Infants are usually born with high levels of lactase. However, in some ethnic groups the activity of lactase experiences a genetically controlled decrease after weaning1. Such an effect is known to lead to lactose malabsorption which refers to the inefficient digestion of lactose2.
Lactose intolerance is the condition that encompasses the symptoms of malabsorption. Since lactose can access the large intestine, it is exposed to a dense community of bacteria which are able to ferment it and produce metabolites and gases. This is reflected in intestinal discomfort in individuals who suffer from bloating, abdominal pain, nausea and diarrhoea2.
In some special cases, individuals can lack lactase due to a rare inherited disorder. Lactose intolerance can also be secondary to some conditions (e.g inflammatory bowel disease) affecting the gut mucosa since lactase is expressed in this region3 .
1. Heymand et al. (2006) Pediatrics 118 (3) 1279-1286
2. Misselwitz et al. (2013) United European Gastroenterol J. 1(3): 151–159.
3. Insel et al. (2004). Nutrition, 2nd edition. Sudbury, MA: Jones and Bartlett.