DEFINITION
Lactose intolerance is the inability or insufficient ability to digest the milk sugar, called lactose, resulting from the absence or deficiency of lactase in the mucosa cells of the gastrointestinal tract. Bacterial fermentation of the unabsorbed lactose leads to symptoms that range from a mild indigestion (dyspepsia) to severe diarrhea. Lactose intolerance is also called lactase deficiency, alactasia or hypolactasia.
Lactose is a disaccharide of glucose and galactose, and its digestion requires the actions of the enzyme lactase (a beta-galactosidase located in the surface membrane of small-intestinal epithelial cells). Many people have low levels of lactase, but most don't experience signs and symptoms of lactose intolerance. Only people with both low lactase levels who also have associated signs and symptoms have, by definition, lactose intolerance.
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. Difference between food intolerance and food allergy
EPIDEMIOLOGY
Of the world's population, 75% is estimated to be lactose-deficient. Lactose intolerance is very common among Asian, South American, and African persons.
Adult-onset lactase deficiency varies widely among countries. Northern Europeans have the lowest prevalence at approximately 5%. Central Europeans have a higher prevalence at approximately 30%, and Southern Europeans have a much higher prevalence at approximately 70%. Hispanic and Jewish populations also have a high prevalence at approximately 70%, while Northern Indians have a much lower prevalence than Southern Indians, at approximately 25% and 65%, respectively. Almost all (90%) Asians and Africans are affected.
Males and females are equally affected by lactose intolerance. Of those women who are lactose intolerant, 44% regain the ability to digest lactose during pregnancy.
SYMPTOMS
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:
* Diarrhea
* Nausea
* Abdominal cramps
* Bloating
* Gas
DIAGNOSIS
The suspect of lactose intolerance is based on symptoms and response to reducing the amount of dairy foods in diet. The diagnosis can be confirmed by conducting one or more of the following tests:
Lactose tolerance 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. Instead, if glucose level doesn't rise, it means the body isn't properly digesting and absorbing the lactose. 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.
Hydrogen breath test. The hydrogen breath test is the most convenient and reliable test for lactase deficiency and lactose intolerance. For the breath test, pure lactose, usually 25 grams (the equivalent of 16 oz of milk), is ingested with water after an overnight fast. In persons who are lactose intolerant, the lactose that is not digested and absorbed in the small intestine reaches the colon and ferments, causing hydrogen and other gases (such as methane) to be released. Small amounts of the hydrogen and methane are absorbed from the colon into the blood and then travel to the lungs where they are excreted in the breath. Samples of breath are collected every 10 or 15 minutes for 3-5 hours after ingestion of the lactose, and the samples are analyzed for hydrogen and/or methane. If hydrogen and/or methane are found in the breath, it means that the small intestine of the person was unable to digest and absorb all of the lactose. The amount of hydrogen or methane excreted in the breath is roughly proportional to the degree of lactase deficiency, that is, the larger the amount of hydrogen and/or methane produced, the greater the deficiency. The amount of hydrogen and/or methane in the breath, however, is not proportional to the severity of the symptoms. In other words, a person who produces little hydrogen and/or methane may have more severe symptoms than a person who produces a large amount hydrogen and/or methane. The breath test is the best test for determining lactase deficiency and lactose intolerance, but it has several weaknesses. The first is that it is a long, boring test. The second is the important issue respect to the quantity of lactose that should be used. Lastly, the breath test can be falsely abnormal when there is spread of bacteria from the colon into the small intestine, a condition called bacterial overgrowth of the small bowel. When overgrowth occurs, the bacteria that have moved up into the small intestine get to the lactose in the intestine before there has been enough time for the lactose to be digested and absorbed normally, and these bacteria produce hydrogen and/or methane. This may lead erroneously to a diagnosis of lactose intolerance. Other conditions also interfere with the breath test. Thus, diseases that markedly speed up transit of lactose through the small intestine prevent lactose from being fully digested and absorbed, leading to a misdiagnosis of lactose intolerance. Recent treatment with antibiotics can suppress colonic bacteria and their production of hydrogen or methane and lead to a misdiagnosis of lactose tolerance. Fortunately, these latter conditions are uncommon and usually can be anticipated on the basis of a person's history or symptoms.
Stool acidity test. For infants and children who can't undergo other tests, a stool acidity test may be used. The infant or child is given a small amount of lactose orally. 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.
