Milk intolerance (lactose intolerance) occurs when the body is unable to produce enough of the enzyme lactase, which is needed to break down lactose for easy digestion. It is important to note that milk intolerance is not the same as a milk allergy, because milk intolerance does not involve the immune system, which is a key player in all types of allergic reactions.

Lactose is a naturally occurring sugar found in milk and dairy products. The lactase enzyme, which is produced by the cells that line the small intestine, breaks lactose down into the sugars glucose and galactose. This reaction occurs in the small intestine as a normal part of digestion. Once lactose has been properly broken down by the lactase, it can be easily absorbed into the bloodstream.

Some individuals are lactase deficient, meaning their small intestines do not produce enough lactase to break down all the lactose. This deficiency could be the result of damage sustained to the lower intestine or due to an inherited intolerant condition. In many cases, a lactase-deficient person may not experience symptoms. However, those individuals who do experience symptoms are considered lactose intolerant.  

If lactose is not properly broken down and digested, water remains in the bowel. This can result in bloating and watery diarrhea. The lactose that passes on into the large intestine comes into contact with naturally occurring bacteria, which ferments the lactose. The result is the production of carbon dioxide, hydrogen and methane. This excess gas leads to additional bloating, cramping and flatulence (passing gas). The hydrogen gas often travels through the bloodstream to the lungs, where it can cause bad breath.

The ability of a person to break down lactose and tolerate dairy products varies greatly by individual. For example, a person who has sustained severe damage to the lower intestine may be unable to tolerate any food containing lactose.

Different types of dairy (e.g., milk, cheese) contain different amounts of lactose. For this reason, some people with milk intolerance can eat some types of dairy without symptoms, while other individuals with milk intolerance must avoid dairy entirely.

According to the National Digestive Diseases Information Clearinghouse, between 30 million and 50 million Americans are intolerant to lactose to some degree. Almost 75 percent of all African Americans and 90 percent of Asian Americans are intolerant to lactose.

Milk intolerance can begin at various ages. In Caucasians, the condition usually begins to affect children over the age of 5. In African Americans, the condition can occur in children as young as 2.