Lactose intolerance is a condition in which people have symptoms due to the decreased ability to digest lactose, a sugar found in dairy products. Those affected vary in the amount of lactose they can tolerate before symptoms develop. Symptoms may include abdominal pain, bloating, diarrhea, gas, and nausea. These symptoms typically start thirty minutes to two hours after eating or drinking milk-based food. Severity typically depends on the amount a person eats or drinks. Lactose intolerance does not cause damage to the gastrointestinal tract.
Lactose intolerance is due to the lack of enzyme lactase in the small intestines to break lactose down into glucose and galactose. There are four types: primary, secondary, developmental, and congenital. Primary lactose intolerance occurs as the amount of lactase declines as people age. Secondary lactose intolerance is due to injury to the small intestine such as from infection, celiac disease, inflammatory bowel disease, or other diseases. Developmental lactose intolerance may occur in premature babies and usually improves over a short period of time. Congenital lactose intolerance is an extremely rare genetic disorder in which little or no lactase is made from birth.
Diagnosis may be confirmed if symptoms resolve following eliminating lactose from the diet. Other supporting tests include a hydrogen breath test and a stool acidity test. Other conditions that may produce similar symptoms include irritable bowel syndrome, celiac disease, and inflammatory bowel disease. Lactose intolerance is different from a milk allergy. Management is typically by decreasing the amount of lactose in the diet, taking lactase supplements, or treating the underlying disease. People are usually able to drink at least one cup of milk per sitting without developing significant symptoms, with greater amounts tolerated if drunk with a meal or throughout the day.
Lactose intolerance primarily refers to a syndrome having one or more symptoms upon the consumption of food substances containing lactose. Individuals may be lactose intolerant to varying degrees, depending on the severity of these symptoms. “Lactose malabsorption” refers to the physiological concomitant of lactase deficiency (i.e., the body does not have sufficient lactase capacity to digest the amount of lactose ingested). Hypolactasia (lactase deficiency) is distinguished from alactasia (total lack of lactase), a rare congenital defect.
Lactose intolerance is not an allergy, because it is not an immune response, but rather a sensitivity to dairy caused by lactase deficiency. Milk allergy, occurring in only 4% of the population, is a separate condition, with distinct symptoms that occur when the presence of milk proteins trigger an immune reaction.
Signs and symptoms
The principal symptom of lactose intolerance is an adverse reaction to products containing lactose (primarily milk), including abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi, and vomiting (particularly in adolescents). These appear one-half to two hours after consumption. The severity of symptoms typically increases with the amount of lactose consumed; most lactose-intolerant people can tolerate a certain level of lactose in their diets without ill effects.
Lactose intolerance is a consequence of lactase deficiency, which may be genetic (primary hypolactasia and primary congenital alactasia) or environmentally induced (secondary or acquired hypoalactasia). In either case, symptoms are caused by insufficient levels of lactase in the lining of the duodenum. Lactose, a disaccharide molecule found in milk and dairy products, cannot be directly absorbed through the wall of the small intestine into the bloodstream, so, in the absence of lactase, passes intact into the colon. Bacteria in the colon can metabolise lactose, and the resulting fermentation produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane) that causes the various abdominal symptoms. The unabsorbed sugars and fermentation products also raise the osmotic pressure of the colon, causing an increased flow of water into the bowels (diarrhea).
The LCT gene provides the instructions for making lactase. The specific DNA sequence in the MCM6 gene helps control whether the LCT gene is turned on or off. At least several thousand years ago, some humans developed a mutation in the MCM6 gene that keeps the LCT gene turned on even after breast feeding is stopped. Populations that are lactose intolerant lack this mutation. The LCT and MCM6 genes are both located on the long arm (q) of chromosome 2 in region 21. The locus can be expressed as 2q21. The lactase deficiency also could be linked to certain heritages. It is more common in Asian Americans, African Americans, Mexican Americans, and Native Americans. Analysis of the DNA of 94 ancient skeletons in Europe and Russia concluded that the mutation for lactose tolerance appeared about 4,300 years ago and spread throughout the European population.
