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Lactose intolerance definition and facts
- Lactose intolerance is an inability to digest lactose, the main sugar in milk, which gives rise to gastrointestinal symptoms.
- Lactose intolerance is caused by a deficiency of the intestinal enzyme lactase that splits lactose into two smaller sugars, glucose and galactose, and allows lactose to be absorbed from the intestine.
- Virtually all individuals are born with lactase and the ability to digest lactose. The disappearance of lactase is either genetically programmed to occur after childhood or is due to diseases of the lining of the intestine that destroys lactase.
- Lactose intolerance that occurs after age 21 (genetically-determined lactase deficiency usually occurs between ages 5-21) is rarely due to genetic lactase deficiency; it suggests another process is interfering with lactose digestion.
- The primary signs and symptoms of lactose intolerance are
- diarrhea,
- flatulence (passing gas),
- abdominal pain, and
- indigestion.
- Other signs and symptoms of lactose intolerance that may occur are:
- Constipation is not a symptom of lactose intolerance.
- The symptoms are similar in adults, toddlers, and infants,
- Among individuals, the severity of the signs and symptoms of lactose intolerance vary and may be provoked by greater or lesser quantities of lactose. Most people are able to tolerate small amounts of lactose even if they are lactase deficient, for example lactose in yogurt. Some people develop severe symptoms with minimal intake of lactose.
- Lactose intolerance can be diagnosed by eliminating lactose from the diet and observing for a disappearance of symptoms or provocation of symptoms with a milk challenge,
- Tests that are useful for diagnosing lactose intolerance or lactase deficiency, include a lactose breath test, blood glucose test, stool acidity test, intestinal biopsy and genetic testing looking for the gene that controls the production of lactase,
- Lactose intolerance is treated with dietary changes, supplements of lactase enzyme, correction of underlying conditions in the small intestine, and, possibly, by adaptation to increasing amounts of milk.
- Lactose intolerance is rarely due to milk allergy in adults. Children with milk allergy, where it is more common, usually outgrow the allergy by adulthood.
- Avoidance of milk and milk-containing products can lead to a dietary deficiency of calcium and vitamin D that, in turn, can lead to bone disease (osteoporosis).
- There is no "cure" for genetically-programmed lactase deficiency with lactose intolerance.
What is lactose intolerance?
Lactose intolerance is the inability to digest and absorb lactose (the sugar in milk) that results in gastrointestinal symptoms when milk or food products containing milk are consumed. It is due either to a genetically-programmed loss of lactase, the intestinal enzyme responsible for the digestion of lactose, or diseases affecting the small intestine that destroy lactase. Genetically programmed loss of lactase occurs between early childhood and age 21. The timing of this loss is determined primarily by ethnicity.
Because diseases of the intestine can occur at any age, lactose intolerance can begin at any age; however, that does not mean that an individual has become genetically lactase deficient.
What causes lactose intolerance?
Lactose is a sugar molecule that is made up of two smaller sugars, glucose and galactose. In order for lactose to be absorbed from the intestine and into the body, it must first be split into glucose and galactose. The glucose and galactose are then absorbed by the cells lining the small intestine. The enzyme that splits lactose into glucose and galactose is called lactase, and it is located on the surface of the cells lining the small intestine.
Lactose intolerance is caused by reduced or absent activity of lactase that prevents the splitting of lactose (lactase deficiency). Lactase deficiency may occur for one of three reasons, congenital, secondary or developmental.
Congenital causes of lactose intolerance
Lactase deficiency may occur because of a congenital absence (absent from birth) of lactase due to a mutation in the gene that is responsible for producing lactase. This is a very rare cause of lactase deficiency, and the symptoms of this type of lactase deficiency begin shortly after birth.
Secondary causes of lactose intolerance
Another cause of lactase deficiency is secondary lactase deficiency. This type of deficiency is due to diseases that destroy the lining of the small intestine along with the lactase. An example of such a disease is celiac disease (sprue).
