13 minute read
Ujvala Jupalli '25
Lactose Intolerance as the “Norm”
BY UJVALA JUPALLI '25
Cover Image: ose with lactose intolerance have to watch their consumption of dairy products and switch out milk, cheese, ice cream, yogurt, and other dairy-containing products. ese changes in lifestyle may require some extra e ort but, overall, lead to better health bene ts for those without the ability to digest lactose.
Image Source: Wikimedia Commons What is Lactose Intolerance?
Lactose intolerance a ects up to 65% to 70% of the world’s adult population and is more common in certain countries than in others (Bayless et al., 2017). e disorder results from an inability to digest lactose, which is a disaccharide found in most dairy products. Lactose is made up of the two monosaccharides: glucose and galactose. Monosaccharides are also known as simple sugars; these molecules can be directly absorbed through the wall of the small intestine and the hepatic portal system. Lactose-intolerant individuals lack the ability to break these two molecules apart, which causes the physical symptoms of the syndrome. Individuals with lactose intolerance face many consequences a er consuming lactose, including stomach cramps, stomachaches, nausea, bloating, high levels of gas, and diarrhea. ese individuals may try to avoid consuming dairy products by switching out cow milk for substitutes like oat milk, almond milk, or soy milk.
Interestingly, lactose intolerant individuals are not lactose intolerant throughout their whole lifetime. Humans, like most mammals, are born with the ability to digest the lactose in breastmilk (the main source of nutrients and calories for a newborn). However, we lose our ability to break down lactose a er weaning; this phenomenon is referred to as lactase non-persistence, or LNP. Individuals who retain the ability to digest lactose a er weaning have a mutation of an autosomal dominant trait that allows them to continue to do so. ere are some individuals who are unable to break down lactose from birth. is is known as congenital lactase de ciency or CLD (Diekmann, 2015). ese infants are at an elevated risk for weight loss and severe dehydration if they are not given lactose-free milk/formula. Infants with this condition in the developing world are at a great risk of developing life-threatening diseases due to the lack of access to adequate healthcare and nutrition. Lastly, there is a group of individuals who are lactose tolerant but lose their ability to digest lactose during childhood or adulthood. is is called secondary lactose intolerance and is due to a decrease in lactase production caused by an injury, illness, or surgery involving the small intestine. For instance, illnesses such as intestinal infections, bacterial overgrowth in the intestines, and in ammation of the digestive tract (also known as Crohn’s disease) can play a major role in the development of secondary lactose intolerance (Luthy et al., 2017). Lactose intolerance can be diagnosed using either a hydrogen breath test or a lactose tolerance test. e hydrogen breath test measures the amount of hydrogen in an individual’s breath a er their consumption of lactose. Breathing out large amounts of hydrogen means that the
lactose wasn’t digested in the small intestine. Instead, that lactose was fermented by bacteria in the colon which resulted in the production of hydrogen and other gases (Catanzaro et al., 2021). e lactose tolerance test measures glucose levels in the bloodstream at various intervals a er the consumption of lactose. If the level of glucose in the blood doesn’t rise, it means that the lactose wasn’t broken down by the lactase enzyme in the small intestine and absorbed into the bloodstream. Although this test doesn’t require expensive equipment, it is more invasive and therefore not as widely used as the hydrogen breath test (Misselwitz et al., 2019).
