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Lactose Intolerance

For medical students2 min readUpdated 2026-10-10

Lactose intolerance (lactase deficiency) is a condition characterized by impaired digestion of milk sugar due to a deficiency or complete absence of the specific intestinal enzyme. Clinically, this pathology presents with prominent gastrointestinal symptoms after the consumption of whole milk.

Main enzymeLactase ($\beta$-galactosidase complex), which hydrolyzes milk sugar.

DiagnosticsLactose tolerance test: assessment of blood glucose levels at 30, 60, and 90 minutes.

SymptomsVomiting, diarrhea, severe cramps, abdominal pain, and flatulence following milk ingestion.

Dietary therapyFermented dairy products are better tolerated thanks to bacterial fermentation.

Biochemical Basis of Lactose Hydrolysis

Normally, digestion of milk sugar is mediated by lactase, an enzyme belonging to the $\beta$-galactosidase complex. This enzyme catalyzes a critical chemical reaction in the lumen of the gastrointestinal tract:

Lactose + $H_2O$ $\rightarrow$ Glucose + Galactose

The monosaccharides produced during hydrolysis (glucose and galactose) are absorbed across the intestinal wall and enter the systemic circulation, providing energy for the body. If the enzyme is absent or its activity is reduced, unabsorbed lactose remains in the intestinal lumen, triggering a cascade of pathological reactions and resulting in 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 lactase deficiency: Relatively rare in the population. Symptoms appear early in life—manifesting immediately after the newborn receives their first feeding containing milk.
Age-dependent down-regulation (Primary adult-type hypolactasia): The most common form, seen in adult patients and older children. Its etiology lies in the physiological decrease in the amount of synthesized lactase during ontogeny.
Secondary lactase deficiency: Temporary and acquired. It develops as a direct consequence of gastrointestinal diseases (e.g., gastritis or colitis) or following digestive tract surgery.

Clinical Presentation and Laboratory Diagnosis

The classic clinical picture includes dyspeptic symptoms shortly after milk consumption. Patients complain of vomiting, diarrhea, cramps, severe abdominal pain, and flatulence. In neonates, profuse diarrhea following feeding is often the leading symptom.

To confirm the diagnosis, the lactose tolerance test is used in clinical biochemistry.

The patient is given an oral load of 50 grams of lactose on an empty stomach.
Blood glucose concentration is then measured at 30, 60, and 90 minutes.
Pathological result: Blood glucose levels do not rise.

The pathogenesis of this phenomenon is straightforward: due to the absence of the enzyme, lactose is not hydrolyzed into monosaccharides, glucose is not formed, and consequently, it is not absorbed into the bloodstream.

Basics of Dietary Therapy

Despite intolerance to whole milk, patients with lactase deficiency can often consume fermented dairy products without discomfort. This is because the bacterial cultures used in their production take over part of the human digestive system's workload, partially fermenting the lactose even before the product enters the gastrointestinal tract.

Mnemonic

Remember the reaction biochemistry easily: "Lactose with water ($H_2O$) breaks down into two 'G's — Glucose and Galactose."

Frequently asked questions

What diseases and conditions lead to the development of secondary lactase deficiency?

Secondary lactase deficiency is caused by conditions that damage enterocytes or cause atrophic changes in the intestinal mucosa. These include:

Infectious processes (intestinal infections, acute viral diarrhea).
Inflammatory bowel diseases (colitis, gastritis).
Immunological reactions (cow's milk protein allergy).
Atrophic mucosal changes (e.g., in celiac disease).
Conditions associated with prolonged parenteral nutrition.
Sequelae of gastrointestinal surgeries.

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. Hydrolysis of the disaccharide lactose occurs at this surface, yielding monosaccharides—galactose and glucose—for subsequent absorption into the blood.

Which gene encodes the human lactase enzyme?

The human lactase enzyme is encoded by the LCT gene. This gene ensures the production of the enzyme required for digesting milk sugar in adults, an evolutionary adaptation that allowed humans to consume domestic animal milk. The development of lactose intolerance is linked to carrying the ancestral version of this gene.

Why does blood glucose remain unchanged during the tolerance test in a patient with lactase deficiency?

Due to the absence of lactase, the disaccharide lactose is not hydrolyzed into monosaccharides. As a result, glucose is not formed in the intestine and cannot be absorbed into the blood, so its concentration does not rise.

Can I drink kefir if I am lactose intolerant?

Yes, fermented dairy products are usually tolerated much better. The bacterial cultures they contain independently ferment and break down a significant portion of the lactose.

Can lactose intolerance develop in adulthood if it was not present during childhood?

Yes, this is the most common form. It is associated with the physiological age-related decline in the expression of the gene responsible for lactase synthesis during ontogeny.

Go deeper

Structure and function of the $\beta$-galactosidase complex
Genetic regulatory mechanisms of lactase gene expression
Effect of unabsorbed lactose on large bowel microflora
Differential diagnosis of primary and secondary lactase deficiency
Pathophysiology of osmotic diarrhea in disaccharidase deficiency