The most frequent cause of lactose malabsorption is lactase non-persistence, whereas acquired conditions include small intestinal bowel overgrowth (SIBO), infectious enteritis (e.g. giardiasis), or mucosal alterations secondary to CD, inflammatory bowel disease (IBD), drugs, surgery, radiations, or other conditions that imply the reduction of absorptive capacity or downregulation of lactase expression in the small intestine. Here, LCT is linked to irritable bowel syndrome.