e Importance of Lactase
e breakdown of lactose involves an enzymatic hydrolysis process. By utilizing the enzyme lactase-phlorizin hydrolase (LPH), also known as lactase, our body breaks down the disaccharide into the digestible molecules: galactose and glucose. Glucose and galactose are monosaccharides that can be directly absorbed by the enterocytes (single-layered columnar epithelial cells) through active transport in the lumen of the small intestine (Tümer et al., 2013). ese molecules can either be used as fuel by the cells in the central nervous system and the periphery or be converted into glycogen and stored for later use in either the liver or muscle cells. Lactase is an enzyme located on the brush border of the small intestine among its three sections. e rst section is the duodenum, the middle is the jejunum, and the last is the ileum. is enzyme has the highest concentration in the jejunum and the lowest concentration in the ileum. e brush border is the inner lining of the small intestine that contains enzymes, plicae circulares (folds in the mucus membrane), and villi capillaries on its surface. is inner lining is responsible for much of the breakdown and absorption of complex sugars, amino acids, fatty acids, and nutrients – including lactose – that were not already absorbed by the duodenum (Collins et al., 2021). e main function of the lactase enzyme is to break down the lactose molecules into its monosaccharide components so they can be better absorbed by our bowels.
e lactase gene is located on chromosome number two, and the regulatory mutation that controls whether this gene is on or o is actually located fourteen thousand base pairs upstream on non-coding DNA that used to be commonly known as “junk DNA.” However, today it is known that these so-called “junk DNA” are far from useless, because they can control whether a gene is turned on or not. In essence, the mutation that controls whether the lactase gene is on or o a er the weaning period and whether our body continues to produce lactase is located several
Image 1: ere are many popular substitutes for cow’s milk including but not limited to coconut milk, almond milk, cashew milk, soy milk, and hemp milk.
Image Source: Wikimedia Commons
Image 2: Lactose can be broken down by lactase a er its reaction with water. is is the process of enzymatic hydrolysis and this is what allows those with the lactase enzyme to digest dairy products. Image Source: Wikimedia Commons
Image 3: e domestication of cows played a major role when it came to lactase gene expression. ere is a high positive correlation between populations with early cow domestication and populations with lactose tolerance.
Image Source: Wikimedia Commons thousand nucleotide bases away from the actual lactase gene in question (Anguita-Ruiz et al., 2020).
Convergent Evolution
An interesting fact about the lactose tolerance mutation is that it occurred in several distinct populations around the world during a similar time period. e domestication of cows about ten thousand years ago greatly contributed to this evolution, since milk can be an extra source of calories and water in times of famines, malnourishment, and drought. erefore, the groups of individuals who were able to digest lactose and use it as a source of energy a er weaning were the ones who survived during tough times, reproduced, and were able to pass their traits onto their o spring. roughout years of evolution, large percentages of these populations were able to develop that same trait. e development of this trait across distinct populations is known as convergent evolution. Another important factor to note is that this selection pressure on lactase gene expression is only present in humans; the lactase gene in other mammals is naturally turned o since they don’t require it a er the weaning period (Anguita-Ruiz et al., 2020).
Today, many studies have shown that there are clear di erences between the percentages of speci c populations in the ability to digest lactose (Anguita-Ruiz et al., 2020; Bayless et al., 2017; Ségurel & Bon, 2017). Populations who have practiced cattle-breeding and dairy farming show the greatest percentages of individuals who are lactose tolerant. e highest rates of lactase persistence are present in individuals in Northern Europe and the Middle East, or those who have migrated from there. e lowest rates are in East Asian countries, with China having 15% of individuals with lactase persistence and South Korea, Vietnam, and Cambodia having 0% to 5%. is may be due to the fact that many tribes or early populations in East Asia were nonpastoralist communities and therefore did not rely heavily on many dairy products for calories and/or nutrition. As for the United States, lactase persistence is present in 83% to 93% of White Americans with origins from Europe or Scandinavia. 12% to 40% of African Americans and about 30% of Mexican Americans from more rural areas also have the ability to digest lactose. Additionally, relatively low levels of lactase persistence have been found in South America with Peru having 6%, Uruguay having 30%, and Colombia having 20% of individuals able to digest dairy (Anguita-Ruiz et al., 2020). In the populations with a higher lactase persistence, there are also a greater number of individuals who do end up losing their ability to digest lactose but many years a er the weaning period ends. For example, some studies of the Finnish population (a country in northern Europe) demonstrated that a majority of Finnish individuals don’t lose their lactase persistence until the age of 10 and others are even able to digest lactose until the age of 20. In ailand (a Southeast Asian country), on the other hand, many children lose their lactase activity in their intestines by the age of two (Kuchay, 2020).
