Diarrhea

Diarrhea: Mechanisms and Causes

Diarrhea, also known as loose stools, is a condition characterized by frequent (three or more times per day) watery or liquid bowel movements. Like vomiting, diarrhea is not a disease but a symptom and a protective mechanism by which the body attempts to quickly eliminate pathogens, toxins, or undigested substances. Depending on duration, diarrhea is classified as acute (up to 2 weeks) or chronic (over 4 weeks).

Why does diarrhea occur? Main mechanisms

The basis of diarrhea is a disruption of the balance between secretion and absorption of fluid in the intestine. There are four main mechanisms:

• Secretory diarrhea: Occurs when intestinal cells begin actively secreting water and electrolytes into its lumen. This is the classic mechanism in infections caused by bacterial toxins (e.g., cholera) or viruses (rotavirus).
• Osmotic diarrhea: Occurs when substances that are poorly absorbed are present in the intestinal lumen and "draw" water by osmosis. Examples include lactose intolerance, consumption of certain laxatives or artificial sweeteners.
• Inflammatory (exudative) diarrhea: Occurs with damage and inflammation of the intestinal mucosa (e.g., Crohn’s disease, ulcerative colitis, bacterial infections). The damaged wall cannot absorb water normally, and mucus, blood, and protein appear in the stool.
• Motility disorder: Too rapid passage of food through the intestine (hypermotility) does not allow enough time for water absorption.

Acute vs. Chronic diarrhea

Acute diarrhea is infectious in 90% of cases (viruses, bacteria, parasites) or results from food poisoning. Chronic diarrhea is more often associated with non‑infectious causes such as irritable bowel syndrome (IBS), inflammatory bowel diseases, celiac disease. The main danger of acute diarrhea is dehydration.