People with lactose intolerance are unable to fully digest the sugar (lactose) in milk. As a result, they have diarrhea, gas and bloating after eating or drinking dairy products. The condition, which is also called lactose malabsorption, is usually harmless, but its symptoms can be uncomfortable.  

Too little of an enzyme produced in your small intestine (lactase) is usually responsible for lactose intolerance. You can have low levels of lactase and still be able to digest milk products. But if your levels are too low you become lactose intolerant, leading to symptoms after you eat or drink dairy.

Colon and small intestine:

The small intestine and colon are parts of your digestive tract, which processes the foods you eat. The intestines take nutrients from the foods. What isn't absorbed by the intestines continues along the digestive tract and is passed as stool during a bowel movement.  

Most people with lactose intolerance can manage the condition without having to give up all dairy foods.  

Symptoms:

The signs and symptoms of lactose intolerance usually begin from 30 minutes to two hours after eating or drinking foods that contain lactose. Common signs and symptoms include:  

• Diarrhea. 
• Nausea, and sometimes, vomiting.
• Stomach cramps.
• Bloating.
• Gas.

When to see a doctor:

Make an appointment with your doctor if you frequently have symptoms of lactose intolerance after eating dairy foods, particularly if you're worried about getting enough calcium.  

Causes:
 

Lactose intolerance occurs when your small intestine doesn't produce enough of an enzyme (lactase) to digest milk sugar (lactose).  

Normally, lactase turns milk sugar into two simple sugars - glucose and galactose - which are absorbed into the bloodstream through the intestinal lining.  

If you're lactase deficient, lactose in your food moves into the colon instead of being processed and absorbed. In the colon, normal bacteria interact with undigested lactose, causing the signs and symptoms of lactose intolerance.  

There are three types of lactose intolerance. Different factors cause the lactase deficiency underlying each type.  

People who develop primary lactose intolerance - the most common type - start life producing enough lactase. Infants, who get all their nutrition from milk, need lactase.

As children replace milk with other foods, the amount of lactase they produce normally drops, but usually remains high enough to digest the amount of dairy in a typical adult diet. In primary lactose intolerance, lactase production falls off sharply by adulthood, making milk products difficult to digest. 

This form of lactose intolerance occurs when your small intestine decreases lactase production after an illness, injury or surgery involving your small intestine. Diseases associated with secondary lactose intolerance include intestinal infection, celiac disease, bacterial overgrowth and Crohn's disease.

Treatment of the underlying disorder might restore lactase levels and improve signs and symptoms, though it can take time. 

It's possible, but rare, for babies to be born with lactose intolerance caused by a lack of lactase. This disorder is passed from generation to generation in a pattern of inheritance called autosomal recessive, meaning that both the mother and the father must pass on the same gene variant for a child to be affected. Premature infants can also have lactose intolerance because of an insufficient lactase level.

Risk factors:

Factors that can make you or your child more prone to lactose intolerance include:  

• Increasing age. Lactose intolerance usually appears in adulthood. The condition is uncommon in babies and young children.
• Ethnicity. Lactose intolerance is most common in people of African, Asian, Hispanic and American Indian descent.
• Premature birth. Infants born prematurely might have reduced levels of lactase because the small intestine doesn't develop lactase-producing cells until 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've had radiation therapy for cancer in your stomach or you have intestinal complications from chemotherapy, your risk of developing lactose intolerance increases.