Where’s My Enzyme? Explaining Lactose Intolerance

By: Alisha Gomes


Image Credit: Flickr @ Arne Hendriks

You’ve most likely heard the term “lactose intolerant” in your life. Maybe your classmate had thrown up after digesting milk. Maybe your cousin couldn’t stop burping at the dinner table after eating cheese. Whether you’ve heard of it or not, lactose intolerance is a fascinating digestive disorder caused by the inability to digest lactose, a common sugar found in dairy products. What’s so fascinating about lactose intolerance? This digestive disorder, like many things, has its roots in evolution. You would think that evolution would lead to the ability to digest milk but it turns out to be the other way around.


Most adults cannot digest milk. Babies, however, can easily digest it. This is because of the enzyme lactase, which can break down lactose. The gene that makes lactase (the enzyme that breaks down lactase into glucose and galactose to make them small enough for intestinal absorption) is turned off once a person becomes an adult—and about 40% of humans lose the ability to digest milk between the ages of 2 to 5. People of Northern/Central European, African, and Middle Eastern descent who have adopted a pastoral lifestyle, however, have developed lactose tolerance in a very short period of time. This characteristic is because milk was abundant during times of famine and the consumption of milk grew in places of poverty. Thus, those who drank milk and could digest it had a selective advantage. Those who were lactose intolerant would have been the least likely to survive and pass on their genes to the next generation and those who were lactose tolerant were the most likely to do such.


Enzymes are biological catalysts that are contained within the class of proteins. Enzymes do not cause reactions to occur—instead, they help to speed up reactions. Starch or amylose is a polymer made of many glucose monomers (hence it is called a polysaccharide). Starch is a carbohydrate and is found in many foods like potatoes, maize, and peas. The monomers are connected together by covalent bonds through dehydration synthesis (which is the formation of long molecules by losing a water molecule). Hydrolysis is the breaking down of polymers into monomers by the adding of water molecules. For biological organisms to use the energy contained in starches, the polysaccharide would have to be broken down into its constituent monomers (glucose). The hydrolysis of starch to glucose uses the enzyme amylase which is found in saliva and in the pancreas. More importantly, it has been supported that certain regional populations contain more copies of the gene that produces amylase. For example, a population that is dependent upon starch-heavy foods will have more copies of the gene that produces amylase.


Understanding lactose intolerance not only helps to understand evolution better but it also helps us understand the people around us better. The next time someone mentions lactose intolerance, be sure to tell them the reason behind it: evolution.


Citations:

https://www.carolina.com/knowledge/2020/01/24/the-science-behind-lactose-intolerance

http://www.evo-ed.org/Pages/Lactase/cellbio.html

https://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/25-enzymes/lactose-intolerance.html

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/lactose-intolerance

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4586575/

https://www.nhs.uk/conditions/lactose-intolerance/causes/

https://www.hopkinsmedicine.org/health/conditions-and-diseases/lactose-intolerance


Image Credit: No changes were made to the following image, https://www.flickr.com/photos/arnehendriks/27043384163/in/photolist-goiLg5-e6iSUs-g6Qsu3-gng8r3-9NsALh-pR54Kg-8NAU5g-gnfUMe-HcJpWc-F9SJAd-tQs7Md-EJRsKK-o9edG-dx7Y9L-3x2f6-nVG2ef-AE143A-KupR2z-LpwGx-7jTxuN-7G1PEn-g8N1d-9DX6TC-j74er7-7yL6jG-8ouyP-tejJv7-2Mt944-9S3q9F-goCFFy-4JxHg8-bf7V8g-9H3y4t-uqXBK-8mXhGq-6Q5fEs-8ActAJ-5F6PZy-B1JD2-9o8twt-6VbER1-6zB6sT-5xyfWS-dAZHeY-2hxMr3-PmMLGn-Dm2G8j-6bEKa, License: https://creativecommons.org/licenses/by/2.0/legalcode


What Did You Learn?

Questions:

1. What causes lactose intolerance?


Lactose intolerance occurs when the small intestine does not produce enough of the enzyme lactase that digests milk sugar called lactose. What causes lactose intolerance is evolution. People of a certain background (Northern/Central European, African, and Middle Eastern) adopted a pastoral lifestyle and eventually developed lactose tolerance in a very short period of time since milk was abundant during times of famine and the consumption of milk grew in places of poverty. That said, those who were lactose intolerant would have been the least likely to survive and pass on their genes to the next generation and those who were lactose tolerant were the most likely to do such.


2. When do people lose the ability to digest milk?


About 40% of humans lose the ability to digest milk between the ages of 2 to 5 because lactase (the enzyme that breaks down lactase into glucose and galactose to make them small enough for intestinal absorption) is turned off once a person becomes an adult.


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