Lactase deficiency

Classification and etiology

The disease is classified by the degree of enzyme deficiency: partial intolerance (hypolactasia) and complete intolerance (alactasia) are distinguished. Depending on the etiology, LD is divided into two fundamentally different groups:

• Primary LD. Characterized by a decrease in enzyme production, while the intestinal cells themselves (enterocytes) remain undamaged. Includes congenital (genetically determined) alactasia, the transient form (occurring in immature and premature newborns), and "adult-type" LD (constitutional decrease in the enzyme after 3–5 years).
• Secondary LD. Always a consequence of destruction of the small intestinal mucosa. Enterocyte damage can be caused by acute intestinal infections, the gastrointestinal form of cow's milk protein allergy, celiac disease, and other inflammatory gastrointestinal diseases.

In addition, absolutely healthy infants may experience relative lactase deficiency. It occurs with maternal hyperlactation, when the volume of lactose exceeds the capacity of the child's yet immature enzymatic systems.

Pathogenesis and clinical presentation

Normally, the lactase enzyme is localized on the apical membrane of mature enterocytes and breaks down milk sugar into monosaccharides (glucose and galactose). In enzyme deficiency, uncleaved lactose passes transitively into the large intestine, where it becomes a nutrient medium for bacteria. During bacterial fermentation, short-chain fatty acids, lactic acid, water, and gases are released, which sharply increases the osmotic pressure in the intestinal lumen.

Clinically, this process manifests with three groups of symptoms:

1. Osmotic "fermentative" diarrhea. Stool frequency increases up to 8–10 times a day. Stool is yellow, foamy, watery (leaves a large water stain on the diaper), and has a sharp sour odor. Evacuation of stool under pressure—the "fountain" sign—is characteristic.
2. Intestinal dyspepsia. Gas accumulation causes pronounced flatulence, abdominal distension, and painful colic. Increased intra-abdominal pressure often provokes profuse spitting up (regurgitation).
3. Systemic disorders. Due to fluid loss with the stool, there is a risk of dehydration (exicosis), and impaired nutrient absorption leads to hypotrophy — delayed physical development.

Diagnostic approaches

The diagnosis is established based on clinical manifestations and laboratory test results. It is important to note that macro- and microscopic mucosal changes are not characteristic of primary LD — the intestine is visually intact.

The diagnostic minimum includes:

• Empirical dietary diagnostics: symptoms are rapidly relieved when the patient is switched to a lactose-free diet (elimination test).
• Coprological markers: detection of elevated carbohydrate levels in the stool, a shift in stool pH to the acidic side (less than 5.5), as well as increased excretion of short-chain fatty acids.
• Tolerance tests: during a lactose glycemic test, there is no adequate increase in blood glucose, since the disaccharide is not broken down in the intestine.

In complex cases, mucosal biopsy is used for direct determination of enzyme activity or genetic testing.

Principles of treatment and diet therapy

Management tactics depend on age, the degree of enzyme deficiency, and the form of LD (primary or secondary). The main principle is a stepwise reduction of the carbohydrate load while searching for the maximum volume of lactose that the child tolerates without symptoms.

• In breastfeeding. Breast milk is preserved whenever possible. The mother's diet is corrected (whole cow's milk is excluded). In case of hyperlactation, it is recommended to express the first portion of milk, which is richest in lactose. Enzyme therapy is prescribed: exogenous lactase preparations are dissolved in 20–30 ml of expressed milk and left for 20 minutes for in vitro fermentation, after which they are given before feeding.
• In formula feeding. For mild manifestations, it is sufficient to switch the infant to fermented milk formulas. For pronounced hypolactasia, specialized low-lactose or lactose-free formulas are prescribed.
• Introduction of complementary foods. The first complementary foods are dairy-free and gluten-free porridges (buckwheat, rice, corn). At 5.5–6 months, meat puree is introduced to prevent protein deficiency. Fruits and juices are prescribed at a later date.

The duration of the diet varies: in secondary LD, restriction is temporary (until enterocytes heal), transient LD resolves by 3–4 months of life, and in congenital alactasia, a low-lactose diet must be followed for life.

Class and exam questions

What else to study on this topic

• Differential diagnosis of LD with gastrointestinal cow's milk protein allergy
• Scheme for introducing complementary foods and stages of diet expansion after relief of LD symptoms
• Features of the intestinal microbiota in relative lactase deficiency
• Genetic testing: detection of polymorphisms associated with primary LD

Other topics in "Chapter 3. Adaptation, Early Childhood Nutrition, Rickets"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.