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Lactase persistence or lactose tolerance is the continued activity of the lactase enzyme in adulthood, allowing the digestion of lactose in milk. In most mammals , the activity of the enzyme is dramatically reduced after weaning . [ 1 ]
Lactase (EC 3.2.1.108) is an enzyme produced by many organisms and is essential to the complete digestion of whole milk. It breaks down the sugar lactose into its component parts, galactose and glucose .
[23] [24] In areas with lactase persistence, it is believed that by domesticating animals, a source of milk became available while an adult and thus strong selection for lactase persistence could occur; [21] [25] in a Scandinavian population, the estimated selection coefficient was 0.09-0.19. [25]
In most mammals, production of lactase diminishes after infants are weaned from maternal milk. However, 5% to 90% of the human population possess an advantageous autosomal mutation in which lactase production persists after infancy. The geographic distribution of lactase persistence is concordant with areas of high milk intake.
Some populations have developed genetic changes to allow the digestion of lactose: lactase persistence. [78] Other populations developed cooking methods like milk fermentation. [78] Lactase persistence in humans evolved relatively recently (in the last 10,000 years) among some populations.
Thus, even in the case of lactase-persistence there is a huge time delay between the onset of a new habit and the spread of the adaptive allele and so milk consumption may have been restricted to children or to lactose-reduced products. Another example of mutation positively selected by the switch to agriculture is the number of AMY1 gene copies.
The enzyme lactase is required for degradation of the milk sugar lactose and is present at high levels in infants, but in most populations will decrease after weaning or during infancy, potentially leading to lactose intolerance in adulthood.
Alpha-lactalbumin, which is expressed in response to prolactin, increases the affinity of N-acetyllactosamine synthase for its substrate, causing increased production of lactose during lactation. The interaction that facilitates lactose biosynthesis consists of a-lactalbumin (the regulatory unit) binding reversibly to the glycosyltransferase.