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Lactose Digestion in Humans

By Matthew Fox, MD ; Updated August 14, 2017

Lactose is a sugar that is common in milk and other dairy products. Since it is a carbohydrate, it can serve as a primary source of energy for the body. In the intestines lactose is broken down by the enzyme lactase. People have different levels of this enzyme. If the enzyme levels are low or absent, then lactose is acted on by bacteria in the intestine, causing bloating and stomach cramps.

Process of Digestion

Lactose passes from the stomach into the small intestine where the enzyme lactase, also known as beta-D-galactosidase, and lactase-phlorizin hydrolase cleaves the sugar molecule in half. The result is two single sugar units, one glucose molecule and one galactose molecule. These single unit sugar molecules are absorbed from the intestine into the bloodstream to be taken up by the cells and used for energy, according to "Lehninger Principles of Biochemistry" by Drs. David L. Nelson and Michael M. Cox.

Digestion Across the Lifespan

Lactose is found in milk, including human milk. Infants are able to digest lactose because their bodies produce lactase. As a person gets older, lactase is no longer produced in most people, and they lose the ability to digest lactose. Scientists believe that this is due to the reliance on milk during youth in mammals, and reliance on nondairy foods in many populations after infancy. Rarely, someone is born without the lactase enzyme. These people are unable to digest lactose from birth; this is called congenital lactase deficiency.

Prevalence of Lactose Digestion and Problems

The majority of people are unable to digest lactose after they pass through early childhood. As noted in a 2007 article in "Nature Genetics" those who can digest lactose have a genetic difference on chromosome 2 that allows the lactase enzyme to continue to be expressed. Most of these people are descendants of people who lived in areas of the world that relied more heavily on dairy products for nutrition, such as western Europe and east Africa. Those who are descended from non-pastoral societies tend to have higher rates of this inability.

Problems with Digestion

If a person is unable to digest lactose they are said to have lactose intolerance. If a lactose-intolerant person consumes lactose, naturally occurring bacteria in the intestines act on the lactose, resulting in bloating, gas, cramps, nausea and diarrhea. Lactose intolerance can be treated by the avoidance of lactose or the ingestion of the enzyme lactase with dairy products.

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