Lactose intolerance is the inability to digest lactose, a type of sugar found in milk and other dairy products. It is caused by not having enough of the enzyme lactase.

Lactose intolerance happens when the small intestine does not make enough of the enzyme lactase. Babies' bodies make 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 make lactase after early childhood.

People from cultures in which dairy farming occurred earliest are less likely to suffer from lactose intolerance, compared with 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 different times in life. In Caucasians, it usually starts to affect children older than age 5. In African-Americans, lactose intolerance often occurs as early as age 2.

When people with lactose intolerance eat or drink 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. Approximately 30 million American adults have some degree of lactose intolerance by age 20.

Lactose intolerance is sometimes seen in premature babies. Children who were born at full term generally do not show signs of lactose intolerance until they are at least 3 years old.

Not having milk in the diet can result in a shortage of calcium, vitamin D, riboflavin, and protein. Therefore, a milk substitute is needed. For infants younger than 2 years, soy formulas are good substitutes. Good alternatives for toddlers are soy or rice milk. Older children may also use lactase-treated cow's milk. Goat's milk is low in lactose, but should only be used in children if it is properly supplemented with essential amino acids and vitamins.

Most people with low lactase levels can tolerate 2-4 ounces of milk at one time (up to half a cup). Larger (8 oz.) servings may cause problems for people with some degree of milk intolerance.

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

• Abdominal cramps
• Bloating
• Flatulence
• Weight loss
• Malnutrition
• Slow growth
• Abdominal distention
• Diarrhea
• Floating stools
• Foul-smelling stools

Symptoms often follow eating or drinking milk products, and are often relieved by withdrawal of milk products. Large doses of milk products may cause worse symptoms.

• Test for stool-reducing substances
• Lactose tolerance test
• Lactose-hydrogen breath test
• Small bowel biopsy