Some human populations have developed lactase persistence, in which lactase production continues into adulthood probably as a response to the benefits of being able to digest milk from farm animals. Some have argued that this links intolerance to natural selection favoring lactase-persistent individuals, but it is also consistent with a physiological response to decrease lactase production when it is not needed in cultures in which dairy products are not an available food source. Although populations in Europe, India, Arabia, and Africa were first thought to have high rates of lactase persistence because of a single mutation, lactase persistence has been traced to a number of mutations that occurred independently. Different alleles for lactase persistence have developed at least three times in East African populations, with persistence extending from 26% in Tanzania to 88% in the Beja pastoralist population in Sudan.
The accumulation of Epigenetic factors, primarily DNA methylation, in the LCT and MCM6 gene may also contribute to the onset of lactose intolerance in adults.
Lactose intolerance is classified according to its causes as:
Primary hypolactasia, or primary lactase deficiency, is genetic, only affects adults, and is caused by the absence of a lactase persistence allele. In individuals without the lactase persistence allele, less lactase is produced by the body over time, leading to hypolactasia in adulthood. The frequency of lactase persistence, which allows lactose tolerance, varies enormously worldwide, with the highest prevalence in Northwestern Europe, declines across southern Europe and the Middle East and is low in Asia and most of Africa, although it is common in pastoralist populations from Africa.
Secondary hypolactasia or secondary lactase deficiency, also called acquired hypolactasia or acquired lactase deficiency, is caused by an injury to the small intestine. This form of lactose intolerance can occur in both infants and lactase persistent adults and is generally reversible. It may be caused by acute gastroenteritis, coeliac disease, Crohn’s disease, ulcerative colitis, chemotherapy, intestinal parasites (such as giardia), or other environmental causes.
Primary congenital alactasia
Primary congenital alactasia, also called congenital lactase deficiency, is an extremely rare, autosomal recessive enzyme defect that prevents lactase expression from birth. People with congenital lactase deficiency cannot digest lactose from birth, so cannot digest breast milk. This genetic defect is characterized by a complete lack of lactase (alactasia). About 40 cases have been reported worldwide, mainly limited to Finland. Before the 20th century, babies born with congenital lactase deficiency often did not survive, but death rates decreased with soybean-derived infant formulas and manufactured lactose-free dairy products.
When lactose intolerance is due to secondary lactase deficiency, treatment of the underlying disease may allow lactase activity to return to normal levels. In people with coeliac disease, lactose intolerance normally reverts or improves several months after starting a gluten-free diet, but temporary dietary restriction of lactose may be needed.
People with primary lactase deficiency cannot modify their body’s ability to produce lactase. In societies where lactose intolerance is the norm, it is not considered a condition that requires treatment. However, where dairy is a larger component of the normal diet, a number of efforts may be useful. There are four general principles in dealing with lactose intolerance: avoidance of dietary lactose, substitution to maintain nutrient intake, regulation of calcium intake, and use of enzyme substitute. Regular consumption of dairy food by lactase deficient individuals may also reduce symptoms of intolerance by promoting colonic bacteria adaptation.
The primary way of managing the symptoms of lactose intolerance is to limit the intake of lactose to a level that can be tolerated. Lactase deficient individuals vary in the amount of lactose they can tolerate, and some report that their tolerance varies over time, depending on health status and pregnancy. However, as a rule of thumb, people with primary lactase deficiency and no small intestine injury are usually able to consume at least 12 grams of lactose per sitting without symptoms, or with only mild symptoms, with greater amounts tolerated if consumed with a meal or throughout the day.
Lactose is found primarily in dairy products, which vary in the amount of lactose they contain:
Milk – unprocessed cow’s milk is about 4.7% lactose; goat’s milk 4.7%; sheep’s milk 4.7%; buffalo milk 4.86%; and yak milk 4.93%.
Sour cream and buttermilk – if made in the traditional way, this may be tolerable, but most modern brands add milk solids.
Butter – the process of making butter largely removes lactose, but it is still present in small quantities; clarified butter contains a negligible amount of lactose.
