LCT and hyperinsulinemic hypoglycemia, familial, 4: Secondary lactase deficiency is the consequence of many conditions leading to either a reduction of absorptive capacity or down-regulation of lactase expression in the small intestine, such as severe malnutrition, mucosal damage due to celiac disease and Inflammatory bowel diseases (IBD), Small bowel bacterial overgrowth (SIBO), etc. [22].