Lactose intolerance is the inability or insufficient ability to digest lactose, a sugar found in milk and milk products. Lactose intolerance is caused by a deficiency of the enzyme lactase, which is produced by the cells lining the small intestine. Lactase breaks down lactose into two simpler forms of sugar called glucose and galactose, which are then absorbed into the bloodstream.
Not all people with lactase deficiency have digestive symptoms, but those who do may have lactose intolerance. Most people with lactose intolerance can tolerate some amount of lactose in their diet.
People sometimes confuse lactose intolerance with cow milk allergy. Milk allergy is a reaction by the body’s immune system to one or more milk proteins and can be life threatening when just a small amount of milk or milk product is consumed. Milk allergy most commonly appears in the first year of life, while lactose intolerance occurs more often in adulthood.
The cause of lactose intolerance is best explained by describing how a person develops lactase deficiency. Primary lactase deficiency develops over time and begins after about age 2 when the body begins to produce less lactase. Most children who have lactase deficiency do not experience symptoms of lactose intolerance until late adolescence or adulthood.
Researchers have identified a possible genetic link to primary lactase deficiency. Some people inherit a gene from their parents that makes it likely they will develop primary lactase deficiency. This discovery may be useful in developing future genetic tests to identify people at risk for lactose intolerance.
Secondary lactase deficiency results from injury to the small intestine that occurs with severe diarrheal illness, celiac disease, Crohn’s disease, or chemotherapy. This type of lactase deficiency can occur at any age but is more common in infancy.
Lactose intolerance is a common condition that is more likely to occur in adulthood, with a higher incidence in older adults. Some ethnic and racial populations are more affected than others, including African Americans, Hispanic Americans, American Indians, and Asian Americans. The condition is least common among Americans of northern European descent.
Infants born prematurely are more likely to have lactase deficiency because an infant’s lactase levels do not increase until the third trimester of pregnancy.
People with lactose intolerance may feel uncomfortable 30 minutes to 2 hours after consuming milk and milk products. Symptoms range from mild to severe, based on the amount of lactose consumed and the amount a person can tolerate. Common symptoms include:
- abdominal pain
- abdominal bloating
Lactose intolerance can be hard to diagnose based on symptoms alone. People may think they suffer from lactose intolerance because they have digestive symptoms; however, other conditions such as irritable bowel syndrome can cause similar symptoms. After taking a medical history and performing a physical examination, the doctor may first recommend eliminating all milk and milk products from the person’s diet for a short time to see if the symptoms resolve. Tests may be necessary to provide more information. Two tests are commonly used to measure the digestion of lactose.
Hydrogen Breath Test. The person drinks a lactose-loaded beverage and then the breath is analyzed at regular intervals to measure the amount of hydrogen. Normally, very little hydrogen is detectable in the breath, but undigested lactose produces high levels of hydrogen. Smoking and some foods and medications may affect the accuracy of the results. People should check with their doctor about foods and medications that may interfere with test results.
Although the body’s ability to produce lactase cannot be changed, the symptoms of lactose intolerance can be managed with dietary changes. Most people with lactose intolerance can tolerate some amount of lactose in their diet. Gradually introducing small amounts of milk or milk products may help some people adapt to them with fewer symptoms. Often, people can better tolerate milk or milk products by taking them with meals.
The amount of change needed in the diet depends on how much lactose a person can consume without symptoms. For example, one person may have severe symptoms after drinking a small glass of milk, while another can drink a large glass without symptoms. Others can easily consume yogurt and hard cheeses such as cheddar and Swiss but not milk or other milk products.
Lactose-free and lactose-reduced milk and milk products, available at most supermarkets, are identical to regular milk except that the lactase enzyme has been added. Lactose-free milk remains fresh for about the same length of time or longer than regular milk if it is ultra-pasteurized. Lactose-free milk may have a slightly sweeter taste than regular milk. Soy milk and other products may be recommended by a health professional.
People who still experience symptoms after dietary changes can take over-the-counter lactase enzyme drops or tablets. Taking the tablets or a few drops of the liquid enzyme when consuming milk or milk products may make these foods more tolerable for people with lactose intolerance.
Lactose intolerance and calcium intake
Milk and milk products are a major source of calcium and other nutrients. Calcium is essential for the growth and repair of bones at all ages. A shortage of calcium intake in children and adults may lead to fragile bones that can easily fracture later in life, a condition called osteoporosis.
Yogurt made with active and live bacterial cultures is a good source of calcium for many people with lactose intolerance. When this type of yogurt enters the intestine, the bacterial cultures convert lactose to lactic acid, so the yogurt may be well-tolerated due to a lower lactose content than yogurt without live cultures. Frozen yogurt does not contain bacterial cultures, so it may not be well-tolerated.
Calcium is absorbed and used in the body only when enough vitamin D is present. Some people with lactose intolerance may not be getting enough vitamin D. Vitamin D comes from food sources such as eggs, liver, and vitamin D-fortified milk and yogurt. Regular exposure to sunlight also helps the body naturally absorb vitamin D. Talking with a doctor or registered dietitian may be helpful in planning a balanced diet that provides an adequate amount of nutrients—including calcium and vitamin D—and minimizes discomfort. A health professional can determine whether calcium and other dietary supplements are needed.
Other lactose products
Milk and milk products are often added to processed foods—foods that have been altered to prolong their shelf life. People with lactose intolerance should be aware of the many food products that may contain even small amounts of lactose, such as:
- bread and other baked goods
- waffles, pancakes, biscuits, cookies, and mixes to make them
- processed breakfast foods such as doughnuts, frozen waffles and pancakes, toaster pastries, and sweet rolls
- processed breakfast cereals
- instant potatoes, soups, and breakfast drinks
- potato chips, corn chips, and other processed snacks
- processed meats, such as bacon, sausage, hot dogs, and lunch meats
- salad dressings
- liquid and powdered milk-based meal replacements
- protein powders and bars
- non-dairy liquid and powdered coffee creamers
- non-dairy whipped toppings
Checking the ingredients on food labels is helpful in finding possible sources of lactose in food products. If any of the following words are listed on a food label, the product contains lactose:
- milk by-products
- dry milk solids
- non-fat dry milk powder
Lactose is also used in some prescription medicines, including birth control pills, and over-the-counter medicines like products to treat stomach acid and gas. These medicines most often cause symptoms in people with severe lactose intolerance.