Yogurt – lactobacilli used in the production of yogurt remove lactose to varying degrees, depending on the type of yogurt. Bacteria found in yogurt produce their own enzyme, lactase, that facilitates digestion in the intestines in lactose intolerant individuals.
Cheese – fermentation also reduces the lactose content of cheeses and aging reduces it further; traditionally made hard cheeses might contain 10% of the lactose found in an equivalent volume of milk. However, manufactured cheeses may be produced using processes that do not have the same lactose-reducing properties.
There is no standardized method for measuring the lactose content of food. The stated dairy content of a product also varies according to manufacturing processes and labelling practices, and commercial terminology varies between languages and regions. As a result, absolute figures for the amount of lactose consumed (by weight) may not be very reliable. Kosher products labeled pareve or fleishig are free of milk. However, if a “D” (for “dairy”) is present next to the circled “K”, “U”, or other hechsher, the food product likely contains milk solids, although it may also simply indicate the product was produced on equipment shared with other products containing milk derivatives.
Lactose is also a commercial food additive used for its texture, flavor, and adhesive qualities. It is found in additives labelled as casein, caseinate, whey, lactoserum, milk solids, modified milk ingredients, etc. As such lactose is found in foods such as processed meats (sausages/hot dogs, sliced meats, pâtés), gravy stock powder, margarines, sliced breads, breakfast cereals, potato chips, processed foods, medications, prepared meals, meal replacements (powders and bars), protein supplements (powders and bars), and even beers in the milk stout style. Some barbecue sauces and liquid cheeses used in fast-food restaurants may also contain lactose. Lactose is often used as the primary filler (main ingredient) in most prescription and non-prescription solid pill form medications, though product labeling seldom mentions the presence of ‘lactose’ or ‘milk’, and neither do product monograms provided to pharmacists, and most pharmacists are unaware of the very wide scale yet common use of lactose in such medications until they contact the supplier or manufacturer for verification.
Further information: Milk § Reduction or elimination of lactose, and Milk substitute
Plant-based “milks” and derivatives such as soy milk, rice milk, almond milk, coconut milk, hazelnut milk, oat milk, hemp milk, macadamia nut milk, and peanut milk are inherently lactose-free. Low-lactose and lactose-free versions of foods are often available to replace dairy-based foods for those with lactose intolerance.
When lactose avoidance is not possible, or on occasions when a person chooses to consume such items, then enzymatic lactase supplements may be used.
Lactase enzymes similar to those produced in the small intestines of humans are produced industrially by fungi of the genus Aspergillus. The enzyme, ß-galactosidase, is available in tablet form in a variety of doses, in many countries without a prescription. It functions well only in high-acid environments, such as that found in the human gut due to the addition of gastric juices from the stomach. Unfortunately, too much acid can denature it, so it should not be taken on an empty stomach. Also, the enzyme is ineffective if it does not reach the small intestine by the time the problematic food does. Lactose-sensitive individuals can experiment with both timing and dosage to fit their particular needs.
While essentially the same process as normal intestinal lactose digestion, direct treatment of milk employs a different variety of industrially produced lactase. This enzyme, produced by yeast from the genus Kluyveromyces, takes much longer to act, must be thoroughly mixed throughout the product, and is destroyed by even mildly acidic environments. Its main use is in producing the lactose-free or lactose-reduced dairy products sold in supermarkets.
Rehabituation to dairy products
Regular consumption of dairy foods containing lactose can promote a colonic bacteria adaptation, enhancing a favorable microbiome, which allows people with primary lactase deficiency to diminish their intolerance and to consume more dairy foods. The way to induce tolerance is based on progressive exposure, consuming smaller amounts frequently, distributed throughout the day. Lactose intolerance can also be managed by ingesting live yogurt cultures containing lactobacilli that are able to digest the lactose in other dairy products. This may explain why many South Asians, though genetically lactose intolerant, are able to consume large quantities of milk without many symptoms of lactose intolerance, since consuming live yogurt cultures is very common among the South Asian population.