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Lactose intolerance is the condition in which lactase, an enzyme needed for proper metabolization of lactose (a constituent of milk and other dairy products), is not produced in adulthood. A hydrogen breath test is required for a clinical diagnosis.

With lactose intolerance, the result of consuming lactose or a lactose-containing food is excess gas production and often diarrhea. In western cultures milk products are nearly ubiquitous and are contained in at least a small amount in almost all recipes, restaurant dishes, and processed food. People with lactose intolerance need to read food ingredient labels carefully if they wish to avoid consuming lactose.

Biology
The normal mammalian condition is for the young to lose the ability to digest effectively milk sugar (lactose) at the end of the weaning period (a species-specific length of time usually equal to roughly 3% of lifespan). In humans, lactase production usually drops about 90% during the first four years of life, though the exact drop and age varies widely. However, certain human populations have undergone a mutation on chromosome 2 which results in a bypass of the normal shutdown in lactase production, allowing members of these groups to continue consumption of fresh milk and fresh milk products throughout their lives.

There is some debate on exactly where and when the mutation(s) occurred, some arguing for separate mutation events in Sweden and the Arabian Peninsula near 4000 BC which converged as they spread, while others argue for a single event in the Middle East at about 4500 BC which radiated from there. Some sources suggest a third and more recent mutation in the East African Tutsi. Whatever the precise origin in time and place, most modern western Eurasians and people of western Eurasian ancestry show the effects of this mutation (that is, they are able to safely consume milk products all their lives) while most modern eastern Eurasians, sub-Saharan Africans and native peoples of the Americas and Pacific Islands do not (making them lactose intolerant as adults).

Lactose intolerance by group

Table Data obtained (in part) from "Lactose and Lactase", Scientific American, October, 1972, by Norman Kretchmer. Statistical significance varies greatly depending on number of people sampled.

Many global cat breeds (Asian breeds in particular) share the mammalian lactose sensitivity, unlike many European breeds that have a mutation similar to the European human mutation.

Aetiology
Without lactase, the lactose in milk remains uncleaved and unabsorbed. Lactose cannot pass easily through the intestinal wall into the bloodstream, so it remains in the intestines. Soon, gut bacteria adapt to the relative abundance of lactose (relative to other sugars like glucose) and switch over to metabolizing lactose. Along the way they produce copious amounts of gas by fermentation.

The gas causes a range of unpleasant abdominal symptoms, including stomach cramps, bloating, flatulence and diarrhoea. Like other unabsorbed sugars, e.g. mannitol, the lactose raises the osmotic pressure of the colon contents, preventing the colon from resorbing water and hence causing a laxative effect to add to the excessive gas production.

Diagnosis
Since the majority of Europeans have the mutation rendering them lactose-tolerant, lactose intolerance is widely regarded as a medical condition in Europe and North America. A fair proportion of patients with symptoms of irritable bowel syndrome actually have lactose intolerance without knowing it.

A simple test can clarify the issue: after an overnight fast, 50 grams of lactose (in a solution with water) is to be swallowed. If the lactose cannot be digested, enteric bacteria will metabolize it and produce hydrogen. This can be detected in the air the patient exhales. The test takes about 2 to 3 hours. A medical condition with similar symptoms is fructose malabsorption.

Measuring the blood glucose level every 10-15 minutes after ingestion will show a "flat curve" in individuals with lactose malabsorption, while the lactose persistent will have a significant "top", with an elevation of typically 50-100 % within 1-2 hours.

A definitive diagnosis for research purposes can be obtained by analysis of an intestinal biopsy for lactase activity.

Although not as precise a test, an estimate of lactose intolerance can be made as follows: Drink two cups of milk on an empty stomach and watch for signs of intestinal discomfort over the next several hours. The following day, eat two ounces of hard cheese or drink two cups of lactose-free milk. If symptoms are experienced only on the first day, the individual may be lactose intolerant. If symptoms are experienced on both days, the individual may have an allergy to dairy products, which is unrelated to lactose intolerance.

Some individuals are able to self-diagnose without intentionally testing themselves, simply by realizing in retrospect that their symptoms always correspond to prior lactose consumption. This, however, is the least reliable of the methods herein described, as peoples' memories are imperfect. In addition, it is harder to isolate one definite cause without a formal test.

Managing lactose intolerance
There is no "treatment" or "cure" to lactose intolerance. There have been some cases where the intolerance has somehow diminished with time; this has not been studied scientifically, however, and whether it is a case of desensitization remains to be seen. It should be remembered that lactose intolerance is not a binary (all-or-nothing) condition: the reduction in lactase production, and hence, amount of lactose that can be tolerated varies from person to person, and may change with age. The management of lactose intolerance involves avoiding lactose-containing products, use of alternative products or artificial lactase enzyme medication.

Avoiding lactose-containing products
Since each individual's tolerance to consumed lactose varies, according to the National Institute of Health, "Dietary control of lactose intolerance depends on people learning through trial and error how much lactose they can handle."

Many people are more tolerant of yoghurt than milk because it contains lactase produced by the bacterial cultures used to make the yogurt. Also, hard cheeses (e.g. Swiss) produce far less reaction than the equivalent amount of milk because the cheese making and ageing processes greatly reduce the amount of lactose. A typical Swiss or Cheddar might contain 5% of the lactose found in whole milk, while long-aged cheeses contain almost no lactose at all.

It is important for lactose intolerant people to especially be careful in avoiding products that whilst not apparently dairy (or are dairy but normally contain low amounts of lactose) nonetheless contain lactose. Such products include commercial sausages (notably frankfurters), medications which may contain lactose as a filler, most meal replacement and protein bars, cottage cheese, and even yogurts containing carageenan or gelatin.

