<p><strong><span style="font-size:20px">Causes</span></strong></p>

<p>Lactose intolerance is a consequence of&nbsp;<a href="https://en.wikipedia.org/wiki/Lactase" title="Lactase">lactase</a>&nbsp;deficiency, which may be genetic (<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#Primary_hypolactasia">primary hypolactasia</a>&nbsp;and&nbsp;<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#Primary_congenital_alactasia">primary congenital alactasia</a>) or environmentally induced (<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#Secondary_hypolactasia_or_acquired_hypoalactasia">secondary or acquired hypoalactasia</a>). In either case, symptoms are caused by insufficient levels of lactase in the lining of the&nbsp;<a href="https://en.wikipedia.org/wiki/Duodenum" title="Duodenum">duodenum</a>. Lactose, a&nbsp;<a href="https://en.wikipedia.org/wiki/Disaccharide" title="Disaccharide">disaccharide</a>&nbsp;molecule found in milk and dairy products, cannot be directly absorbed through the wall of the small intestine into the bloodstream, so, in the absence of lactase, passes intact into the&nbsp;<a href="https://en.wikipedia.org/wiki/Colon_(anatomy)" title="Colon (anatomy)">colon</a>.&nbsp;<a href="https://en.wikipedia.org/wiki/Gut_flora" title="Gut flora">Bacteria</a>&nbsp;in the colon can metabolise metabolize lactose, and the resulting&nbsp;<a href="https://en.wikipedia.org/wiki/Fermentation_(biochemistry)" title="">fermentation</a>&nbsp;produces copious amounts of gas (a mixture of&nbsp;<a href="https://en.wikipedia.org/wiki/Hydrogen" title="Hydrogen">hydrogen</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Carbon_dioxide" title="Carbon dioxide">carbon dioxide</a>, and&nbsp;<a href="https://en.wikipedia.org/wiki/Methane" title="Methane">methane</a>) that causes the various abdominal symptoms. The unabsorbed sugars and fermentation products also raise the&nbsp;<a href="https://en.wikipedia.org/wiki/Osmotic_pressure" title="Osmotic pressure">osmotic pressure</a>&nbsp;of the colon, causing an increased flow of water into the bowels (diarrhea).</p>

<p>The&nbsp;<em>LCT</em>&nbsp;gene provides the instructions for making lactase. The specific DNA sequence in the&nbsp;<em>MCM6</em>&nbsp;gene helps control whether the&nbsp;<em>LCT</em>&nbsp;gene is turned on or off.&nbsp;At least several thousand years ago, some humans developed a mutation in the&nbsp;<em>MCM6</em>&nbsp;gene that keeps the&nbsp;<em>LCT</em>&nbsp;gene turned on even after breastfeeding is stopped.&nbsp;Populations that are lactose intolerant lack this mutation. The&nbsp;LCT and&nbsp;<em>MCM6</em>&nbsp;genes are both located on the long arm (q) of chromosome 2 in region 21. The locus can be expressed as 2q21.<span style="font-size:10.8333px"> </span>The lactase deficiency also could be linked to a certain heritage. It is more common in Asian Americans, African Americans, Mexican Americans, and Native Americans.&nbsp;Analysis of the DNA of 94 ancient skeletons in Europe and Russia concluded that the mutation for lactose tolerance appeared about 4,300 years ago and spread throughout the European population.</p>

<p>Some human populations have developed&nbsp;<a href="https://en.wikipedia.org/wiki/Lactase_persistence" title="Lactase persistence">lactase persistence</a>, in which lactase production continues into adulthood probably as a response to the benefits of being able to digest milk from farm animals.&nbsp;Some have argued that this links intolerance to&nbsp;<a href="https://en.wikipedia.org/wiki/Natural_selection" title="Natural selection">natural selection</a>&nbsp;favoring lactase-persistent individuals, but it is also consistent with a physiological response to decrease decreasing lactase production when it is not needed in cultures in which dairy products are not an available food source.&nbsp;Although populations in Europe, India, Arabia, and Africa were first thought to have high rates of lactase persistence because of a single mutation, lactase persistence has been traced to a number of mutations that occurred independently.&nbsp;Different alleles for lactase persistence have developed at least three times in East African populations, with persistence extending from 26% in&nbsp;<a href="https://en.wikipedia.org/wiki/Tanzania" title="Tanzania">Tanzania</a>&nbsp;to 88% in the&nbsp;<a href="https://en.wikipedia.org/wiki/Beja_people" title="Beja people">Beja</a>&nbsp;pastoralist population in&nbsp;<a href="https://en.wikipedia.org/wiki/Sudan" title="Sudan">Sudan</a>.</p>

<h3>Primary hypolactasia[<a hrefspan style="httpsfont-size:16px">Primary <//en.wikipedia.org/w/index.php?titlespan><span style=Lactose_intolerance&amp;action=edit&amp;section=4" title="Edit sectionfont-size: Primary hypolactasia16px">edithypolactasia</aspan>]</h3>

