Lactose Intolerance

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. 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 be very careful reading food ingredient labels if they wish to avoid consuming lactose.

Biology

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). 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)[1].

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.

Etiology

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 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.

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.

Treatment

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. 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 yogurt (see below). 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 Asian breeds are particularly sensitive to lactose.

Lactose intolerance is not all or nothing: the amount of lactose that can be tolerated varies from person to person, and decreases with age. 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."^[NIH]

Many people are more tolerant of yogurt 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 cheesemaking and aging 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.[2]

In 2001, a Finnish research group developed "HYLA" products, which are milk products low in lactose, ranging from ice cream to cheese. These are cow's milk products, but lactose is removed from the milk by using a Valio patented chromatographic separation method. The ultra-pasteurization process combined with aseptic packaging ensures a long shelf-life. Due to these innovations, people who are lactose intolerant have been able to maintain the same diet as those who are lactose tolerant, without having to purchase medication or making significant changes in their eating habits. In recent years (1990–2000) there has been an increase in the number of lactose-reduced and lactose-free dairy products. Examples 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 lactase pills which provide the missing enzyme, allowing a person to tolerate milk products for about 30-45 minutes after taking a pill.

History

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 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-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 metabolizable and digestible by humans, thus some form of human-bacteria symbiosis was proposed as a means of producing lactase in the human digestive track; 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.

See also

• Gastroenterology

External links

• OMIM 223100
• OMIM 150220
• Got Milk But Can't Stomach It?
• United States National Institute of Health page regarding lactose intolerance

References

• Annu Rev Genet. 2003;37:197-219