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Lactose intolerance is the name given to the condition (found in the majority of humans) in which lactase, an enzyme needed for proper metabolization of lactose (a constituent of milk and other dairy products), is not produced in adulthood. 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. Someone with lactose intolerance needs to be very careful reading food ingredient labels if they wish to avoid consuming lactose.
The normal mammalian condition is for the young to lose the ability to effectively digest milk sugar (lactose) at the end of the weaning period (a species-specific length of time usually equal to roughly 3% of lifespan). It has been established that 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. Whatever the precise origin, most western Eurasians and people of western Eurasian ancestry show the effects of this mutation while most eastern Eurasians, sub-saharan Africans and native peoples of the Americas and Pacific Islands do not .
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, flatulence and diarrhea. 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.
One solution to this problem (other than avoiding milk) is lactose-free milk, which is produced by passing milk over lactase enzyme bound to an inert carrier: once the molecule is cleaved, there are no lactose ill-effects, whatever the milk drinker's ancestry. The milk sold for pet cats is another example of lactose-reduced milk. Oddly, many European cat breeds have a mutation similar to the human mutation, also prevalent in Europe, which allows symptom-free adult lactose consumption. Most oriental breeds are particularly sensitive to lactose.
In recent years (1990-2000) there has been an increase in the number of lactose-reduced and lactose-free dairy products. Some of these products are cottage cheese, American cheese and ice cream. These products are made using milk substitutes such as soy milk, almond milk, or rice milk. Another recent solution has been a pill which artificially provides the missing enzyme, allowing a person to tolerate milk products for a period of a few hours after taking the pill.
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 g of lactose (in a solution with water) are 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.
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.
It was a simple accident of history that the first nations to industrialize and develop modern scientific medicine happened to be dominated by persons of Western and Northern European descent, who took adult dairy consumption for granted. It took them a long time to understand that the majority of the human species did not. 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 diversity of the human species (and also both genders and all ages) did not become well-established until after the civil rights movements of the 1960s.
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-European persons.
Approximately 70% of the human species cannot tolerate lactose in adulthood. It is thus argued 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.
- United States National Institute of Health page regarding lactose intolerance
- Annu Rev Genet. 2003;37:197-219
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