Lactose Intolerance
<p ><b>Lactose intolerance</b>, also called <b>lactase deficiency</b> and <b>hypolactasia</b>, is the inability to digest <a href="http://en.wikipedia.org/wiki/Lactose" title="Lactose" >lactose</a>, a sugar found in <a href="http://en.wikipedia.org/wiki/Milk" title="Milk" >milk</a> and to a lesser extent milk-derived <a href="http://en.wikipedia.org/wiki/Dairy_products" title="Dairy products" class="mw-redirect" >dairy products</a>.</p>
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Lactose intolerant individuals have insufficient levels of <a href="http://en.wikipedia.org/wiki/Lactase" title="Lactase" >lactase</a>, the <a href="http://en.wikipedia.org/wiki/Enzyme" title="Enzyme" >enzyme</a> that<a href="http://en.wikipedia.org/wiki/Metabolism" title="Metabolism" >metabolizes</a> lactose into <a href="http://en.wikipedia.org/wiki/Glucose" title="Glucose" >glucose</a> and <a href="http://en.wikipedia.org/wiki/Galactose" title="Galactose" >galactose</a>, in their digestive system. In most cases this causes symptoms such as abdominal bloating and cramps, flatulence, diarrhea, nausea, <a href="http://en.wikipedia.org/wiki/Borborygmi" title="Borborygmi" class="mw-redirect" >borborygmi</a> (rumbling stomach), or vomiting<sup id="cite_ref-NDDIC_0-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-NDDIC-0" >[1]</a></sup> after consuming significant amounts of lactose. Some studies in the U.S. and elsewhere<sup class="noprint Inline-Template" >[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:WikiProject_Countering_systemic_bias" title="Wikipedia:WikiProject Countering systemic bias" >where?</a></i>]</sup> suggest that milk consumption by lactose intolerant individuals may be a significant cause of<a href="http://en.wikipedia.org/wiki/Irritable_bowel_syndrome" title="Irritable bowel syndrome" >irritable bowel syndrome</a>.<sup id="cite_ref-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-1" >[2]</a></sup></p>
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Most <a href="http://en.wikipedia.org/wiki/Mammal" title="Mammal" >mammals</a> normally become lactose intolerant after <a href="http://en.wikipedia.org/wiki/Weaning" title="Weaning" >weaning</a>, but some human populations have developed <a href="http://en.wikipedia.org/wiki/Lactase_persistence" title="Lactase persistence" >lactase persistence</a>, in which lactase production continues into adulthood. It is estimated that 75% of adults worldwide show some decrease in lactase activity during adulthood.<sup id="cite_ref-2" class="reference" ><a href="http://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-2" >[3]</a></sup> The frequency of decreased lactase activity ranges from 5% in northern Europe through 71% for Sicily to more than 90% in some African and Asian countries.<sup id="cite_ref-autogenerated3_3-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-autogenerated3-3" >[4]</a></sup> This distribution is now thought to have been caused by recent<a href="http://en.wikipedia.org/wiki/Natural_selection" title="Natural selection" >natural selection</a> favoring lactase persistant individuals in cultures that rely on dairy products.<sup id="cite_ref-4" class="reference" ><a href="http://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-4" >[5]</a></sup> While it was first thought that this would mean that populations in Europe, India and Africa had high frequencies of lactase persistence because of a particular mutation, it has now been shown that lactase persistence is caused by several independently occurring mutations.<sup id="cite_ref-5" class="reference" ><a href="http://en.wikipedia.org/wiki/Lactose_intolerance#cite_note-5" >[6]</a></sup></p>