Lactose intolerance is the inability to metabolize lactose, a sugar found in milk and other dairy products, because the required enzyme lactase is absent in the intestinal system or its availability is lowered. It is estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood. The frequency of decreased lactase activity ranges from as little as 5% in northern Europe, up to 71% for Sicily, to more than 90% in some African and Asian countries.
Overview
Disaccharides cannot be absorbed through the wall of the small intestine into the bloodstream, so in the absence of lactase, lactose present in ingested dairy products remains uncleaved and passes intact into the colon. The operons of enteric bacteria quickly switch over to lactose metabolism, and the resulting in-vivo fermentation produces copious amounts of gas (a mixture of hydrogen, carbon dioxide, and methane). This, in turn, may cause a range of abdominal symptoms, including stomach cramps, bloating, and flatulence. In addition, as with other unabsorbed sugars (such as sorbitol, mannitol, and xylitol), the presence of lactose and its fermentation products raises the osmotic pressure of the colon contents.
Classification
There are three major types of lactose intolerance:
- Primary lactose intolerance . Environmentally induced when weaning a child in non–dairy consuming societies. This is found in many Asian and African cultures, where industrialized and commercial dairy products are uncommon.
- Secondary lactose intolerance . Environmentally induced, resulting from certain gastrointestinal diseases, including exposure to intestinal parasites such as Giardia lamblia. In such cases the production of lactase may be permanently disrupted. A very common cause of temporary lactose intolerance is gastroenteritis, particularly when the gastroenteritis is caused by rotavirus. Another form of temporary lactose intolerance is lactose overload in infants.
- Congenital lactase deficiency . A genetic disorder which prevents enzymatic production of lactase. Present at birth, and diagnosed in early infancy.
Lactase biology
The normal mammalian condition is for the young of a species to experience reduced lactase production at the end of the weaning period (a species-specific length of time). In humans, in non-dairy consuming societies, lactase production usually drops about 90% during the first four years of life, although the exact drop over time varies widely.
However, certain human populations have a mutation on chromosome 2 which eliminates the shutdown in lactase production, making it possible for members of these populations to continue consumption of fresh milk and other dairy products throughout their lives without difficulty. This appears to be an evolutionarily recent adaptation to dairy consumption, and has occurred independently in both northern Europe and east Africa in populations with a historically pastoral lifestyle. Lactase persistence, allowing lactose digestion to continue into adulthood, is a dominant allele, making lactose intolerance a recessive genetic trait. A noncoding variation in the MCM6 gene has been strongly associated with adult type hypolactasia (lactose intolerance).
Some cultures, such as that of Japan, where dairy consumption has been on the increase, demonstrate a lower prevalence of lactose intolerance in spite of a genetic predisposition.
Pathological lactose intolerance can be caused by coeliac disease, which damages the villi in the small intestine that produce lactase. This lactose intolerance is temporary. Lactose intolerance associated with coeliac disease ceases after the patient has been on a gluten-free diet long enough for the villi to recover.
Certain people who report problems with consuming lactose are not actually lactose intolerant. In a study of 323 Sicilian adults, Carroccio et al. (1998) found only 4% were both lactose intolerant and lactose maldigesters, while 32.2% were lactose maldigesters but did not test as lactose intolerant. However, Burgio et al. (1984) found that 72% of 100 Sicilians were lactose intolerant in their study and 106 of 208 northern Italians (i.e., 51%) were lactose intolerant.
Lactose intolerance by group
The statistical significance varies greatly depending on number of people sampled.
Lactose intolerance levels also increase with age. At ages 2 - 3 yrs., 6 yrs., and 9 - 10 yrs., the amount of lactose intolerance is, respectively:
- 6% to 15% in white Americans and northern Europeans
- 18%, 30%, and 47% in Mexican Americans
- 25%, 45%, and 60% in black South Africans
- approximately 10%, 20%, and 25% in Chinese and Japanese
- 30–55%, 90%, and >90% in Mestizos of Peru
Chinese and Japanese populations typically lose between 20 and 30 percent of their ability to digest lactose within three to four years of weaning. Some studies have found that most Japanese can consume 200 ml (8 fl oz) of milk without severe symptoms (Swagerty et al., 2002).
Ashkenazi Jews can keep 20 - 30 percent of their ability to digest lactose for many years. Of the 10% of the Northern European population that develops lactose intolerance, the development of lactose intolerance is a gradual process spread out over as many as 20 years.
Diagnosis
To assess lactose intolerance, the intestinal function is challenged by ingesting more dairy than can be readily digested. Clinical symptoms typically appear within 30 minutes but may take up to 2 hours, depending on other foods and activities. Substantial variability of the clinical response (symptoms of nausea, cramping, bloating, diarrhea, and flatulence) is to be expected, as the extent and severity of lactose intolerance varies between individuals.
When considering the need for confirmation, it is important to distinguish lactose intolerance from milk allergy, which is an abnormal immune response (usually) to milk proteins. Since lactose intolerance is the normal state for most adults on a worldwide scale and is not considered a disease condition, a medical diagnosis is not normally required. However, if confirmation is necessary, three tests are available.
Hydrogen breath test
In a hydrogen breath test, after an overnight fast, 50 grams of lactose (in a solution with water) is swallowed. If the lactose cannot be digested, enteric bacteria metabolize it and produce hydrogen. This, along with methane, can be detected in the patient's breath by a clinical gas chromatograph or a compact solid state detector. The test takes about 2 to 3 hours. A medical condition with similar symptoms is fructose malabsorption.
In conjunction, measuring the blood glucose level every 10 – 15 minutes after ingestion will show a "flat curve" in individuals with lactose malabsorption, while the lactase persistent will have a significant "top", with an elevation of typically 50 to 100% within 1 – 2 hours. However, given the need for frequent blood draws, this approach has been largely supplanted by breath testing.
Stool acidity test
This test can be used to diagnose lactose intolerance in small infants, for whom other forms of testing are risky or impractical.
Intestinal biopsy
An intestinal biopsy can confirm lactose intolerance following discovery of elevated hydrogen in the hydrogen breath test. However, given the invasive nature of this test, and the need for a highly specialized laboratory to measure lactase enzymes or mRNA in the biopsy tissue, this approach is used almost exclusively in clinical research.
History of diagnosis
The ancient Greek physician Hippocrates (460-370 B.C.) first noted gastrointestinal upset and skin problems in some who consumed milk; patients experiencing the former symptom may likely have been suffering from lactose intolerance. However, it was only in the last few decades that the syndrome was more widely described by modern medical science.
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. Holzel et al. (1959) and Durand (1959) produced two of the earliest studies of lactose intolerance. As anecdotes of embarrassing dairy-induced discomfort increased, the First World donor countries could no longer ascribe the reports to spoilage in transit or inappropriate food preparation by the Third World recipients.
Because the first nations to industrialize and develop modern scientific medicine were dominated by people of European descent, adult dairy consumption was long taken for granted. Westerners for some time did not recognize that the majority of the human ethno-genetic groups could not consume dairy products 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 ethnic diversity of the human populations (as well as all genders and 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 p
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