<p>Primary hypolactasia, or primary lactase deficiency, is genetic, only affects adults, and is caused by the absence of a lactase persistence allele. In individuals without the lactase persistence allele, less lactase is produced by the body over time, leading to hypolactasia in adulthood.^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-Heyman2006-2">[2]</a>}^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-variant-25">[25]</a>}&nbsp;The frequency of lactase persistence, which allows lactose tolerance, varies enormously worldwide, with the highest prevalence in Northwestern Europe, declines across southern Europe and the Middle East and is low in Asia and most of Africa, although it is common in pastoralist populations from Africa.^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-Deng2015-3">[3]</a>}^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-Swallow_2003-26">[26]</a>}</p>

<h3>Secondary hypolactasia[<a hrefspan style="httpsfont-size:16px">Secondary <//en.wikipedia.org/w/index.php?titlespan><span style=Lactose_intolerance&amp;action=edit&amp;section=5" title="Edit sectionfont-size: Secondary hypolactasia16px">edithypolactasia</aspan>]</h3>

<p>Secondary hypolactasia or secondary lactase deficiency, also called acquired hypolactasia or acquired lactase deficiency, is caused by an injury to the&nbsp;<a href="https://en.wikipedia.org/wiki/Small_intestine" title="Small intestine">small intestine</a>. This form of lactose intolerance can occur in both infants and lactase persistent adults and is generally reversible.^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-27">[27]</a>}&nbsp;It may be caused by acute&nbsp;<a href="https://en.wikipedia.org/wiki/Gastroenteritis" title="Gastroenteritis">gastroenteritis</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Coeliac_disease" title="Coeliac disease">coeliac disease</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Crohn%27s_disease" title="Crohn's disease">Crohn&#39;s disease</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Ulcerative_colitis" title="Ulcerative colitis">ulcerative colitis</a>,^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-28">[28]</a>}&nbsp;<a href="https://en.wikipedia.org/wiki/Chemotherapy" title="Chemotherapy">chemotherapy</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Intestinal_parasites" title="Intestinal parasites">intestinal parasites</a>&nbsp;(such as&nbsp;<a href="https://en.wikipedia.org/wiki/Giardia" title="Giardia">giardiaGiardia</a>), or other environmental causes.^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-Heyman2006-2">[2]</a>}^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-SwagertyWalling2002-29">[29]</a>}^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-30">[30]</a>}^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-eMedicine_pediatric-31">[31]</a>}^{<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-SwagertyWalling2002-29">[29]</a>}</p>

<h3>Primary congenital alactasia[<a hrefspan style="httpsfont-size:16px">Primary congenital <//en.wikipedia.org/w/index.php?title=Lactose_intolerance&amp;action=edit&amp;section=6" titlespan><span style="Edit sectionfont-size: Primary congenital alactasia16px">editalactasia</aspan>]</h3>

