Lactose — chemical composition, dietary sources, and clinical indications for exclusion. Who may benefit from a lactose-free nutritional regimen?

Lactose is a disaccharide unique to mammalian milk, composed of D-galactose and D-glucose linked by a beta-1,4-glycosidic linkage.

Under normal brush-border kinetics, lactase cleaves this bond, facilitating SGLT1-mediated monosaccharide absorption. In hypolactasia, unabsorbed lactose enters the cecum, exerting an osmotic gradient that draws water into the colon (osmotic diarrhea). Colonic anaerobic microflora rapidly ferment the substrate into hydrogen, carbon dioxide, methane, and short-chain fatty acids, triggering meteorism and painful intestinal cramping.

Distinguishing carbohydrate malabsorption from immune-mediated hypersensitivity is critical for patient safety:

• Lactose Intolerance: A non-immune enzymatic defect confined to carbohydrate maldigestion. Symptoms are strictly gastrointestinal. Lactose-free dairy products are completely tolerated.
• Cow's Milk Protein Allergy (CMPA): An immune-mediated hypersensitivity (IgE or non-IgE driven) targeting milk proteins: caseins or whey fractions (beta-lactoglobulin). Symptoms range from urticaria and angioedema to anaphylactic shock. Lactose-free dairy retains all intact allergenic milk proteins and is strictly contraindicated in CMPA!

Definitive clinical evaluation involves objective diagnostic testing:

• Hydrogen Breath Test (HBT): The non-invasive gold standard. Following oral ingestion of a standardized lactose load (25 g), exhaled hydrogen is measured serially. A baseline rise exceeding 20 ppm diagnosticates colonic bacterial fermentation of malabsorbed lactose.
• Genetic LCT Polymorphism Testing (-13910 C/T): Delineates genetically programmed primary lactase non-persistence (homozygous C/C genotype) from lactase persistence phenotypes (T/T or C/T).
• Clinical Elimination-Rechallenge Protocol: Complete resolution of dyspepsia during strict dairy restriction and reproducible symptom recurrence upon blinded challenge.