The symptoms begin thirty minutes to two hours after diary products are consumed, and last a full day or longer. People who suffer from lactose intolerance either lack or don't produce enough of an enzyme called lactase. Cells in the digestive tract that line the small intestine normally produce lactase. These sugars can be readily absorbed into the bloodstream. Without lactase, the lactose in dairy products just lingers in the intestine where it ferments and causes the unpleasant symptoms.
Lactose intolerance tends to develop slowly over time.
The human body naturally begins to produce less lactase after the age of 2, and people become more and more lactase deficient as they grow older. But for some people, this process is accelerated. They eventually begin to suffer symptoms every time they drink milk or eat certain kinds of dairy products. Lactase deficiency is also associated with some types of digestive disease. It can also develop following an injury to the small intestine.