Biochemical basis of lactose hydrolysis
Normally, the digestion of milk sugar is ensured by lactase, an enzyme belonging to the $\beta$-glycosidase complex. This enzyme catalyzes a crucial chemical reaction in the lumen of the gastrointestinal tract:
Lactose + $H_2O$ $\rightarrow$ Glucose + Galactose
The monosaccharides formed during hydrolysis (glucose and galactose) are absorbed through the intestinal wall and enter the systemic circulation, supplying energy to the body. If the enzyme is absent or its activity is reduced, unhydrolyzed lactose remains in the intestinal lumen, triggering a cascade of pathological reactions and leading to impaired carbohydrate absorption.
Classification of enzyme deficiency
In clinical practice and biochemistry, there are three main forms of lactase deficiency, differing in etiology and time of manifestation:
- Congenital deficiency. Relatively rare in the population. Symptoms appear at an early age—manifestation occurs immediately after the child's birth as soon as they receive their first feeding of milk.
- Age-related decline in gene expression. The most common form, typical of adult patients and older children. Its etiology lies in the physiological decrease in the amount of synthesized lactase during ontogenesis.
- Secondary deficiency. Temporary and acquired in nature. It develops as a direct consequence of previous gastrointestinal diseases (e.g., gastritis or colitis), as well as after digestive tract surgeries.
Clinical presentation and laboratory diagnostics
The classic clinical picture includes the development of dyspeptic symptoms shortly after milk consumption. Patients complain of vomiting, diarrhea, cramps, sharp abdominal pain, and flatulence. In newborns, the leading symptom is often severe diarrhea after feeding.
To confirm the diagnosis in clinical biochemistry, the lactose tolerance test is used.
- The patient is given a load: 50 grams of lactose on an empty stomach.
- Blood glucose concentration is then measured at 30, 60, and 90 minutes.
- Result in pathology: blood glucose levels do not increase.
The pathogenesis of this phenomenon is simple: due to the absence of the enzyme, lactose is not broken down into monosaccharides, glucose is not formed and, accordingly, is not absorbed into the bloodstream.
Basics of dietary therapy
Despite whole milk intolerance, patients with lactase deficiency can often consume fermented dairy products without symptoms. This is explained by the fact that the bacterial starter culture used in their production takes over part of the work of the human digestive system and partially ferments the lactose before the product even enters the gastrointestinal tract.
Briefly: answers to topical questions
What diseases and conditions lead to the development of secondary lactase deficiency?
Secondary lactase deficiency is caused by conditions that lead to enterocyte damage or atrophy of the intestinal mucosa. Such diseases and pathological factors include:
- Infectious processes (intestinal infections, acute viral diarrhea).
- Inflammatory bowel diseases (colitis, gastritis).
- Immunological reactions (cow's milk protein allergy).
- Atrophic changes of the mucosa (e.g., in celiac disease).
- Conditions associated with prolonged parenteral nutrition.
- Consequences of gastrointestinal surgery.
In which specific part of the gastrointestinal tract and on which cells is lactase localized?
Lactase is localized in the small intestine on the surface of epithelial cells—mature enterocytes. This enzyme is located directly on their brush border, where it carries out membrane (contact) digestion. The hydrolysis of the disaccharide lactose takes place on this surface, resulting in the formation of monosaccharides—galactose and glucose—for subsequent absorption into the blood.
Which specific gene encodes the lactase enzyme in humans?
The lactase enzyme in humans is encoded by the LCT gene. This gene ensures the production of the enzyme required to digest milk sugar in adults, which evolutionarily allowed humans to consume domestic animal milk. The development of lactose intolerance is associated with having the ancestral version of this gene.
It is easy to remember the biochemistry of the reaction: 'Lactose with water ($H_2O$) breaks down into two 'G's—Glucose and Galactose.'
Class and exam questions
Why does blood glucose remain unchanged during a tolerance test in a patient with lactase deficiency?
Due to the absence of the lactase enzyme, the disaccharide lactose is not hydrolyzed into monosaccharides. Therefore, glucose is simply not formed in the intestine and cannot be absorbed into the blood, meaning its concentration does not rise.
Can you drink kefir if you have lactose intolerance?
Yes, fermented dairy products are generally much better tolerated. The bacterial starter culture they contain independently ferments and breaks down a significant portion of the lactose.
Can lactose intolerance develop in adulthood if it wasn't present in childhood?
Yes, this is the most common form. It is associated with the physiological age-related decrease in the expression of the gene responsible for lactase synthesis during ontogenesis.
What else to study on this topic
- Structure and functions of the $\beta$-glycosidase complex
- Genetic mechanisms regulating lactase gene expression
- Effects of unhydrolyzed lactose on the large intestine microflora
- Differential diagnosis of primary and secondary lactase deficiency
- Pathophysiology of osmotic diarrhea in disaccharidase deficiency
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