Lactose intolerance (also milk sugar intolerance, hypolactasia) is caused by lactose (also known as milk sugar) malabsorption due to reduced activity or the enzyme lactase. Due to this, a smaller or larger portion of lactose remains unprocessed and unabsorbed in the small intestines. As a result, the microbes in the large intestine cause gas and acids, which may give rise to abdominal discomfort: excessive flatulence and bloating, stomach growling, stomach pain and diarrhoea.
In most people, the body produces lactase throughout their life, but the activity of its production is the highest among infants and toddlers, as in this phase milk is the primary food that the body needs. When other foods are introduced in the menu, the need for a large amount of lactase disappears and its activity is gradually reduced in time. This is why older people may be less tolerant to dairy products than children and teens. Some lactase activity (usually 50–70%) is preserved.