This question originally appeared on Quora.
Answer by Ti Zhao, biomedical engineer:
Sort of. It's somewhat complicated since there are genetic as well as environmental components to this.
Natural lactase production is genetic, but is not the only thing that determines lactose tolerance. Illness, damage to the small intestines, and the composition of your gut bacteria can all affect your level of lactose tolerance.
When lactose enters the body, it's cleaved by the enzyme lactase into glucose and galactose via a process known as hydrolysis.
Once lactose has been cleaved into glucose and galactose, those two simple sugars can be absorbed by the body and used as energy. However, when lactose isn't broken down in the small intestines, it moves to the colon, where it interacts with the bacteria there. Bacteria in the colon can break down some of the lactose, but it produces various gases in the process. The rest of the intact lactose draws water into the colon via osmosis as it passes through. I don't think I need to go into detail as to what excess gas and water in the colon can lead to ... But there you have it, hallmark symptoms of "lactose intolerance."
Lactase (the enzyme that breaks down lactose) is produced by cells in the lining of the small intestine. All humans have the ability to produce lactase during infancy, but for many, that ability is turned off in adulthood. In Caucasians, the rs4988235 variant in the MCM6 gene acts as the switch to regulate the expression of the lactase gene (LCT). Individuals can have one of three genotypes (GG, AG, and AA). Those with the GG genotype are known as lactase non-persistent (aka likely, though not necessarily, lactose intolerant). Lactose intolerance resulting from the downregulation of LCT is known as primary lactose intolerance. If you're really curious,
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