The ability to digest lactose is produced by the enzyme β-galactosidase called lactase-phlorizin hydrolase (LPH). The fact that in adulthood this enzyme can still be synthesized appears to be a genetic trait that follows a dominant inheritance pattern and presents a complex genetic basis, as well as being especially common in the descendants of populations that have traditionally practiced cattle domestication.

The LPH enzyme is encoded by the lactase gene (LCT), expressed in the small intestine and although the exact reason for the natural decline in lactase levels over the years is not known. Two genetic variants have been identified that are related to the expression of the LPH enzyme and that its production is not reduced in adulthood. Interestingly, these are not located within the TBI gene, but in a regulatory region called MCM6 that modulates TBI expression.

Certain variants in the MCM6 gene can lead to decreased expression of the TBI gene in intestinal cells which, over the years, can eventually lead to lactose intolerance of foods and intestinal discomfort when lactose is consumed in a certain amount.

It is important to differentiate a person with reduced LPH enzyme activity from other pathologies such as congenital lactase deficiency and secondary hypolactasia. Congenital lactase deficiency is extremely rare and appears at the onset of lactation, caused by severe mutations in both alleles of the LCT gene. On the other hand, secondary hypolactasia is caused by direct damage to the intestinal mucosa resulting in a transient reduction of lactase activity (e.g. celiac disease, Crohn's disease, viral infections and radiation/chemotherapy-induced enteritis), and has no genetic basis.

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