Absence of Lactase and its Treatments
Another term for the absence or low levels of lactase in the small intestine is hypolactasia. Without lactase in the small intestine, our bodies are not able to break down and absorb lactose. e undigested lactose becomes fermented by colonic microbiota (bacteria in the colon). is increases the solute concentration of digestive uids, which causes a counterbalancing in ux of water into the lumen, therefore contributing to the unfavorable symptoms of lactose intolerance (Kuchay, 2020). is form of digestion leads to the creation of gases like hydrogen, carbon dioxide, and methane in the intestine (Misselwitz et al., 2019). Gas in the intestine leads to additional symptoms, including stomach cramps, stomach pain, and diarrhea.
e most common way many individuals avoid the symptoms of lactose intolerance is by restricting their intake of lactose. However, it is o en encouraged that these individuals include some form of dairy in their diet instead of completely avoiding lactose in order to build some tolerance (Szilagyi & Ishayek, 2018). e nondairy substitutes are mostly derived from plants and do not contain nearly as many nutritional bene ts as the dairy products themselves. us, it is essential for those who are lactose intolerant to make sure they are getting enough calcium, Vitamin D, and Vitamin A from the other foods in their diet. Additionally, the lactase enzyme can be added to dairy products in the form of liquids, capsules, or tablets prior to consumption. is enzyme will begin to break down the lactose before it even enters the gastrointestinal tract and therefore allow those individuals with lactose intolerance to consume dairy (Szilagyi & Ishayek, 2018). References
Anguita-Ruiz, A., Aguilera, C. M., & Gil, Á. (2020). Genetics of Lactose Intolerance: An Updated Review and Online Interactive World Maps of Phenotype and Genotype Frequencies. In Nutrients (Vol. 12, Issue 9, p. 2689). MDPI AG. https://doi.org/10.3390/nu12092689
Bayless, T. M., Brown, E., & Paige, D. M. (2017). Lactase Non-persistence and Lactose Intolerance. In Current Gastroenterology Reports (Vol. 19, Issue 5). Springer Science and Business Media LLC. https://doi.org/10.1007/s11894-017-0558-9
Catanzaro, R., Sciuto, M., & Marotta, F. (2021). Lactose intolerance: An update on its pathogenesis, diagnosis, and treatment. Nutrition research (New York, N.Y.), 89, 23–34. https://doi. org/10.1016/j.nutres.2021.02.003
Collins JT, Nguyen A, Badireddy M. Anatomy, Abdomen and Pelvis, Small Intestine. [Updated 2021 Aug 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/ books/NBK459366
Diekmann, L., Pfei er, K., & Naim, H. Y. (2015). Congenital lactose intolerance is triggered by severe mutations on both alleles of the lactase
Conclusion
Although lactose intolerance is present in a majority of adults, the symptoms and their severity di er based on the individual’s age, gender, amount of lactose ingested, bowel motor abnormalities, and visceral sensitivity (He et al., 2008). Lactose malabsorption can be a ected by environmental factors as well and can change throughout the lifetime of an individual. Nevertheless, there are numerous substitutes today for dairy products that are just as avorsome and accessible as the dairy products themselves. All things considered, lactose intolerance is a common biological pattern because it is natural for human beings to not be able to digest lactose a er the weaning period (Swagerty et al., 2002). In fact, since lactose tolerance is caused by a mutation, it is therefore the “real disease”, despite coming with many bene ts. All in all, it is important to recognize that lactose intolerance is a naturally occurring phenomenon that a ects a huge portion of the human population.
Image 4: is gure demonstrates the distribution of lactose intolerance around the world. It can be seen that Northern Europe has the highest lactase persistence (LP) and East Asia has the lowest lactase persistence Image Source: Wikimedia Commons
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