Genetically programmed causes of lactose intolerance
The most common cause of lactase deficiency is a decrease in the amount of lactase that occurs after childhood and persists into adulthood, referred to as adult-type hypolactasia. This decrease in lactase is genetically programmed. Lactase deficiency (and lactose intolerance) is most common among Asians, affecting more than 90% of adults in some communities. People with ancestry from Northern Europe, on the other hand, have a 5% rate of lactase deficiency. In addition to variability in the prevalence of lactase deficiency among ethnic groups, there also is variability in the age at which symptoms of lactase deficiency (and lactose intolerance) appear.
As people age they may develop lactose intolerance; however, the extent of the intolerance appears to be mild and not associated with clinical symptoms. Therefore, the development of lactose intolerance in the elderly should not be made lightly.
Food Allergies vs. Food Intolerance
Sometimes people become sick from eating a particular
food, because they cannot properly process or digest the food, or because they
have a true allergic (immune) reaction to the food. Food allergies and food intolerance are sometimes confused with each other, but they are quite different in terms of their origin,
symptoms and treatment.
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Other tests to diagnose lactose intolerance
Genetic testing
The appearance and disappearance of lactase in the intestinal lining is controlled by genes. It is possible to analyze DNA, the substance of genes, from cells in the blood to determine if an individual has the genes that program disappearance of lactase. If they do, then they are very likely to be lactose intolerant. Genetic testing is the most direct way of diagnosing congenital or developmental lactase deficiency. Unfortunately, the test is relatively complex, expensive, and often not easily available. Moreover, it usually is not necessary to know an individual’s lactase genetics to diagnose and treat.
Intestinal biopsy
The most direct test for lactase deficiency is biopsy of the intestinal lining with measurement of lactase levels in the lining. The biopsy can be obtained by endoscopy or by special capsules that are passed through the mouth or nose and into the small intestine. The analysis of lactase levels in the biopsy requires specialized procedures that are not often available, and, as a result, lactase levels are not often measured except for research purposes.
Blood glucose test
The blood glucose test is an older test for lactase deficiency and lactose intolerance. For the blood glucose test, lactose is ingested (usually 0.75 to 1.5
gm. of lactose per kg of body weight) after an overnight fast, and serial blood samples are drawn and analyzed for glucose. If the level of blood glucose rises more than 25 mg/100ml, it means that the lactose has been split in the intestine and the resulting glucose has been absorbed into the blood. This implies that lactase levels are normal. Unfortunately, the blood glucose test, though simple in principle, requires the collection of multiple samples of blood. Moreover, the test has many real and potential problems, the most common of which is false positive tests, that is, an abnormal test in people who have normal lactase levels and no lactose intolerance. For these reasons, the blood glucose test is not often used.
Is there a test for lactose intolerance in infants and young children?
The stool acidity test is a test for lactase deficiency in infants and young children. For the stool acidity test, the infant or child is given a small amount of lactose orally. Several consecutive stool samples then are tested for acidity. With a deficiency of lactase, unabsorbed lactose enters the colon and is split into glucose and galactose. Some of the glucose and galactose is broken down by the bacteria into acids, for example, lactic acid. Lactic acid turns the stool acidic. Therefore, a lactase deficient infant or child will develop an acidic stool following the test dose of lactose.
Despite the availability of the stool acidity test, the superiority of breath testing has led to modifications in the equipment for collecting breath samples that makes it easier to do breath testing in young children and even infants. As a result, the stool acidity test is not done frequently.
What is the treatment for lactose intolerance?
Dietary changes
The most obvious means of treating lactose intolerance is by reducing the amount of lactose in the diet. Fortunately, most people who are lactose intolerant can tolerate small or even moderate amounts of lactose. It often takes only elimination of the major milk-containing products to obtain sufficient relief from their symptoms. Thus, it may be necessary to eliminate only milk, yogurt, cottage cheese, and ice cream. Though yogurt contains large amounts of lactose, it often is well-tolerated by lactose intolerant people. This may be so because the bacteria used to make yogurt contain lactase, and the lactase is able to split some of the lactose during storage of the yogurt as well as after the yogurt is eaten (in the stomach and intestine). Yogurt also has been shown to empty more slowly from the stomach than an equivalent amount of milk. This allows more time for intestinal lactase to split the lactose in yogurt, and, at least theoretically, would result in less lactose reaching the colon.
