Lactose, the sugar found in dairy products, is digested by the lactase enzyme. Lactase is produced in the small intestine and is responsible for breaking down lactose into two smaller and more readily absorbable sugars – glucose and galactose.
Most mammals produce high levels of the lactase enzyme as infants, and then lactase production decreases after weaning. However, due to our reliance on dairy products as a food source, many humans now have lactase persistence, where the lactase enzyme is continually produced throughout adulthood. These lactase persistent individuals can continue to digest lactose throughout their lifetime.
Lactase persistence occurs in more than 75% of Caucasians around the world, and approximately 95% of people of Northern European ancestry. Other ethnicities have a much lower proportion of lactase persistence (e.g. approximately 50% of Mexicans, only 10% of Asians and less than 5% of Native Americans).