Lactase deficiency

In recent years, the topic of milk intolerance has been actively discussed in the media, in social networks, and on forums. “Milk is harmful”, “milk should be given up” – these and similar statements are often not supported by any convincing evidence.

By “harm” is most often meant the appearance of certain unpleasant symptoms (nausea, bloating, diarrhea) after consuming milk and dairy products as food. Since such a situation is indeed possible, especially in adults, milk is automatically recognized in the eyes of the general public as an undesirable foodstuff. So how are things really? Are special mixes so necessary for newborns, for example, Let’s look at it from an evidence-based perspective.

Lactose and lactase: what happens to them in the human body?
Putting aside a true allergy to milk protein (most commonly cow’s milk), the so-called lactase deficiency accounts for the vast majority of cases of poor milk tolerance. What kind of disease is this? Let’s start at the beginning.

Mammalian milk (including humans) contains carbohydrates in addition to fats and proteins, primarily the milk sugar lactose. Lactose consists of two molecules of simple sugars – glucose and galactose – connected by a special bond. In order for lactose to be absorbed by humans, this bond must be broken. Only then can the simple sugars glucose and galactose be easily absorbed in the small intestine.

The process of breaking the bond inside the lactose molecule is only possible with a special enzyme – lactase. The names are similar: the enzyme lactase splits the milk sugar lactose. This enzyme is located on the surface of the villi of the small intestine.

In humans and most mammals, the activity of the lactase enzyme is maximal after birth, allowing the absorption of large quantities of milk, which in the first months is the only source of food for the newborn. After breastfeeding is discontinued, lactase activity gradually fades as the individual transitions to an adult diet in which milk and dairy products are only a small part of the diet.

In some individuals the decrease in the activity of this enzyme occurs very quickly, in some people gradually, and a certain proportion of people retain high lactase activity for a long time. What does it depend on?

It is known that the ability of lactase to break down the milk sugar lactose is regulated by a specific gene. More specifically, two genes. One gene (LCT) encodes the lactase enzyme, without it it simply could not be produced. The second gene next to it is called MSM6, and its task is to regulate the activity of the LCT gene.

For example, it is known that the variant (genotype) of the MSM6 gene, designated as C/C, is associated with low LCT gene activity, which leads to early intolerance of milk sugar. In the T/T genotype, LCT gene activity is high, even in adulthood. The C/T genotype is the most common variant that is most likely to cause lactose intolerance in middle age.

This information is very important because it is now possible to evaluate the MSM6 gene in laboratories.

So, the activity of the lactase enzyme is regulated by the gene, and with age, the likelihood of this activity decreasing increases. So what happens to the milk sugar lactose if there is not enough enzyme to break it down in the small intestine? Some lactose will still be broken down and absorbed in the small intestine. The remaining uncracked lactose molecules will travel further in the small intestine and eventually enter the large intestine. During this movement, the high osmotic activity of lactose pulls water molecules on itself, increasing the amount of fluid in the intestinal lumen, which increases the risk of diarrhea. Milk sugar in the colon is an excellent nutrient substrate for local bacteria, which actively begin to break it down, producing gas. Increased gas and bloating are a frequent symptom in these patients. In addition, persons with impaired lactose breakdown and absorption may experience mild abdominal pain, nausea and other unpleasant symptoms.

To summarize all of the above: low activity of the lactase enzyme leads to impaired absorption of lactose in the small intestine. This condition is referred to in medicine as “lactase deficiency”. Lactase deficiency, which occurs over time in adults, also has a second name – adult hypolactasia. This article will focus on this particular variant of lactase deficiency, which is the most common. We will leave aside congenital lactase deficiency (an extremely rare disease of newborns) and secondary lactase deficiency (occurs as a consequence of a number of gastroenterological, infectious and other diseases).