Lactose intolerance is the inability to metabolize lactose, because of a lack of the required enzyme lactase in the digestive system. 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.
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.
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.
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, diarrhoea, 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.
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.
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.
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.
Managing lactose intolerance
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 4 general principles: 1) avoidance of dietary lactose; 2) substitution to maintain nutrient intake; 3) regulation of calcium intake; 4) 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).
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 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|
Lactose in non-dairy products
Lactose (also present when labels state lactoserum, whey, milk solids, modified milk ingredients, etc.) is a commercial food additive used for its texture, flavour and adhesive qualities, and is found in foods such as processed meats (sausages/hot dogs, sliced meats, pâtés), gravy stock powder, margarines sliced breads, breakfast cereals, potato chips, dried fruit, processed foods, medications, pre-prepared meals, meal replacement (powders and bars), and protein supplements (powders and bars).
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).
Plant based milks and derivatives are inherently lactose free: soy milk, rice milk, almond milk, hazelnut milk, oat milk, hemp milk, peanut milk, horchata.
The dairy industry has created low-lactose or lactose-free products to replace regular dairy. Lactose-free milk can be produced by passing milk over lactase enzyme bound to an inert carrier; once the molecule is cleaved, there are no lactose ill-effects. A form is available with reduced amounts of lactose (typically 30% of normal), and alternatively with nearly 0%.
Finland, where approximately 17% of the Finnish-speaking population has hypolactasia, has had "HYLA" (acronym for hydrolysed lactose) products available for many years. These low-lactose level cow's milk products, ranging from ice cream to cheese, use a Valio patented chromatographic separation method to remove lactose. The ultra-pasteurization process, combined with aseptic packaging, ensures a long shelf-life.
Recently, the range of low-lactose products available in Finland has been augmented with milk and other dairy products (such as ice cream, butter, and buttermilk) that contain no lactose at all. The remaining about 20% of lactose in HYLA products is taken care of enzymatically. These typically cost slightly more than equivalent products containing lactose. Valio also markets these products in Sweden and in Estonia.
In the UK, where an estimated 15% of the population are affected by lactose intolerance, Lactofree produces milk, cheese, and yogurt products which contain only 0.03% lactose.
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).
When lactose avoidance is not possible, or on occasions when a person chooses to consume such items, then enzymatic lactase supplements may be used.
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 can experiment with both timing and dosage to fit their particular needs.
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. 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.
Rehabituation to dairy products
For healthy individuals with secondary lactose intolerance, it may be possible in some cases for the bacteria in the large intestine to adapt to an altered diet and break down small quantities of lactose more effectively by habitually consuming small amounts of dairy products several times a day over a period of time. 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.