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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.
A deficiency of lactase — an enzyme produced in your small intestine — is usually responsible for lactose intolerance. Many people have low levels of lactase but are able to digest milk products without problems. If you're actually lactose intolerant, though, your lactase deficiency leads to symptoms after you eat dairy foods.
Most people with lactose intolerance can manage the condition without having to give up all dairy foods.
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
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.
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.
Primary lactose intolerance
This is the most common type of lactose intolerance. People who develop primary lactose intolerance start life producing plenty of lactase — a necessity for infants, who get all their nutrition from milk. As children replace milk with other foods, their lactase production normally decreases, but remains high enough to digest the amount of dairy in a typical adult diet.
In primary lactose intolerance, lactase production falls off sharply, making milk products difficult to digest by adulthood. Primary lactose intolerance is genetically determined, occurring in a large proportion of people with African, Asian or Hispanic ancestry. The condition is also common among those of Mediterranean or Southern European descent.
Secondary lactose intolerance
This form of lactose intolerance occurs when your small intestine decreases lactase production after an illness, injury or surgery involving your small intestine. Among the diseases associated with secondary lactose intolerance are celiac disease, bacterial overgrowth and Crohn's disease. Treatment of the underlying disorder may restore lactase levels and improve signs and symptoms, though it can take time.
Congenital or developmental 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, meaning that both the mother and the father must pass on the same gene variant for a child to be affected. Premature infants may also have lactose intolerance because of an insufficient lactase level.
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 may 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 have received radiation therapy for cancer in your abdomen or have intestinal complications from chemotherapy, you have an increased risk of lactose intolerance.
Your doctor may suspect lactose intolerance based on your symptoms and your response to reducing the amount of dairy foods in your diet. Your doctor can confirm the diagnosis by conducting one or more of the following tests:
Lactose tolerance test. The lactose tolerance test gauges your body's reaction to a liquid that contains high levels of lactose. Two hours after drinking the liquid, you'll undergo blood tests to measure the amount of glucose in your bloodstream. If your glucose level doesn't rise, it means your body isn't properly digesting and absorbing the lactose-filled drink.
Hydrogen breath test. This test also requires you to drink a liquid that contains high levels of lactose. Then your doctor measures the amount of hydrogen in your breath at regular intervals. Normally, very little hydrogen is detectable. However, if your body doesn't digest the lactose, it will ferment in the colon, releasing hydrogen and other gases, which are absorbed by your intestines and eventually exhaled. Larger than normal amounts of exhaled hydrogen measured during a breath test indicate that you aren't fully digesting and absorbing lactose.
Stool acidity test. For infants and children who can't undergo other tests, a stool acidity test may be used. The fermenting of undigested lactose creates lactic acid and other acids that can be detected in a stool sample.
There's currently no way to boost your body's production of lactase, but you can usually avoid the discomfort of lactose intolerance by:
Avoiding large servings of milk and other dairy products
Including small servings of dairy products in your regular meals
Eating and drinking lactose-reduced ice cream and milk
Drinking regular milk after you add a liquid or powder to it to break down the lactose
Lifestyle and home remedies
With some trial and error, you may be able to predict your body's response to different foods containing lactose and figure out how much you can eat or drink without discomfort. Few people have such severe lactose intolerance that they have to cut out all milk products and be wary of nondairy foods or medications that contain lactose.
Maintain good nutrition
Reducing the dairy products doesn't mean you can't get enough calcium. Calcium is found in many other foods, such as:
Calcium-fortified products, such as breads and juices
Milk substitutes, such as soy milk and rice milk
Also make sure you get enough vitamin D, which is typically supplied in fortified milk. Eggs, liver and yogurt also contain vitamin D, and your body makes vitamin D when you spend time in the sun. Even without restricting dairy foods, though, many adults don't get enough vitamin D. Talk to your doctor about taking vitamin D and calcium supplements to be sure.
Limit dairy products
Most people with lactose intolerance can enjoy some milk products without symptoms. It may be possible to increase your tolerance to dairy products by gradually introducing them into your diet. Some people find that they can tolerate full-fat dairy products, such as whole milk and cheese, more easily than dairy products with no or reduced fat.
Ways to change your diet to minimize symptoms of lactose intolerance include:
Choosing smaller servings of dairy. Sip small servings of milk — up to 4 ounces (118 milliliters) at a time. The smaller the serving, the less likely it is to cause gastrointestinal problems.
Saving milk for mealtimes. Drink milk with other foods. This slows the digestive process and may lessen symptoms of lactose intolerance.
Experimenting with an assortment of dairy products. Not all dairy products have the same amount of lactose. For example, hard cheeses, such as Swiss or cheddar, have small amounts of lactose and generally cause no symptoms. You may be able to tolerate cultured milk products, such as yogurt, because the bacteria used in the culturing process naturally produce the enzyme that breaks down lactose.
Buying lactose-reduced or lactose-free products. You can find these products at most supermarkets in the refrigerated dairy section.
Using lactase enzyme tablets or drops. Over-the-counter tablets or drops containing the lactase enzyme (Dairy Ease, Lactaid, others) may help you digest dairy products. You can take tablets just before a meal or snack. Or the drops can be added to a carton of milk. Not everyone with lactose intolerance is helped by these products.
Probiotics are living organisms present in your intestines that help maintain a healthy digestive system. Probiotics are also available as active or "live" cultures in some yogurts and as supplements in capsule form. They are sometimes used for gastrointestinal conditions, such as diarrhea and irritable bowel syndrome. They may also help your body digest lactose. Probiotics are generally considered safe and may be worth a try if other methods don't help.
Preparing for an appointment
Start by seeing your family doctor or a general practitioner if you have signs or symptoms that suggest you may have lactose intolerance. Because appointments can be brief, it's a good idea to be prepared for your appointment. Here's some information to help you get ready, and know what to expect from your doctor.
What you can do
Be aware of any preappointment restrictions. At the time you make the appointment, be sure to ask if there's anything you need to do in advance, such as restrict your diet.
Write down any symptoms you're experiencing, including any that may seem unrelated to the reason for which you scheduled the appointment.
Make a list of all medications, vitamins or supplements that you're taking.
Write down questions to ask your doctor.
Questions you may want to ask your doctor about lactose intolerance include:
Are my symptoms caused by lactose intolerance?
Are there other possible causes for my symptoms?
What kinds of tests do I need? Do these tests require any special preparation?
Is lactose intolerance a lifelong condition, or could it go away?
What are my treatment options?
Must I stop eating all dairy products?
How can I be certain that I'm getting enough calcium in my diet?
Should I see a dietitian?
I have these other health conditions. How can I best manage these conditions together?
Are there any brochures or other printed material that I can take with me? What websites do you recommend?
Do I need to come in for periodic follow-up visits? If so, how often?
What you can do in the meantime
Keep track of your daily servings of different dairy foods, including milk, ice cream, yogurt and cottage cheese, along with when you have them and what you eat with them. Also let your doctor know which dairy foods, in what amounts, are likely to give you symptoms. This information can help your doctor make a diagnosis.
If you think you may have lactose intolerance, try cutting dairy products from your diet for a few days to see if your symptoms ease. Let your doctor know if your symptoms got better on the days you didn't have dairy products.