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, nausea, bloating, acid reflux 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.

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.

Some cultures, such as that of Japan, where dairy consumption has been on the increase compared with traditional dietary patterns, 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 (BMJ Textbook of Gastroenterology, Chapter 11, Celiac Disease, Dr.Jamie Gregor & Dr. Diamond Sherin Alidina).

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

'

Human group	Individuals examined	Intolerance (%)	Reference	Allele frequency
Basque people|Basques	85	0.3		N/A
Dutch people|Dutch	N/A	1		N/A
Danish people|Danes	N/A	4		N/A
Anglo-Celtic Australian|Europeans in Australia	160	4		0.20
Swedish people|Swedes	N/A	5-7		N/A
Demographics of the United Kingdom|British	N/A	5???15		0.184-0.302
German people|Germans	1805	6-23		N/A
Swiss	N/A	10		0.316
European Americans	245	12		0.346
Tuareg	N/A	13		N/A
Finnish people|Finns	N/A	14-23		N/A
Eastern Slavs (Russians, Belarusians, Ukrainians)	N/A	15		N/A
Austrians	N/A	15???20		N/A
Spanish people|Spaniards (non-Basque)	N/A	15		N/A
French people|Northern French	N/A	17		N/A
Central Italian people|Italians	65	19		N/A
Demographics of India|Indians	N/A	20		N/A
African Tutsi	N/A	20		0.447
African Fulani	N/A	23		0.48
Bedouins	N/A	25		N/A
Portuguese people|Portuguese adults	102	35		N/A
Southern Italian people|Italians	51	41		N/A
African American Children	N/A	45		N/A
Sami people|Saami (in Russia and Finland)	N/A	25???60		N/A
Northern Italian people|Italians	89	52		N/A
North American Hispanics	N/A	53		N/A
Balkans	N/A	55		N/A
Mexican American Males	N/A	55		N/A
Cretans	N/A	56		N/A
African Maasai	21	62		N/A
French people|Southern French	N/A	65		N/A
Greek Cypriots	N/A	66		N/A
Mizrahi Jews|Jews, Mizrahi (Iraq, Iran, etc)	N/A	85		N/A
American Jews|Jews, North American	N/A	68.8		N/A
Sephardi Jews|Jews, Sephardic	N/A	62		N/A
Yemenite Jews|Jews, Yemenite	N/A	44		N/A
Sicily#People|Sicilians	100	71		N/A
South Americans	N/A	65???75		N/A
Rural Mexicans	N/A	73.8		N/A
African Americans	20	75		0.87
Kazakhs from northwest Xinjiang	195	76.4%
Demographics of Lebanon|Lebanese	75	78		N/A
Central Asians	N/A	80		N/A
Alaska Natives|Alaskan Eskimo	N/A	80		N/A
Australian Aborigines	44	85		0.922
Inner Mongolians	198	87.9
African Bantu peoples|Bantu	59	89		0.943
Asian Americans	N/A	90		N/A
Northeastern Han Chinese	248	92.3
Chinese people|Chinese	71	95		0.964
Southeast Asians	N/A	98		N/A
Thai people|Thais	134	98		0.99
Native Americans in the United States|Native Americans	24	100		1.00

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.

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 a milk allergy, which is an abnormal immune response (usually) to milk proteins. This may be done by giving lactose-free milk to a person displaying a reaction to milk. If the condition is purely lactose intolerance, the sufferer's symptoms will vanish, but if it is a milk allergy, the person will have the same reaction to the lactose-free milk as to normal milk. (An intermediate result might suggest that the person has both conditions.) 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 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 20th century 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. The term Milk drinking syndrome stands for a phenomenon when European experience is extrapolated to the other populations of the world.

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 hypothesised 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. Some form of human-bacteria symbiosis was proposed as a means of producing lactase in the human digestive tract.

Nomenclature

According to Heyman (2006), 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 groups should be labeled as having lactase persistence . A counter-argument to this is that the cultures that don't generally consume unmodified milk products have little need to discuss their intolerance to it, leaving the cultures for which lactose intolerance is a significant dietary issue to define its terminology.

The ability to digest lactose into adulthood (lactase persistence) would have only been useful to humans after the invention of animal husbandry and the domestication of animal species that could provide a consistent source of milk. Hunter-gatherer populations before the Neolithic revolution were overwhelmingly lactose intolerant,

Several genetic markers for lactase persistence have been identified, and these show that the allele has multiple origins in different parts of the world (i.e. it is an example of convergent evolution).

