Causes and pathophysiology of diarrhea

Diarrhea is a consequence, mainly, of excess water in the feces, which can be caused by infection, medication, food, surgery, inflammation, accelerated passage through the intestine or malabsorption. These causes can cause diarrhea 4 different mechanisms: increased osmotic pressure in the lumen of the intestine, increased secretion, inflammation and reduced absorption time. Paradoxical diarrhea is a consequence of coprostasis and is a leakage of fluid around the stool. Acute diarrhea (<4 days) is usually caused by etiologically determined causes, such as food poisoning or infection.

Complications can be the result of diarrhea of any etiology. There may be a loss of fluid with dehydration, loss of electrolytes (Na, K, Mg, CI) and even sometimes vascular collapse. Collapse can develop rapidly in patients with severe diarrhea (eg, cholera), very young, elderly or emaciated patients. Loss of NSO ₃ can cause metabolic acidosis. Hypokalemia can occur with severe or chronic diarrhea or if there is a lot of mucus in the stool. Hypomagnesemia after prolonged diarrhea can cause tetany.

[1], [2], [3], [4], [5], [6],

Osmotic diarrhea

Osmotic diarrhea is observed in the case of a delay in the intestine of non-absorbed, water-soluble substances that retard water. Such solutions include polyethylene glycol, salts of Mg (hydroxide and sulfate) and Na phosphate, which are used as laxatives. Osmotic diarrhea develops with sugar intolerance (eg, lactose intolerance caused by a deficiency of lactase). The use of large amounts of hexitols (eg sorbitol, mannitol, xylitol), used as sweeteners in candies and chewing gums, causes osmotic diarrhea due to poor absorption of these substances. Lactulose, which is used as a laxative, causes diarrhea in a similar mechanism. Excess absorption of certain fruits can also cause osmotic diarrhea.

Secretory diarrhea

Secretory diarrhea develops when the intestines secrete electrolytes and water more than they are absorbed. Secretogens include bacterial toxins (eg cholera and colitis caused by Clostridium difficile), enteropathogenic viruses, bile acids (eg, after resection of the ileum), unabsorbed edible fats and many medications (eg quinidine, quinine, colchicine, selective inhibitors of serotonin, cholinesterase inhibitors, anthraquinone laxatives, castor oil, prostaglandins). Different endocrine tumors produce secretogens, including vipoma (vasoactive intestinal peptide), gastrin (gastrin), labrocytosis (histamine), medullary thyroid carcinoma (calcitonin and prostaglandins) and carcinoid tumors (histamine, serotonin and polypeptides). Microscopic colitis (collagen or lymphocytic) rarely causes secretory diarrhea, especially in women over 60 years of age.

Inflammatory diarrhea

Inflammatory diarrhea develops in certain infections and diseases that cause inflammation of the mucous membrane or ulceration (eg Crohn's disease, ulcerative colitis, tuberculosis, lymphoma, cancer). Emerging exudation in the lumen of the intestine of plasma, whey proteins, blood and mucus increases the volume of intestinal contents and fluid. Involving the rectal mucosa in the inflammatory process can cause sudden and frequent stools, since the inflamed rectum is more sensitive to stretching.

Nutritional factors that cause diarrhea

Nutritional factor	A source
Caffeine	Coffee, tea, cola, non-prescription medicines used for headache
Fructose (in quantities exceeding the absorptive capacity of the gut)	Apple juice, pear juice, grapes, honey, dates, nuts, fig tree fruit, soft drinks (especially with fruit)
Hexites, sorbitol and mannitol	Apple juice, pear juice, chewing gum without sugar, mint candies
Lactose	Milk, ice cream, frozen yogurt, yoghurt, soft cheeses
Magnesium-containing antacids	Antacids
Sucrose	Table sugar

Diarrhea due to reduced suction time

Diarrhea due to reduced suction time is observed with insufficient contact of the chyme with the active absorbent surface of the gastrointestinal tract, leading to too much water retention in the stool. Factors that reduce contact time include small or extensive intestinal resections, stomach resection, pyloroplasty, vagotomy, bypass gut anastomoses, medications (eg, Mg-containing antacids, laxatives) or humoral substances (eg prostaglandins, serotonin) that cause accelerated passage by stimulating the smooth muscles of the intestine.

[7], [8], [9], [10], [11], [12],

Diarrhea associated with malabsorption

Diarrhea associated with malabsorption may result from osmotic or secretory mechanisms. The mechanism can be osmotic, if not absorbed, water-soluble substances with low molecular weight enter the intestine. Lipids are not osmotic substances, but some (fatty acids, bile acids) act as stimulants of secretion and cause secretory diarrhea. Generalized malabsorption (eg, gluten enteropathy), malabsorption of fat causes the secretion of the colon, and malabsorption of carbohydrates causes osmotic diarrhea. Diarrhea associated with malabsorption can also develop in the case of delayed passage of chyme and reproduction of bacteria contained in small intestinal contents, which is observed with intestinal narrowing, scleroderma with gastrointestinal disease and adhesions, after surgical interventions.

[13], [14], [15]