The signs and symptoms of lactose intolerance usually begin 30 minutes to two hours after eating or drinking foods that contain lactose. Common signs and symptoms include:
- Nausea, and sometimes, vomiting
- Abdominal cramps
Symptoms are usually mild, but may sometimes be severe.
When to see a doctor
Make an appointment with your doctor if you or your child has any signs or symptoms that worry you.
Lactose intolerance is usually caused by low levels of the enzyme lactase in your small intestine that lead to signs and symptoms.
Normally, the cells that line your small intestine produce an enzyme called lactase. The lactase enzyme attaches to lactose molecules in the food you eat and breaks them into two simple sugars — glucose and galactose — which can be absorbed into your bloodstream.
Without enough of the lactase enzyme, most of the lactose in your food moves unprocessed into the colon, where the normal intestinal bacteria interact with it. This causes the hallmarks of lactose intolerance — gas, bloating and diarrhea.
There are three types of lactose intolerance.
Normal result of aging for some people (primary lactose intolerance)
Normally, your body produces large amounts of lactase at birth and during early childhood, when milk is the primary source of nutrition. Usually your lactase production decreases as your diet becomes more varied and less reliant on milk. This gradual decline may lead to symptoms of lactose intolerance.
Result of illness or injury (secondary lactose intolerance)
This form of lactose intolerance occurs when your small intestine decreases lactase production after an illness, surgery or injury to your small intestine. It can occur as a result of intestinal diseases, such as celiac disease, gastroenteritis and an inflammatory bowel disease like Crohn's disease. Treatment of the underlying disorder may restore lactase levels and improve signs and symptoms, though it can take time.
Condition you're born with (congenital lactose intolerance)
It's possible, but rare, for babies to be born with lactose intolerance caused by a complete absence of lactase activity. This disorder is passed from generation to generation in a pattern of inheritance called autosomal recessive. This means that both the mother and the father must pass on the defective form of the gene for a child to be affected. Infants with congenital lactose intolerance are intolerant of the lactose in their mothers' breast milk and have diarrhea from birth. These babies require lactose-free infant formulas. Premature infants may also have lactose intolerance because of an insufficient lactase level. In babies who are otherwise healthy, this doesn't lead to malnutrition.
Factors that can make you or your child more prone to lactose intolerance include:
- Increasing age. Lactose intolerance becomes more common as you age — the condition is uncommon in babies and young children.
- Ethnicity. Lactose intolerance is most common in black, Asian, Hispanic and American Indian people.
- Premature birth. Infants born prematurely may have reduced levels of lactase because this enzyme increases in the fetus late in the third trimester.
- Diseases affecting the small intestine. Small intestine problems that can cause lactose intolerance include bacterial overgrowth, celiac disease and Crohn's disease.
- Certain cancer treatments. If you have received radiation therapy for cancer in your abdomen or have intestinal complications from chemotherapy, you have an increased risk of lactose intolerance.