<p>Lactose intolerance is due to the lack of&nbsp;<a href="https://en.wikipedia.org/wiki/Enzyme" title="Enzyme">enzyme</a>&nbsp;<a href="https://en.wikipedia.org/wiki/Lactase" title="Lactase">lactase</a>&nbsp;in the&nbsp;<a href="https://en.wikipedia.org/wiki/Small_intestines" title="Small intestines">small intestines</a>&nbsp;to break lactose down into&nbsp;<a href="https://en.wikipedia.org/wiki/Glucose" title="Glucose">glucose</a>&nbsp;and&nbsp;<a href="https://en.wikipedia.org/wiki/Galactose" title="Galactose">galactose</a>.&nbsp;There are four types: primary, secondary, developmental, and congenital.&nbsp;Primary lactose intolerance occurs as the number of lactase declines as people age.<span style="font-size:10.8333px"> </span>Secondary lactose intolerance is due to injury to the small intestine such as from infection,&nbsp;<a href="https://en.wikipedia.org/wiki/Celiac_disease" title="Celiac disease">celiac disease</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Inflammatory_bowel_disease" title="Inflammatory bowel disease">inflammatory bowel disease</a>, or other diseases.<span style="font-size:10.8333px"> </span>Developmental lactose intolerance may occur in&nbsp;<a href="https://en.wikipedia.org/wiki/Premature_babies" title="Premature babies">premature babies</a>&nbsp;and usually improves over a short period of time.&nbsp;Congenital lactose intolerance is an extremely rare&nbsp;<a href="https://en.wikipedia.org/wiki/Genetic_disorder" title="Genetic disorder">genetic disorder</a>&nbsp;in which little or no lactase is made from birth.</p>

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

<p>Lactose intolerance is classified according to its causes as:</p>

<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.&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.</p>

<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.&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>,&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">Giardia</a>), or other environmental causes.</p>

<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;<a 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/Infant_formula" title="Infant formula">infant formulas&nbsp;</a>and manufactured lactose-free dairy products.</p>

<p>People with primary lactase deficiency cannot modify their body&rsquo;s ability to produce lactase.&nbsp;In societies where lactose intolerance is the norm, it is not considered a condition that requires treatment. However, where dairy is 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>

<p>The primary way of managing the symptoms of lactose intolerance is to limit the intake of lactose to a level that can be tolerated.&nbsp;Lactase deficient individuals vary in the amount of lactose they can tolerate,&nbsp;and some report that their tolerance varies over time, depending on health status and pregnancy.&nbsp;However, as a rule of thumb, people with primary lactase deficiency and no small intestine injury are usually able to consume 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>

<p>Further information:&nbsp;<a href="https://en.wikipedia.org/wiki/Milk#Reduction_or_elimination_of_lactose" title="Milk">Milk &sect;&nbsp;Reduction or elimination of lactose</a>, and&nbsp;<a href="https://en.wikipedia.org/wiki/Milk_substitute" title="Milk substitute">Milk substitute</a></p>

<p><a href="https://en.wikipedia.org/wiki/Plant_milk" title="Plant milk">Plant-based &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">coconut milk</a>,&nbsp;<a href="https://en.wikipedia.org/wiki/Hazelnut" title="Hazelnut">hazelnut</a>&nbsp;milk,&nbsp;<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, 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 lactose intolerance.</p>

<p>When lactose avoidance is not possible, or on occasions when a person chooses to consume such items, then enzymatic lactase supplements may be used.</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/Yeast" title="Yeast">yeast</a>&nbsp;from the genus&nbsp;<em><a href="https://en.wikipedia.org/wiki/Kluyveromyces" title="Kluyveromyces">Kluyveromyces</a></em>, 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.</p>

<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>