Lactose (also referred to as milk sugar) is a sugar that is found most notably in milk. Lactose makes up around 2–8% of milk (by weight). The name comes from the Latin word for milk, plus the -ose ending used to name sugars.


Infant mammals are fed on milk by their mothers, which is rich in the carbohydrate Lactose. To digest it, an enzyme called lactase (β-D-galactosidase) is secreted by the intestinal villi. This enzyme cleaves the lactose molecule into its two subunits; glucose and galactose for absorption.

Since lactose occurs mostly in milk, in most mammals the production of its digestive enzyme, lactase, gradually decreases with maturity, due to lack of constant consumption.

Many people who live in Europe, the Middle East, India, and parts of East Africa, maintain normal lactase production into adulthood. In many of these areas, milk from mammals such as cattle, goats, and sheep is used as a large source of food. Hence, it was in these regions that genes for lifelong lactase production first evolved. The genes of lactose tolerance have evolved independently in various ethnic groups.

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