PATHOGENESIS
Lactose intolerance is caused by low levels of the enzyme lactase in small intestine that lead to signs and symptoms.
Normally, the cells that line small intestine produce an enzyme called lactase. The lactase enzyme attaches to lactose molecules in the food and breaks them into two simple sugars, glucose and galactose, which can be absorbed into bloodstream.
Without enough of the lactase enzyme, most of the lactose in food moves unprocessed into the colon, where the normal intestinal bacteria interact with it. This causes the hallmarks of lactose intolerance: gas, bloating and diarrhea.
There are three types of lactose intolerance.
Primary lactose intolerance: normal result of aging for some people
Normally, the body produces large amounts of lactase at birth and during early childhood, when milk is the primary source of nutrition. Usually lactase production decreases as diet becomes more varied and less reliant on milk. This gradual decline may lead to symptoms of lactose intolerance.
Secondary lactose intolerance: result of illness or injury
This form of lactose intolerance occurs when small intestine decreases lactase production after an illness, surgery or injury to small intestine. It can occur as a result of intestinal diseases, such as celiac disease, gastroenteritis and an inflammatory bowel disease like Crohn's disease. Treatment of the underlying disorder may restore lactase levels and improve signs and symptoms, though it can take time.
Congenital lactose intolerance: condition you're born with
It's possible, but very rare, for babies to be born with lactose intolerance caused by a complete absence of lactase activity. Infants with congenital lactose intolerance are intolerant of the lactose in their mothers' breast milk and have diarrhea from birth. These babies require lactose-free infant formulas.
Congenital lactase deficiency is caused by mutations in the LCT gene. The LCT gene provides instructions for making the lactase enzyme. Mutations that cause congenital lactase deficiency are believed to interfere with the function of lactase, causing affected infants to have a severely impaired ability to digest lactose in breast milk or formula. The type of lactose intolerance that occurs in infants is inherited in an autosomal recessive pattern, which means both copies of the LCT gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.
Lactose intolerance in adulthood is caused by gradually decreasing activity (expression) of the LCT gene after infancy, which occurs in most humans. LCT gene expression is controlled by a DNA sequence called a regulatory element, which is located within a nearby gene called MCM6. The ability to digest lactose into adulthood depends on which variations in the regulatory element within the MCM6 gene individuals have inherited from their parents. The variations that promote continued lactase production are considered autosomal dominant, which means one copy of the altered regulatory element in each cell is sufficient to sustain lactase production. People who have not inherited these variations from either parent will have some degree of lactose intolerance.
PATIENT RISK FACTORS
Factors that can make people more prone to lactose intolerance include:
* Increasing age: lactose intolerance becomes more common aging.
* Ethnicity: lactose intolerance is most common in black, Asian, Hispanic and American Indian people.
* Premature birth: infants born prematurely may have reduced levels of lactase, because this enzyme increases in the fetus 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.
* Radiation to the abdomen.
TREATMENTS
Treatments can't cure lactose intolerance.
There's currently no way to boost body's production of the lactase enzyme.
Although the body's ability to produce lactase cannot be changed, the symptoms of lactose intolerance can be managed with dietary changes. The amount of change needed in the diet depends on how much lactose a person can consume without symptoms. 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.
Ways to change the diet to minimize symptoms of lactose intolerance include:
* Choosing smaller servings of dairy.
* Saving milk for mealtimes. Drink milk with other foods, rather than alone.
* 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. Cultured milk products, such as yogurt, may be tolerate because the bacteria used in the culturing process naturally produce the enzyme that breaks down lactose.
* Buying lactose-reduced or lactose-free products.
* Watching out for hidden lactose. Milk and lactose are often added to prepared foods, such as cereal, instant soups, salad dressings, nondairy creamers, processed meats and baking mixes. Also look for other words that indicate lactose, such as whey, milk byproducts, fat-free dry milk powder and dry milk solids. Lactose is also used in medications.
* Using lactase enzyme tablets or drops. Over-the-counter tablets or drops containing the lactase enzyme may help digest dairy products. Tablets can be taken 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.
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. The amount of calcium a person needs to maintain good health varies by age.
Getting enough calcium is important for people with lactose intolerance when the intake of milk and milk products is limited. Many foods can provide calcium and other nutrients the body needs. Non-milk products that are high in calcium include fish with soft bones such as salmon and sardines and dark green vegetables such as spinach.
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 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.
