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 resultant 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, thereby preventing the colon from reabsorbing water, and causing osmotic diarrhea.
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.
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.
|Human groups||Individuals Examined||Percent Intolerant||Allele frequency|
|Europeans in Australia||160||4%||0.20|
|Northern Europeans and Scandinavians||N/A||5%||N/A|
|Eastern Slavs (Russians, Belarusians, Ukrainians)||N/A||15%||N/A|
|African American Children||N/A||45%||N/A|
|Saami (in Russia and Finland)||N/A||25–60%||N/A|
|North American Hispanics||N/A||53%||N/A|
|Mexican American Males||N/A||55%||N/A|
|North American Jews||N/A||68.8%||N/A|
|Kazakhs from northwest Xinjiang||195||76.4%|
|Northeastern Han Chinese||248||92.3%|
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:
Chinese and Japanese populations typically lose between 80 and 90 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.
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 not considered a disease condition, a medical diagnosis is not normally required. However, if confirmation is necessary, three tests are available:
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 drawns, this approach has been largely supplanted by breath testing.
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 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. Genetics and protein analysis techniques by the early 1970s revealed this to be untrue; humans produce their own lactase enzyme natively in intestine cells.
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.
Roman authors recorded that the people of northern Europe, particularly Britain and Germany drank unprocessed milk (as opposed to the Romans who made cheese). 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.
The African Fulani have a nomadic origin and their culture once completely revolved around cow, goat, and sheep herding. Dairy products were once a large source of nutrition for them. As might be expected if lactase persistence evolved in response to dairy product consumption, they are particularly tolerant to lactose (about 77% of the population). Many Fulani live in Guinea-Conakry, Burkina Faso, Mali, Nigeria, Niger, Cameroon, and Chad.
There is some debate on exactly where and when genetic mutation(s) occurred. Some argue for separate mutation events in Sweden (which has one of the lowest levels of lactose intolerance in the world) and the Arabian Peninsula around 4000 BC. However, others argue for a single mutation event in the Middle East at about 4500 BC which then subsequently radiated. Some sources suggest a third and more recent mutation in the East African Tutsi. Whatever the precise origin in time and place, most modern Northern Europeans and people of European ancestry show the effects of this mutation (that is, they are able to safely consume milk products all their lives) while most modern East Asians, sub-Saharan Africans and native peoples of the Americas and Pacific Islands do not (making them lactose intolerant as adults). The Maasai ability to consume dairy without exhibiting symptoms may be due to a different genetic mutation. Or it may be due to the fact that they curdle their milk before they consume it, removing the lactose.
A thorough scientific overview of genetic polymorphisms of intestinal lactase activity in adult hypolactasia, is in chapter 76 of OMMBID. A noncoding variation in the MCM6 gene has been strongly associated with adult type hypolactasia.
Lactose is present in 2 large food categories: Conventional dairy products, and as a food additive (in dairy and non dairy products).
Milk. Human milk has the highest lactose percentage at around 9%. Unprocessed cow milk has 4.7% lactose. Unprocessed milk from other mammals 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 be present in small quantities in the butter unless it is also fermented to produce cultured butter.
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. However, many commercial brands contain milk solids, increasing the lactose content.
Cheeses. Traditionally made hard cheese (such as Swiss cheese) 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 Swiss 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 and ice cream, like yogurt, if made 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.
|Dairy product||Lactose Content|
|Yogurt, plain, low-fat, 240 mL||5 g|
|Milk, reduced fat, 240 mL||11 g|
|Swiss cheese, 28 g||1 g|
|Ice cream, 120 mL||6 g|
|Cottage cheese, 120 mL||2–3 g|
Kosher products labeled pareve are free of milk. However, if a "D" (for "Dairy) is present next to the circled "K," "U," or other hechsher, the food likely contains milk solids (although it may also simply indicate that the product was produced on equipment shared with other products containing milk derivatives).
Alternatively, a bacterium such as L. acidophilus may be added, which affects the lactose in milk the same way it affects the lactose in yogurt (see above).
Lactase enzymes similar to those produced in the small intestines of humans are produced industrially by fungi of the genus aspergillus. The enzyme, β-galactosidase, is available in tablet form in a variety of doses, in many countries without a prescription. It functions well only in high-acid environments, such as that found in the human gut due to the addition of gastric juices from the stomach. Unfortunately, too much acid can denature it, and it therefore should not be taken on an empty stomach. Also, the enzyme is ineffective if it does not reach the small intestine by the time the problematic food does. Lactose-sensitive individuals should experiment with both timing and dosage to fit their particular need. But supplements such as these may not be able to provide the accurate amount of lactase needed to adequately digest the lactose contained in dairy products, which may lead to symptoms similar to the existing lactose intolerance.
While essentially the same process as normal intestinal lactose digestion, direct treatment of milk employs a different variety of industrially produced lactase. This enzyme, produced by yeast from the genus kluyveromyces, takes much longer to act, must be thoroughly mixed throughout the product, and is destroyed by even mildly acidic environments. It therefore has been much less popular as a consumer product (sold, where available, as a liquid) than the aspergillus-produced tablets, despite its predictable effectiveness. Its main use is in producing the lactose-free or lactose-reduced dairy products sold in supermarkets.
Enzymatic lactase supplementation may have an advantage over avoiding dairy products, in that alternative provision does not need to be made to provide sufficient calcium intake, especially in children.
For healthy individuals with Secondary lactose intolerance, it may be possible to train bacteria in the large intestine to break down lactose more effectively by consuming small quantities of dairy products several times a day over a couple of weeks. Reintroducing dairy in this way to people who have an underlying or chronic illness, however, is not recommended, as certain illnesses damage the intestinal tract in a way which prevents the lactase enzyme from being expressed.
Some studies indicate that environmental factors (more specifically, the consumption of lactose) may "play a more important role than genetic factors in the etio-pathogenesis of milk intolerance", but some other publications suggest that lactase production does not seem to be induced by dairy/lactose consumption.
Plant based milk substitutes are not naturally rich in calcium, potassium, or vitamins A or D (and, like all non-animal products, contain no vitamin B12). However, prominent brands are often voluntarily fortified with many of these nutrients.
An increasing number of calcium-fortified breakfast foods, such as orange juice, bread, and dry cereal have been appearing on supermarket shelves. Many fruits and vegetables are rich in potassium and vitamin A; animal products like meat and eggs are rich in vitamin B12, and the human body itself produces some vitamin D from exposure to direct sunlight. Finally, a dietitian or physician may recommend a vitamin or mineral supplement to make up for any remaining nutritional shortfall.
Lactose-reduced dairy products have the same nutritional content as their full-lactose counterparts, but their taste and appearance may differ slightly.
Most infants with gastroenteritis due to rotavirus do not develop lactose intolerance, so these infants do not benefit from being put on a lactose-free diet unless symptoms of lactose intolerance are severe and persistent.