12.9: Lactose Intolerance

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Lactose intolerance is the inability of the small intestine to digest lactose, a sugar found in milk, due to a lack of the enzyme lactase.

Master this section and you'll be able to:

• Explain what causes lactose intolerance and how undigested lactose leads to GI symptoms.

• Distinguish lactose intolerance from milk allergy and describe basic diagnostic and dietary management options.

Food intolerance occurs when a person has difficulty digesting a specific food or nutrient, causing unpleasant GI symptoms such as gas, bloating, flatulence, cramping, and diarrhea. Food intolerances are commonly caused by the body not producing enough of a particular digestive enzyme, so the symptoms generally involve the digestive system, and the severity of symptoms usually correlates with how much of the food was eaten. Unlike food allergies, the immune system does not play a role in food intolerance, and while the symptoms are unpleasant, they are generally not dangerous and will subside once the food passes out of the GI tract. People with food intolerances can also often consume small amounts of the offending food without symptoms.

The most common food intolerances in the US, affecting a significant portion of the population, are lactose, wheat, and shellfish sensitivities.  Let's take a closer look at the first one.

Cause of Lactose Intolerance

In people with lactose intolerance, the absorptive enterocytes of the small intestine do not produce enough of the enzyme lactase, which is responsible for digesting the milk sugar lactose into single sugar molecules (glucose and galactose) that can be absorbed in the small intestine.

Figure \(\PageIndex{1}\): Lactase and the Breakdown of Lactose. The brush border enzyme lactase, located on the apical surface of small-intestinal enterocytes, splits the disaccharide lactose into its two absorbable monosaccharides, glucose and galactose. Lactase sits in the microvilli of the small intestine and performs the final step of carbohydrate digestion before these simple sugars are transported into the bloodstream. When lactase levels are low, undigested lactose moves into the large intestine and may cause gas, bloating, and diarrhea.

Undigested lactose can’t be absorbed, so it continues on to the large intestine. There, it draws more water into the large intestine, and bacteria metabolize the lactose, resulting in gas and acid production. These conditions cause the uncomfortable symptoms of gas, bloating, and diarrhea within about 30 minutes to two hours of consuming dairy foods.

Primary lactase deficiency is genetic, only affects adults, and is caused by the absence of a lactase persistence allele. It is the most common cause of lactose intolerance as a majority of the world’s population lacks these alleles. Secondary, acquired, or transient lactase deficiency is caused by an injury to the small intestine, usually during infancy, from acute gastroenteritis, diarrhea, chemotherapy, intestinal parasites, or other environmental causes. Congenital lactase deficiency is a very rare, autosomal recessive genetic disorder that prevents lactase expression from birth. It is particularly common in Finland. People with congenital lactase deficiency who are unable to digest lactose from birth are unable to digest breast milk.

As with most food intolerances, people with lactose intolerance can often consume some amount of lactose without discomfort, although this varies from person to person. Aged hard cheese, buttermilk, and yogurt (as long as it doesn’t include added milk solids) are often well-tolerated because they are low in lactose, which is consumed by bacteria during fermentation and aging. In addition, lactose-free milk and lactase enzyme supplements are available. Dairy products are some of the main dietary sources of calcium and vitamin D, so people who avoid dairy need to take special care to include other sources of these nutrients in their diets.²

The vast majority of humans are born with the ability to digest lactose. All mammalian milk, including human milk, contains lactose, so historically, infants with lactose intolerance wouldn’t have survived. (Today, infants with lactose intolerance can consume soy-based infant formula.) Beyond infancy, lactose intolerance depends on your genes. In much of the world, it’s common for the activity of the lactase gene to decline with age, resulting in less lactase production and more lactose intolerance. Worldwide, 65% of the human population has some degree of lactose intolerance in adulthood. On the other hand, lactose tolerance is common in cultures where early domestication of dairy animals provided an important source of nutrition. In the U.S., adults of European descent can often tolerate lactose, whereas lactose intolerance is common among Asian Americans, African Americans, Mexican Americans, and Native Americans.

Figure \(\PageIndex{2}\): Lactase Enzyme Supplement. Taking a lactase enzyme supplement allows many people with lactose intolerance to eat dairy products without suffering symptoms. ( “Home-breakfast (lactaid photo)” by Ernesto Andrade is licensed under CC BY-ND 2.0)
 

Symptoms of Lactose Intolerance

Since in people with lactose intolerance, the lactose in chyme is not digested, bacteria in the large intestine ferment the undigested lactose, a process that produces gas. In addition to gas, symptoms include abdominal cramps, bloating, and diarrhea, and/or vomiting after consuming significant amounts of lactose.

Since individuals may be lactose intolerant to varying degrees, symptom severity ranges from mild discomfort to severe pain. All symptoms resolve once the lactose is eliminated in feces. 
 

Assessment and Treatment of Lactose Intolerance

To assess lactose intolerance, intestinal function is challenged by ingesting more dairy products than can be readily digested. Clinical symptoms typically appear within 30 minutes, but may take up to two hours, depending on other foods and activities. Substantial variability in response (symptoms of nausea, cramping, bloating, diarrhea, and flatulence) is to be expected, as the extent and severity of lactose intolerance varies among individuals.

It is important to distinguish lactose intolerance from milk allergy, an abnormal immune response (usually) to milk proteins. This may be done in diagnosis by giving lactose-free milk, producing no symptoms in the case of lactose intolerance, but the same reaction as to normal milk if it is a milk allergy. An intermediate result might suggest that the person has both conditions. Since lactose intolerance is the normal state for most adults worldwide, it is not considered a disease and a medical diagnosis is not normally required, although tests are available if confirmation is necessary.

Lactose intolerance is not considered a condition that requires treatment in societies where the diet contains relatively little dairy. However, those living among societies that are largely lactose-tolerant may find lactose intolerance troublesome. Although there are still no methodologies to reinstate lactase production, some individuals have reported that their intolerance varies over time, depending on health status and pregnancy. About 44% of lactose intolerant women regain the ability to digest lactose during pregnancy. This might be caused by slow intestinal transit and intestinal flora changes during pregnancy.

The hydrogen breath test is used to help diagnose lactose intolerance. Lactose-tolerant people have very little hydrogen in their breath. Those with lactose intolerance exhale hydrogen, which is one of the gases produced by the bacterial fermentation of lactose in the colon. After the hydrogen is absorbed from the intestine, it is transported through blood vessels into the lungs. There are a number of lactose-free dairy products available in grocery stores. In addition, dietary supplements are available. Taken with food, they provide lactase to help digest