Basically, it is the inability of the body to digest lactose
As with most nutrients in food, lactose must be split into smaller units before the body can
properly absorb it. Our bodies make special substances - called enzymes - that help break down
the food. The enzyme which acts specifically on lactose is called lactase. Some
people don't produce enough lactase enzyme, so the lactose in thier food is not properly
broken down, and the undigested lactose goes on to cause troublesome symptoms.
