Lactose intolerance occurs when the small intestine does not produce enough of the enzyme lactase. Babies' bodies produce this enzyme so they can digest milk, including breast milk. Before humans became dairy farmers, most people did not continue to drink milk, so their bodies did not produce lactase after early childhood.

People from cultures in which adult consumption of milk and milk products occurred earliest are less likely to suffer from lactose intolerance than those from areas where dairy farming began more recently. As a result, lactose intolerance is more common in Asian, African, African-American, Native American, and Mediterranean populations than it is among northern and western Europeans.

Lactose intolerance can begin at various times in life. In Caucasians, it usually starts to affect children older than five years of age. In African-Americans, lactose intolerance often occurs as early as two to three years of age.

When people with lactose intolerance consume milk products, they may have symptoms such as abdominal bloating, excessive intestinal gas, nausea, diarrhea, and abdominal cramping.

Lactose intolerance is very common in adults and is not dangerous. Many adults have some degree of lactose intolerance by age 20 (approximately 30 million Americans).

Lactose intolerance is sometimes seen in premature babies. Full-term babies generally do not show signs of lactose intolerance until they are at least three years old.

Eliminating milk from the diet can result in a deficiency of calcium, vitamin D, riboflavin, and protein. Therefore, a milk substitute is a necessity. For infants younger than two years old, soy formulas are adequate substitutes. Good alternatives for toddlers are soy or rice milk. Older children may also use lactase-treated cow's milk.

Lactase deficiency may also occur as a result of intestinal diseases such as celiac sprue and gastroenteritis, or it may follow gastroduodenal surgery. Temporary lactase deficiency can result from viral and bacterial enteritis, especially in children, when the mucosal cells of the intestine are injured.