Most supermarkets carry milk that has had the lactose already split by the addition of lactase. Substitutes for milk also are available, including soy and rice milk. Acidophilus-containing milk is not beneficial since it contains as much lactose as regular milk, and acidophilus bacteria do not split lactose.
For individuals who are intolerant to even small amounts of lactose, the dietary restrictions become more severe. Any purchased product containing milk must be avoided. It is especially important to eliminate prepared foods containing milk purchased from the supermarket and dishes from restaurants that have sauces.
Another means to reduce symptoms of lactose intolerance is to ingest any milk-containing foods during meals. Meals (particularly meals containing fat) reduce the rate at which the stomach empties into the small intestine. This reduces the rate at which lactose enters the small intestine and allows more time for the limited amount of lactase to split the lactose without being overwhelmed by the full load of lactose at once. Studies have shown that the absorption of lactose from whole milk, which contains fat, is greater than from non-fat milk, perhaps for this very reason. Nevertheless, the substitution of whole milk or yogurt for non-fat milk or yogurt does not seem to reduce the symptoms of lactose intolerance.
Lactase enzyme
Caplets or tablets of lactase are available to take with milk-containing foods.
Adaptation
Some people find that by slowly increasing the amount of milk or milk-containing products in their diets they are able to tolerate larger amounts of lactose without developing symptoms. This adaptation to increasing amounts of milk is not due to increases in lactase in the intestine. Adaptation probably results from alterations in the bacteria in the colon. Increasing amounts of lactose entering the colon change the colonic environment, for example, by increasing the acidity of the colon. These changes may alter the way in which the colonic bacteria handle lactose. For example, the bacteria may produce less gas. There also may be a reduction in the secretion of water and, therefore, less diarrhea. Nevertheless, it is not clear how frequently or how much progressive increases in milk intake increase the quantities of milk that can be ingested.
Calcium and vitamin D supplements
Milk and milk-containing products are the best sources of dietary calcium, so it is no wonder that calcium deficiency is common among lactose intolerant persons. This increases the risk and severity of osteoporosis and the resulting bone fractures. It is important, therefore, for lactose intolerant persons to supplement their diets with calcium. A deficiency of vitamin D also causes disease of the bones and fractures. Milk is fortified with vitamin D and is a major source of vitamin D for many people. Although other sources of vitamin D can substitute for milk, it is a good idea for lactose-intolerant persons to take supplemental vitamin D to prevent vitamin D deficiency.
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Which specialties of doctors treat lactose intolerance?
Since internists, pediatricians, and family practitioners all see patients with gastrointestinal symptoms, they all are called upon to diagnose and treat lactose intolerance. If their attempts to diagnose and treat do not result in adequate clinical benefit, patients usually will be referred to a gastroenterologist, an internist or pediatrician specially trained in diseases of the gastrointestinal tract. These specialists will be able to diagnose and treat causes of symptoms other than lactose intolerance.
What are the long-term consequences of lactose intolerance?
The important long-term health consequence of lactose intolerance is calcium deficiency that leads to osteoporosis. Less commonly, vitamin D deficiency may occur and compound the bone disease. Both of these health issues can be prevented easily by calcium and vitamin D supplements. The real problem is that many lactose intolerant people who consciously or unconsciously avoid milk do not realize that they need supplements.
What is new in lactose intolerance?
Genetic testing of DNA of individuals to make a diagnosis of lactase deficiency has already been discussed. This is likely to be an important research tool for studying lactase deficiency. It is still too early to know how helpful this sophisticated test will be in the clinical evaluation and treatment of patients. It is an expensive test. The most important question to answer usually is, does lactose cause symptoms, and not, whether an individual is lactase deficient.
In 1998, scientists were able to make lactose intolerant rats tolerant to lactose by transferring the gene for
lactase production to their intestinal lining cells. It is unlikely that this type of gene therapy will find much of an application in people. Nevertheless, it is a fascinating example of what science can accomplish.