Lactose intolerance is defined as a disorder that appears in the patient after ingesting a product containing lactose. (A disaccharide found in dairy products).
It is a condition widely prevalent in the world, because depending on the age group and ethnicity analyzed, up to 65% of the population may have it. It is interesting to know that the prevalence of this intolerance largely depends on the ethnicity of the individual, as African Americans, Asians and Latinos are more likely to suffer from it. This is due to genetic factors which 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 therefore not surprising that countless investigations have been carried out on this pathology since its discovery in the 1960s.
Join us on this review journey through the world of lactose intolerance as this condition dates back to ancient times and presents some features that will surprise you.
The biological basis of lactose intolerance
Before assessing the overall situation of this disorder and its distribution by the population, we need to lay some groundwork regarding the causes of the disease.
As we have already said, lactose intolerance is a condition that occurs when the affected individual is unable to digest the sugars present in the milk, which causes intestinal disturbances, abdominal bloating, gas and blood pressure. diarrhea.
It is interesting to know that lactose is the main carbohydrate in milk (including that of human origin). Physiologically speaking, this disaccharide sugar cannot be absorbed by the gut of our species, so we need the action of the enzyme lactase, a type of β-galactosidase that is responsible for breaking down this complex sugar in the monosaccharides glucose and galactose.
This very important enzyme is synthesized in the brush edges of the villi (villi) cells of 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 be obtained in a non-industrialized country. world. For this reason, it is normal for the body to save energy by skipping the abundant production of this enzyme.
Lactase enzyme deficiency can be congenital or acquired over the years. Depending on the reason for the pathology, four types of lactose intolerance can be described:
1. Primary lactase deficiency
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 adulthood, as the decline of the lactase enzyme begins in childhood and continues throughout an individual’s life.
Although it is an acquired disease, it is essential to emphasize that it is a genetic disorder, that is to say that there is a predisposition from the birth of the patient to it , although this does not manifest itself in the first years of development.
2. Secondary lactase deficiency
Various diseases can damage the lining of the small intestine, which naturally results in decreased production of lactase enzymes.. 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 cite two other causes, which although much less frequent, are present in the population. Congenital deficiency is a special case in which the newborn is unable to digest dairy products due to an autosomal recessive mutation..
On the other hand, a deficiency during development finds the cause in children born prematurely, because their small intestine has not finished forming at all, and the hydrolysis activity of lactose is compromised.
How does this affect different populations around the world?
It is a subject of particular interest for any epidemiologist, because few disorders manifest themselves with such a clear pattern according to the ethnic group studied. As much the concept of “race” in the human sphere is obsolete due to the lack of consensus and moral application, the fact that our species reports genetic variations according to the place of development and the family line is undeniable.
This is why, over the past 10,000 years, a series of mutations have been observed in certain groups of the population which allow its members to secrete lactase at appropriate levels during their adult life. The variant of the gene encoding lactase activity is inherited in a dominant Mendelian form, so it is not surprising that this resistance is very present in some regions.
It’s fascinating to know that this series of mutations has been caused in populations that have been in close contact with dairy cattle for thousands of years. devolution. Due to this mechanism of natural selection, lactose intolerance is broken down as follows:
- Populations in northern Europe have only 15% lactose intolerant limbs (maximum).
- In some population groups of Asian and Indo-American descent, the prevalence of this disorder reaches almost 100%.
- It is estimated that 65% of the world’s population is lactose intolerant.
As can be seen, this data set shows that the consumption of dairy products over the centuries appears to be an important factor in quantifying lactase production in an adult individual. This is a clear mechanism of natural selection from a theoretical point of view, because useful mutations are fixed over generations, And the deletions disappear.
According to the article, theories based on a mechanism of natural selection make perfect sense: adults who can properly digest milk from cattle would receive more calories with less effort than others, it is expected. so that they have more and better offspring. in general and these genetic mutations will be introduced in subsequent generations.
Lactose intolerance in ancient times
Beyond the data of great interest presented above, scientific reports have shown that lactose intolerance was already present in humans who played in the Bronze Age, around 3000 years ago.
The study of DNA from the bone remains of 14 individuals belonging to this era has enabled modern scientists to discover that, indeed, these humans did not carry the relevant mutations for adequate production of lactase in the adult individual.
Other studies point out that the first European farmers did not show these mutations either 8000 years ago. The most accepted theory is that these genetic variations spread throughout the population as cattle have become a general practice and dairy products are increasingly consumed on a recurring basis. However, these new reports indicate that at least 6,000 years have passed between the first farmers and the widespread development of lactose resistance (at least in the populations analyzed), which is very difficult to explain.
As we have seen, lactose intolerance it is a disorder that goes far beyond the patient’s gastrointestinal upset. It is a pathology of great epidemiological and evolutionary interest, because it is very difficult to explain the fact that certain human groups began to drink milk assiduously in adulthood if, obviously, they were not there. prepared as a species.
Studies like those we have shown you today pose more questions than answers, because explaining the exact origin of the mutations that allowed the development of lactose tolerance and its role in different societies is at least complex. Of course, spaces like this only show the thousands of unknowns that the human body continues to shut off.
- Harvey, L., Ludwig, T., Hou, AQ, Hock, QS, Tan, ML, Osatakul, S., … and Muhardi, L. (2018). Prevalence, cause and diagnosis of lactose intolerance in children aged 1 to 5 years: a systematic review of the literature 1995-2015. Asia Pacific Journal of Clinical Nutrition, 27 (1), 29.
- Lactose intolerance, MayoClinic.org. Retrieved October 6 from https://www.mayoclinic.org/es-es/diseases-conditions/lactose-intolerance/symptoms-causes/syc-20374232
- National Institutes of Health. (2006). Lactose intolerance: information for healthcare providers. US Department of Health and Human Services, 1-6.
- Swagerty Jr, DL, Walling, A. and Klein, RM (2002). Lactose intolerance. American family physician, 65 (9), 1845.
- Warrior skeletons reveal Bronze Age Europeans could not drink milk, sciencemag.org. Retrieved October 6, from https://www.sciencemag.org/news/2020/09/warrior-skeletons-reveal-bronze-age-europeans-couldn-t-drink-milk