland and Western Russia. Small intestinal lactase activity is completely absent. The small intestinal mucosa is otherwise normal.
Lactase non-persistence (hypolactasia) Physiological gradual decline of lactase activity after weaning. This occurs in about 70% of the global population. Significant gastrointestinal symptoms generally do not occur before 5 years of age. The peak onset is in teenagers and young adults. Small amounts of lactose are tolerated by most affected individuals if taken in divided amounts during the day (up to 24 g per day in older children and adults).
Secondary lactose intolerance May occur as a consequence of small bowel injury due conditions such as viral gastro-enteritis, giardiasis, celiac disease or Crohn’s disease. Rare causes of secondary lactose intolerance include epithelial dysplasia syndromes (e.g. microvillus inclusion disease, tufting enteropathy) which present with severe malabsorption and intestinal failure in early infancy. Infants with glucose-galactose malabsorption have normal lactase activity but present with osmotic diarrhea due to the inability to absorb glucose and galactose (derived from lactose).