Prolonged diarrhea is more serious in children than adults because:

Here are some common functional gastrointestinal disorders (FGIDs) in children: Functional dyspepsia, Irritable bowel syndrome, Functional abdominal pain, and Cyclic vomiting.  FGIDs can have a significant emotional and economic impact.  Some other alterations of digestive function in children include: Volvulus: A blockage in the intestine that can prevent food from being digested well. It can also cut off blood supply to the twisted part of the intestine, which can lead to the death of that part of the intestine. Preterm infants: Have reduced digestive and absorption capacities,... Show more

Here are some common functional gastrointestinal disorders (FGIDs) in children: Functional dyspepsia, Irritable bowel syndrome, Functional abdominal pain, and Cyclic vomiting. 

FGIDs can have a significant emotional and economic impact. 
Some other alterations of digestive function in children include:
Volvulus: A blockage in the intestine that can prevent food from being digested well. It can also cut off blood supply to the twisted part of the intestine, which can lead to the death of that part of the intestine.
Preterm infants: Have reduced digestive and absorption capacities, prolonged gastric emptying times, and limited intestinal motility compared to term infants. 

Paediatric functional abdominal pain disorders (FAPDs) are some of the most common disorders in children, affecting up to 25% of all children and infants worldwide. FAPDs include: Abdominal migraine and Functional abdominal pain not otherwise specified. 
The main functions of the GI system include: ingestion and digestion of food, nutrient absorption, secretion of water and enzymes, and excretion of waste products.
 

Related Test: Pathophysiology Practice Test: Alterations of Digestive Function

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