<p>Primary congenital alactasia, also called congenital lactase deficiency, is an extremely rare,&nbsp;<a href="https://en.wikipedia.org/wiki/Autosomal_recessive" title="Autosomal recessive">autosomal recessive</a>&nbsp;enzyme defect that prevents lactase expression from birth.&nbsp;People with congenital lactase deficiency cannot digest lactose from birth, so cannot digest breast milk. This genetic defect is characterized by a complete lack of lactase (alactasia). About 40 cases have been reported worldwide, mainly limited to&nbsp;<supa href="https://en.wikipedia.org/wiki/Finland" title="Finland">Finland</a>.&nbsp;Before the 20th century, babies born with congenital lactase deficiency often did not survive, but death rates decreased with&nbsp;<a href="https://en.wikipedia.org/wiki/Soybean" title="Soybean">soybean</a>-derived&nbsp;<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_noteInfant_formula" title="Infant formula">infant formulas&nbsp;</a>and manufactured lactose-Heyman2006free dairy products.</p> <p><span style="font-2size:20px">[2]<strong>Management</astrong></span></supp> <suphr /><p>When lactose intolerance is due to secondary lactase deficiency, treatment of the underlying disease may allow lactase activity to return to normal levels.&nbsp;In people with coeliac disease, lactose intolerance normally reverts or improves several months after starting a&nbsp;<a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_noteGluten-Deng2015free_diet" title="Gluten-3free diet">[3]gluten-free diet</a>, but temporary dietary restriction of lactose may be needed.</supp> <p>&nbsp;People with congenital primary lactase deficiency cannot digest modify their body&rsquo;s ability to produce lactase.&nbsp;In societies where lactose from birthintolerance is the norm, so cannot digest breast milkit is not considered a condition that requires treatment. This genetic defect However, where dairy is characterized a larger component of the normal diet, a number of efforts may be useful. There are four general principles in dealing with lactose intolerance: avoidance of dietary lactose, substitution to maintain nutrient intake, regulation of calcium intake, and use of enzyme substitute.&nbsp;Regular consumption of dairy food by lactase-deficient individuals may also reduce symptoms of intolerance by promoting colonic bacteria adaptation.</p> <h3><span style="font-size:16px">Dietary avoidance</span></h3> <p>The primary way of managing the symptoms of lactose intolerance is to limit the intake of lactose to a complete lack level that can be tolerated.&nbsp;Lactase deficient individuals vary in the amount of lactase (alactasia)lactose they can tolerate,&nbsp;and some report that their tolerance varies over time, depending on health status and pregnancy. About 40 cases have been reported worldwide&nbsp;However, as a rule of thumb, mainly limited people with primary lactase deficiency and no small intestine injury are usually able toconsume at least 12 grams of lactose per sitting without symptoms, or with only mild symptoms, with greater amounts tolerated if consumed with a meal or throughout the day.</p> <h3><span style="font-size:16px">Milk substitutes</span></h3> <p>Further information:&nbsp;<a href="https://en.wikipedia.org/wiki/FinlandMilk#Reduction_or_elimination_of_lactose" title="FinlandMilk">FinlandMilk &sect;&nbsp;Reduction or elimination of lactose</a>, and&nbsp;<a href="https://en.wikipedia.org/wiki/Milk_substitute" title="Milk substitute">Milk substitute<sup/a></p> <p><a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_notePlant_milk" title="Plant milk">Plant-Deng2015-3based &quot;milks&quot;</a>&nbsp;and derivatives such as&nbsp;<a href="https://en.wikipedia.org/wiki/Soy_milk" title="Soy milk">soy milk</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Rice_milk" title="Rice milk">rice milk</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Almond_milk" title="Almond milk">almond milk</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Coconut_milk" title="Coconut milk">[3]coconut milk</a>,&nbsp;<a href="https://supen.wikipedia.org/wiki/Hazelnut" title="Hazelnut">hazelnut</a>&nbsp;milk,&nbsp;Before the 20th century<a href="https://en.wikipedia.org/wiki/Oat_milk" title="Oat milk">oat milk</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Hemp_milk" title="Hemp milk">hemp milk</a>, macadamia nut milk, babies born and&nbsp;<a href="https://en.wikipedia.org/wiki/Peanut_milk" title="Peanut milk">peanut milk</a>&nbsp;are inherently lactose-free. Low-lactose and lactose-free versions of foods are often available to replace dairy-based foods for those with congenital lactase deficiency often did lactose intolerance.</p> <h3><span style="font-size:16px">Lactase supplements</span></h3> <p>When lactose avoidance is not survivepossible, or on occasions when a person chooses to consume such items,then enzymatic lactase supplements may be used.</p> <p>Lactase enzymes similar to those produced in the small intestines of humans are produced industrially by&nbsp;<a href="https://en.wikipedia.org/wiki/Fungi" title="Fungi">fungi</a>&nbsp;of the&nbsp;<a href="https://en.wikipedia.org/wiki/Genus" title="Genus">genus</a>&nbsp;<supem><a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-Heyman2006-2Aspergillus" title="Aspergillus">[2]Aspergillus</a></supem>. The enzyme,&nbsp;but death rates decreased <a href="https://en.wikipedia.org/wiki/Beta-galactosidase" title="Beta-galactosidase">&beta;-galactosidase</a>, 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,&nbsp;so it 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 withboth timing and dosage to fit their particular needs.</p> <p>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&nbsp;<a href="https://en.wikipedia.org/wiki/SoybeanYeast" title="SoybeanYeast">soybeanyeast</a>-derived&nbsp;from the genus&nbsp;<em><a href="https://en.wikipedia.org/wiki/Infant_formulaKluyveromyces" title="Infant formulaKluyveromyces">infant formulasKluyveromyces</a></em>, takes much longer to act, must be thoroughly mixed throughout the product, and manufactured is destroyed by even mildly acidic environments. Its main use is in producing the lactose-free or lactose-reduced dairy productssold in supermarkets.<sup/p> <h3><span style="font-size:16px">Rehabituation to dairy products</span></h3> <p>Regular consumption of dairy foods containing lactose can promote a colonic bacteria adaptation, enhancing a favorable microbiome, which allows people with primary lactase deficiency to diminish their intolerance and to consume more dairy foods.&nbsp;The way to induce tolerance is based on progressive exposure, consuming smaller amounts frequently, distributed throughout the day. Lactose intolerance can also be managed by ingesting live yogurt cultures containing&nbsp;<a href="https://en.wikipedia.org/wiki/Lactobacillus_acidophilus" title="Lactobacillus acidophilus">lactobacilli</a>&nbsp;that are able to digest the lactose in other dairy products. This may explain why many South Asians, though genetically lactose intolerant, are able to consume large quantities of milk without any symptoms of lactose intolerance, since consuming live yogurt cultures is very common among the South Asian population.</p> <p>&nbsp;</p> <p><span style="font-size:20px"><strong>Reference</strong></span></p> <hr /><p>1. <a href="https://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-32Vandenplas2015-5">[32]&quot;Lactose intolerance&quot;</a></sup>, from Wikipedia</p>