By Thomas Yuang Li, IV Form

The Evolution of Lactose Tolerance

Some people are lactose tolerant because they are lactase persistent (LP). Lactase is an enzyme responsible for the hydrolysis reaction or breakdown of lactose, the sugar in milk, and it helps human digest milk. LP is the continued activity of lactase in human adulthood. LP occurs because of the inheritance of genetic mutations that result in LP. It became prevalent in some societies due to evolution through natural selection.  As of 2018, around 65% of the world population experienced lactose intolerance (NIH). The distribution gradient is uneven, for about 90% of the population in East Asia are lactose intolerant while about only 10% of the population in North Western Europe are lactose intolerant.

Lactose is a disaccharide that contains a galactose and a glucose unit. Lactose makes up 6-8% of milk. Humans can only digest lactose with the help of lactase, making lactase critical in the digestion of milk. Lactase is produced by small intestine cells. It accelerates the hydrolysis reaction or breakdown of lactose. Through hydrolysis, lactose is broken down into glucose and galactose, which are monosaccharides, or simple sugar molecules, that can be absorbed into the human bloodstream. Lactase catalyzes this hydrolysis reaction by lowering the activation energy, the minimum energy required to initiate a chemical reaction. Without sufficient lactase, humans cannot fully digest lactose. As shown in Figure 1, the undigested lactose then passes on to the large intestine, where it interacts with bacteria and ferments. The fermentation causes symptoms of lactose intolerance, which range from bloating to diarrhea.