Carbohydrate malabsorption may occur when digestive enzymes, such as lactase, are deficient or when intestinal mucosal integrity is compromised. In lactose intolerance, undigested lactose passes into the colon, where it is fermented by bacteria, leading to bloating, diarrhoea, and abdominal discomfort. In infants, elevated reducing substances in stool may indicate temporary lactose intolerance following viral gastroenteritis or more complex disorders such as congenital disaccharidase deficiencies. In rare cases, persistent reducing substances may signal inherited metabolic conditions. Identifying carbohydrate malabsorption helps guide dietary management, including lactose restriction or specialised infant formulas where appropriate.