The evolutionary processes driving the rapid spread of lactase persistence in some populations are not known. In some East African ethnic groups lactase persistence has gone from negligible to near-ubiquitous frequencies in just three thousand years, suggesting a very strong selective pressure. Competing theories on why the ability to digest lactose might be selected for include nutritional benefits, milk as a water source in times of drought, and increased calcium absorption helping to prevent rickets and osteomalacia in low-light regions.

Roman authors recorded that the people of northern Europe, particularly Britain and Germany , drank unprocessed milk. 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. This tolerance is thought to be advantageous, as the nomads do not settle down long enough to process mature cheese. Given that their prime source of income is generated through horses, to ignore their milk as a source of calories would be greatly detrimental. The nomads also make an alcoholic beverage, called Kumis, from horse milk, although the fermentation process reduces the amount of lactose present.

For persons living in societies where the diet contains relatively little dairy, lactose intolerance is not considered a condition that requires treatment. However, those living among societies that are largely lactose-tolerant may find lactose intolerance troublesome. Although there are still no methodologies to reinstate lactase production, some individuals have reported their intolerance to vary over time (depending on health status and pregnancy).

Lactose intolerance is not usually an all-or-nothing condition: the reduction in lactase production???and hence, the amount of lactose that can be tolerated???varies from person to person. Since lactose intolerance poses no further threat to a person's health, managing the condition consists of minimizing the occurrence and severity of symptoms. Berdanier and Hargrove recognise four general principles: avoidance of dietary lactose, substitution to maintain nutrient intake, regulation of calcium intake, and use of enzyme substitute.

Avoiding lactose-containing products

Since each individual's tolerance to lactose varies, according to the US National Institute of Health, "Dietary control of lactose intolerance depends on people learning through trial and error how much lactose they can handle." Label reading is essential, as commercial terminology varies according to language and region.

Lactose is present in two large food categories: conventional dairy products, and as a food additive (in dairy and non dairy products).

Dairy products

Lactose is a water-soluble molecule. Therefore, fat percentage and the curdling process have an impact on which foods may be tolerated. After the curdling process, lactose is found in the water portion (along with whey and casein), but is not found in the fat portion. Dairy products which are "fat reduced" or "fat free" generally have a slightly higher lactose percentage. Additionally, low fat dairy foods also often have various dairy derivatives such as milk solids added to them to enhance sweetness, increasing the lactose content.

Milk . Human milk has the highest lactose percentage at around 9%. Unprocessed cow milk has 4.7% lactose. Unprocessed milk from other bovids contains similar lactose percentages ( goat milk 4.1%, buffalo 4.86%, yak 4.93%, sheep milk 4.6%)

Butter . The butter-making process separates the majority of milk's water components from the fat components. Lactose, being a water soluble molecule, will still be present in small quantities in the butter unless it is also fermented to produce cultured butter.

Yogurt , Frozen Yogurt and kefir. People can be more tolerant of traditionally made yogurt than milk, because it contains lactase enzyme produced by the bacterial cultures used to make the yogurt. Frozen yogurt, if cultured similarly to its unfrozen counterpart, will contain similarly reduced lactose levels. However, many commercial brands contain milk solids, increasing the lactose content.

Cheeses . Traditionally made hard cheese (such as Emmental ) and soft ripened cheeses may create less reaction than the equivalent amount of milk because of the processes involved. Fermentation and higher fat content contribute to lesser amounts of lactose. Traditionally made Emmental or Cheddar might contain 10% of the lactose found in whole milk. In addition, the traditional aging methods of cheese (over 2 years) reduces their lactose content to practically nothing. Commercial cheese brands, however, are generally manufactured by modern processes that do not have the same lactose reducing properties, and as no regulations mandate what qualifies as an "aged" cheese, this description does not provide any indication of whether the process used significantly reduced lactose.

Sour cream if made in the traditional way, may be tolerable, but most modern brands add milk solids. Consult labels.

Examples of lactose levels in foods . As scientific consensus has not been reached concerning lactose percentage analysis methods (non-hydrated form or the mono-hydrated form), and considering that dairy content varies greatly according to labeling practices, geography and manufacturing processes, lactose numbers may not be very reliable. The following are examples of lactose levels in foods which commonly set off symptoms. These quantities are to be treated as guidelines only.