According to the National Institute for Health (NIH), approximately 65% of the human population has a reduced ability to digest lactose after infancy. Lactose is a natural sugar commonly found in milk and dairy products. Lactase is an enzyme found in the small intestines which breaks lactose down into more simple sugars to be absorbed into the body.  When there is a deficiency in lactase, undigested lactose in the intestines pulls in water and gets fermented by bacteria which results in gas production.