Alternative Products
Reduced or entirely lactose-free products (using milk substitutes such as soy milk, almond milk, or rice milk) are available allowing lactose-intolerant people to maintain approximately the same diet as those who are tolerant, without having to purchase medication or significantly alter their eating habits.
The food industry has successfully managed to create low-lactose or entirely lactose-free products to replace the regular items, without loss in quality. Lactose-free milk can be produced by passing milk over lactase enzyme bound to an inert carrier: once the molecule is cleaved, there are no lactose ill-effects. Alternatively, a harmless bacterium such as L. acidophilus may be added, which affects the lactose in milk the same way it affects the lactose in yoghurt (see above).

With many pet cats sharing human lactose sensitivity, similar lactose-reduced milk is sold for them.

Finland has had "HYLA" (acronym for hydrolysed lactose) products available for many years, even though the number of lactose intolerant people there is relatively small. These low-lactose level cow's milk products, ranging from ice cream to cheese, use a Valio patented chromatographic separation method to remove lactose. The ultra-pasteurization process, combined with aseptic packaging ensures a long shelf-life. Recently, the range of low-lactose products available in Finland has been completed with milk and other dairy products, even ice cream, that contain no lactose at all: the remaining about 20% of lactose in HYLA products is taken care of enzymatically. These typically cost 2-4 times more than equivalent products containing lactose.

Many countries have similar product lines, and new consumer products continue to become available. In America over recent years (1990-2000) there has been a notable increase of available lactose-reduced and lactose-free dairy products; examples being cottage cheese, American cheese and ice cream.

Lactase-enzyme Medication
These medicinal products aim to replace the deficiency in lactase production and so allow consumption of normal lactose-containing dairy products. Their cost may offset their benefit compared to using lactose-free products in ones own home, but they give dietary freedom when eating out.

Most commonly these are packaged in tablet form allowing a person to tolerate milk products for about 30-45 minutes after taking a pill. In addition solutions of lactase enzyme can be obtained; a few drops being added to a bottle of normal milk to cleave the contained lactose.

History of diagnosis
The condition was first recognized in the 1950s and 1960s when various organizations like the United Nations began to engage in systematic famine-relief efforts in countries outside Europe for the first time. As anecdotes of embarrassing dairy-induced discomfort piled up, the First World donor countries could no longer ascribe the reports to spoilage in transit or inappropriate food preparation at the recipient end in the Third World.

Since the first nations to industrialize and develop modern scientific medicine were dominated by people of Western and Northern European descent, adult dairy consumption was long taken for granted. Westerners for some time did not recognize that the majority of the human ethnogenetic groups could not consume dairy during adulthood. Although there had been regular contact between Europeans and non-Europeans throughout history, the notion that large-scale medical studies should be representative of the racial diversity of the human populations (and also both genders and all ages) did not become well-established until after the American Civil Rights Movement.

Since then, the relationship between lactase and lactose has been thoroughly investigated in food science due to the growing market for dairy products among non-Europeans.

Originally it was hypothesized that gut bacteria such as E. coli produced the lactase enzyme needed to cleave lactose into its constituent monosaccharides and thus become metabolisable and digestible by humans, thus some form of human-bacteria symbiosis was proposed as a means of producing lactase in the human digestive tract; Genetics and protein analyzation techniques by the early 1970s revealed this to be untrue and that humans produce their own lactase enzyme natively in intestine cells.

Approximately 70% of the global population cannot tolerate lactose in adulthood. Thus, some argue that the terminology should be reversed, lactose intolerance should be seen as the norm, and the minority Western European group should be labeled as having lactase persistence.

History of genetic prevalence
Lactose intolerance has been studied as an aid in understanding ancient diets and population movement in prehistoric societies. Milking an animal vastly increases the efficiency of raising it in regards to the calories that can be extracted compared to consumption of its meat alone. It is not surprising then, that consuming milk products became an important part of the agricultural way of life in the Neolithic. Given that at this time the majority of the population was lactose intolerant, it is believed that most of the milk was used to make mature cheeses, which loose most of the lactose and can be safe to eat.

However, cheese takes a long time to produce and one of the theories for the prevalence of lactose tolerance in northern Europe and certain parts of the near east is that, at a time of famine, it became advantageous to consume the milk directly, without having to wait for it to mature. Others believe that the practical advantages of the mutation have been over emphasized and that it is no more than chance that this gene was allowed to flourish in certain societies.

In Europe, Roman sources attest that milk was often used as a purgative (to induce vomiting and diarrhea), and horse milk is suggested as the best, with goat's milk as the worst. This corresponds to the amount of lactose in the milk; horse milk has a great deal of lactose, and goat's milk not very much. Roman authors also remark that the people of northern Europe, particularly Britain and Germany drank unprocessed milk (as opposed to themselves who made cheese and butter). This corresponds very closely with modern European distributions of lactose intolerance, where the people of Britain, Germany and Scandinavia have a good tolerance, and those of southern Europe, especially Italy have a poorer tolerance.

In east Asia, historical sources also attest that the Chinese did not consume milk, whereas the nomads that lived on the borders did. Again, this reflects modern distributions of intolerance. China is particularly notable as a place of poor tolerance, whereas in Mongolia and the Asian steppes horse milk is drunk regularly. Here they even make an alcoholic beverage, called Kumis, from horse milk (although the fermentation process reduces the amount of lactose present). This tolerance is thought to be advantageous as the nomads do not settle down long enough to process mature cheese or may find themselves regularly going through brief periods of starvation; and given that their prime source of income is generated through horses, to ignore milk as a source of calories would be a huge detriment.