Lactose intolerance
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, is not produced in adulthood.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 [1].
Without lactase, the lactose in milk remains uncleaved and unabsorbed, and instead gut bacteria metabolise it, producing copious amounts of gas by fermentation. This 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.
See also: Gastroenterology
| Health science - Medicine - Gastroenterology |
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| Peptic ulcer - Malabsorption (e.g. celiac disease, lactose intolerance, fructose malabsorption, Whipple's disease) - Lymphoma |
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