Lactose intolerance: what is it, types and causes of this condition

Lactose intolerance is defined as a disorder that appears in the patient after he ingests a product with lactose (a disaccharide that is present in dairy products).

It is a condition widely distributed in the world, because according to the age group and ethnic group analyzed, up to 65% of the population can present it. It is interesting to know that the prevalence of this intolerance largely depends on ethnicity. of the individual, as African-American, Asian and Latino people have a greater predisposition to suffer it. This is due to genetic factors that we will see later.

It is estimated that in the United States alone there are 30 to 50 million people who are lactose intolerant. It is not surprising, therefore, that countless investigations have been carried out regarding this pathology since its discovery in the 1960s.

Join us on this review trip through the world of lactose intolerance, because this condition dates back to ancient times and it presents certain characteristics that will surprise you.

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The Biological Basis of Lactose Intolerance

Before evaluating the global situation of this disorder and how it is distributed throughout the population, we have to establish certain bases as regards the causes of the disease.

As we have already said, lactose intolerance is a pathology that occurs when the affected individual cannot digest the sugars in milk, causing intestinal discomfort, bloating, gas, and diarrhea.

It is interesting to know that lactose is the primary carbohydrate that makes up milk (including that of human origin). Physiologically speaking, this disaccharide sugar cannot be absorbed by the intestine of our species, so we require the action of the enzyme lactase, a type of β-galactosidase that is responsible for breaking down this complex sugar into the monosaccharides glucose and galactose.

This all-important enzyme is synthesized on the brush edges of the villi cells (villi) in the small intestine.But unfortunately, it has been observed that in mammals there is a physiological decrease in lactase secretion with age. This responds to a clear mechanism at an evolutionary level: milk is the first food we eat at birth, but it is natural that over the years an adult individual uses less and less this source of nutrients so difficult to obtain in a world not industrialized. For this reason, it is normal for the body to save energy by skipping the abundant production of this enzyme.

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Types

Lactase enzyme deficiency can be congenital or acquired over the years. Depending on the reason underlying the pathology, four types of lactose intolerance can be described:

1. Primary lactase deficiency

It is the most common cause of lactose intolerance. Epidemiological studies show that, for example in Mexico, 30% of adults have this type of deficiency. Symptoms begin to appear after ingestion of dairy products in adolescence or during age adult, since the decrease of the lactase enzyme begins in childhood and continues throughout the life of the individual.

Despite the fact that it is an acquired condition, it is essential to emphasize that it is a genetic disorder, that is, there is a predisposition to it from the birth of the patient, although it is not manifested during the first years of growth.

2. Secondary lactase deficiency

Various diseases can damage the mucosa of the small intestine, which naturally results in a decrease in lactase enzyme production.. Some pathologies that can trigger this secondary deficiency are gastroenteritis, celiac disease, Crohn's disease, ulcerative colitis or the use of certain drugs and chemotherapy.

3. Other causes

We can list two other causes, which although they are much less common, are present in the population. Congenital deficiency is a special case in which the newborn is not able to digest dairy products due to an autosomal recessive mutation.

On the other hand, deficiency during development finds its cause in infants who are born prematurely, since your small intestine is not fully formed and the hydrolyzing activity of lactose is seen engaged.

How does it affect the different populations in the world?

This is a subject of special interest to any epidemiologist, since few disorders manifest with such a clear pattern according to the ethnic group studied. As much as the concept of "race" in the human sphere is in disuse due to the lack of consensus and moral application, the fact that our species reports genetic variations according to the place of development and family lineage is undeniable.

This is why a series of mutations have been observed in certain groups over the last 10,000 years. that allow their members to secrete lactase at the appropriate levels during their lifetime adult. The variant of the gene that encodes lactase activity is inherited in a dominant Mendelian way, so it is not surprising that this resistance is very present in certain regions.

It is fascinating to know that this series of mutations have been promoted in populations that have been in intimate contact with dairy cattle for thousands of years of evolution. Due to this mechanism of natural selection, lactose intolerance is distributed as follows:

• Northern European populations only have 15% lactose intolerance (maximum).
• In certain population groups of Asian and Indo-American descent, the prevalence of this disorder is almost 100%.
• It is estimated that 65% of the world's population is lactose intolerant.

As we can see, this series of data shows that the consumption of dairy products throughout the centuries seems to be an important factor in quantifying lactase production in an individual adult. Is about a clear mechanism of natural selection from a theoretical point of view, since useful mutations are fixed throughout the generations, and the deleterious disappear.

On paper, theories based on a mechanism of natural selection make all the sense in the world: adults able to properly digest milk from cattle would get more. calories with less effort than the rest, so it is to be expected that they had more and better offspring in general and these genetic mutations were presented in the following generations.

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Lactose intolerance in ancient times

Beyond the data of great interest exposed previously, scientific reports have shown that lactose intolerance was already present in humans who starred in the Bronze Age, about 3000 years ago.

The study of the DNA of the skeletal remains of 14 individuals belonging to this time allowed modern scientists to discover that, indeed, these human beings did not carry the pertinent mutations for a correct lactase production in the individual adult.

Other studies indicate that the first European farmers did not show these mutations either 8,000 years ago. The most widely accepted theory is that these genetic variations spread throughout the population as that livestock farming was established as a general practice and the products were consumed more and more frequently dairy products. In any case, these new reports indicate that at least 6,000 years passed between the first ranchers and the generalized development of lactose resistance (at least in the populations analyzed), a fact very difficult to explain.

Resume

As we have seen, lactose intolerance it is a disorder that goes far beyond the gastrointestinal discomfort of the patient. It is a pathology of great epidemiological and evolutionary interest, as it is very difficult to explain the fact that certain human groups began to drink milk assiduously as adults if, clearly, they were not prepared for it as species.

Studies like the ones we have shown you today raise more questions than answers, since explaining the exact origin of the mutations that allowed the development of tolerance to lactose and its role in different societies is to say the least complex. Of course, spaces like this only show the thousands of unknowns that the human body continues to enclose.

Bibliographic references:

• Harvey, L., Ludwig, T., Hou, A. Q., Hock, Q. S., Tan, M. L., Osatakul, S.,... & Muhardi, L. (2018). Prevalence, cause and diagnosis of lactose intolerance in children aged 1-5 years: a systematic review of 1995-2015 literature. Asia Pacific Journal of Clinical Nutrition, 27 (1), 29.
• Lactose Intolerance, MayoClinic.org. Picked up on October 6 at https://www.mayoclinic.org/es-es/diseases-conditions/lactose-intolerance/symptoms-causes/syc-20374232
• National Institutes of Health. (2006). Lactose Intolerance: Information for Health Care Providers. US Department of Health and Human Services, 1-6.
• Swagerty Jr, D. L., Walling, A., & Klein, R. M. (2002). Lactose intolerance. American family physician, 65 (9), 1845.
• Warrior skeletons reveal Bronze Age Europeans couldn't drink milk, sciencemag.org. Picked up on October 6 at https://www.sciencemag.org/news/2020/09/warrior-skeletons-reveal-bronze-age-europeans-couldn-t-drink-milk