Splitting lactose into absorbable sugars
Lactase is an enzyme on the brush border of the small intestine that splits lactose, the disaccharide in milk, into glucose and galactose. Only the single sugars can be absorbed across the intestinal wall, so lactase is the gateway for getting energy from dairy. Lactase activity is high in infancy and, in much of the world's adult population, declines after weaning.
When lactase is insufficient, unabsorbed lactose passes into the colon. There it draws in water osmotically and is fermented by bacteria, producing gas and the bloating, cramping, and loose stools of lactose intolerance.
Intolerance is not a milk allergy
Lactose intolerance is an enzyme shortfall and is dose-dependent, so many affected people tolerate small amounts or fermented dairy. It is distinct from cow's milk protein allergy, which is an immune reaction with different and potentially more serious risks. Lactase supplements taken with dairy can help some people manage symptoms.
This is general educational information, not medical advice. Symptoms overlap with other gut conditions, so a clinician should confirm the cause rather than assuming dairy is the culprit.