What Are the 4 Types of Diarrhea?

Diarrhea can be classified into four types: osmotic, secretory, inflammatory or infectious, and motility-related

Diarrhea can be classified into four types:

1. Osmotic diarrhea

Water absorption in the intestines depends on the proper absorption of solutes, which is hindered if there are too many solutes in the intestinal lumen, which leads to diarrhea. Osmotic diarrhea may result from one of the following two situations:

• Ingestion of a poorly absorbed substrate: Some carbohydrate or divalent ions such as mannitol and sorbitol, Epsom salt (MgSO4), and certain antacids (MgOH2) may be absorbed poorly, leading to diarrhea.
• Malabsorption: Although the most common type of malabsorption is the inability to absorb specific carbohydrates, there are other types of malabsorption too. A common form of malabsorption affecting humans and animals is lactose intolerance. This is caused by a lack of the brush border enzyme lactase, which prevents the breakdown of lactose into simpler carbohydrates, such as glucose and galactose. The intestinal lumen retains lactose, which leads to more water retention. The unabsorbed lactose is then absorbed into the large intestinal lumen, where it gets fermented and produces excessive gas.

2. Secretory diarrhea

In the gastrointestinal tract, a large amount of water is secreted into the small intestine lumen, but a large majority gets absorbed before reaching the large intestine. Diarrhea occurs when water secreted in the intestine exceeds absorption. Cholera caused by Vibrio cholerae produces toxins responsible for excess water secretion in the intestine, which causes diarrhea. Apart from cholera, other agents that cause secretory diarrhea include:

• Certain laxatives
• Hormones secreted by certain tumors
• Various drugs
• Certain metals, organic toxins, and plant products (for example, arsenic, insecticides, mushroom toxins, and caffeine)

3. Inflammatory or infectious diarrhea

The digestive tract has an epithelium that is protected by a barrier. This barrier can be disrupted by invading microbial and viral pathogens, leading to diarrhea. When the epithelium gets disrupted, serum and blood are released into the intestine and cause widespread destruction of the epithelium. Some of the pathogens associated with inflammatory and infectious diarrhea include

• Bacteria: Salmonella, E. coli, and Campylobacter
• Viruses: Rotaviruses, coronaviruses, parvoviruses (canine and feline), and noroviruses
• Protozoa: Coccidia species, Cryptosporium, and Giardia

When these pathogens attack the epithelium, the immune system sends inflammatory mediators and cytokines to the site, thereby damaging the epithelial cells and replacing them with immature cells that lack lactase and other transporters necessary for absorbing nutrients and water. 

4. Motility-related diarrhea

Nutrients and water are efficiently absorbed if they get adequate exposure to the intestinal epithelium and remain in the intestine for sufficient time (transit time). Any condition that increases transit time could lead to diarrhea, even if the absorption isn’t affected much.