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Diarrhea is an increase in the volume, wateriness, or frequency of bowel movements.

The frequency of bowel movements alone is not the defining feature of diarrhea. Some people normally move their bowels 3 to 5 times a day. People who eat large amounts of vegetable fiber may produce more than a pound of stool a day, but the stool in such cases is well formed and not watery. Diarrhea occurs when not enough water is removed from the stool, making the stool loose and poorly formed. Diarrhea is often associated with gas, cramping, an urgency to defecate, and, if the diarrhea is caused by an infectious organism or a toxic substance, nausea and vomiting.

Diarrhea can lead to dehydration and a loss of electrolytes, such as sodium, potassium, magnesium, chloride, and bicarbonate, from the blood. If large amounts of fluid and electrolytes are lost, the person feels weak, and blood pressure can drop enough to cause fainting (syncope), heart rhythm abnormalities (arrhythmias), and other serious disorders.

At particular risk are the very young, the very old, the debilitated, and people with very severe diarrhea. Diarrhea is a major cause of infant mortality in developing countries and results in many hospitalizations in the United States.


Normally, stool is 60 to 90% water. Diarrhea mainly occurs when the percentage is over 90%. Stool may contain too much water if it travels too quickly through the digestive tract, if certain components of the stool prevent the large intestine from absorbing water, or if water is being secreted by the large intestine into the stool. There are many different causes, including drugs and chemicals; infection with viruses, bacteria, or parasites (gastroenteritis; some foods; stress; tumors; and chronic disorders such as irritable bowel syndrome, inflammatory bowel disease, and malabsorption syndromes.

Rapid passage (transit) of stool is one of the most common causes of diarrhea. For stool to have normal consistency, it must remain in the large intestine for a certain amount of time. Stool that leaves the large intestine too quickly is watery. Many medical conditions and treatments can decrease the amount of time that stool stays in the large intestine, including an overactive thyroid (hyperthyroidism); Zollinger-Ellison syndrome (a condition of over-production of acid secondary to a tumor); surgical removal of part of the stomach, small intestine, or large intestine; surgical bypass of part of the intestine; and drugs such as antacids containing magnesium, laxatives, prostaglandins, serotonin, and even caffeine. Many foods, especially those that are acidic, can increase the rate of transit. Some people are intolerant of specific foods and always develop diarrhea after eating them. Stress and anxiety are also common causes.

Osmotic diarrhea occurs when certain substances that cannot be absorbed through the colon wall remain in the intestine. These substances cause excessive amounts of water to remain in the stool, leading to diarrhea. Certain foods (such as some fruits and beans) and hexitols, sorbitol, and mannitol (used as sugar substitutes in dietetic foods, candy, and chewing gum) can cause osmotic diarrhea. Also, lactase deficiency can lead to osmotic diarrhea. Lactase is an enzyme normally found in the small intestine that converts lactose (milk sugar) to glucose and galactose, so that it can be absorbed into the bloodstream. When people with lactase deficiency drink milk or eat dairy products, lactose is not digested. As lactose accumulates in the intestine, it causes osmotic diarrhea—a condition known as lactose intolerance. The severity of osmotic diarrhea depends on how much of the osmotic substance is consumed. Diarrhea stops soon after the person stops eating or drinking the substance. Blood in the digestive tract also acts as an osmotic agent and results in black, tarry stools (melena). Another cause of osmotic diarrhea is an overgrowth of normal intestinal bacteria or the growth of bacteria normally not found in the intestines. Antibiotics can cause osmotic diarrhea by destroying the normal intestinal bacteria.

Secretory diarrhea occurs when the small and large intestines secrete salts (especially sodium chloride) and water into the stool. Certain toxins—such as the toxin produced by a cholera infection or during some viral infections—can cause these secretions. Infections by certain bacteria (for example, Campylobacter) and parasites (for example, Cryptosporidium) can also stimulate secretions. The diarrhea can be massive—more than a quart of stool an hour in cholera. Other substances that cause salt and water secretion include certain laxatives, such as castor oil, and bile acids (which may build up after surgery to remove part of the small intestine). Certain rare tumors—such as carcinoid, gastrinoma, and vipoma—also can cause secretory diarrhea, as can some polyps.

Inflammatory diarrhea occurs when the lining of the large intestine becomes inflamed, ulcerated, or engorged and releases proteins, blood, mucus, and other fluids, which increase the bulk and fluid content of the stool. This type of diarrhea can be caused by many diseases, including ulcerative colitis, Crohn's disease (regional enteritis), tuberculosis, and cancers such as lymphoma and adenocarcinoma. When the lining of the rectum is affected, the person often feels an urgent need to move his bowels and has frequent bowel movements because the inflamed rectum is more sensitive to expansion (distention) by stool.


The evaluation depends on whether the diarrhea is acute (sudden and present for a short time) or chronic (persistent).
For acute diarrhea that lasts for more than 72 hours (or sooner if blood is present, or the person is weak or has a fever, rash, or severe pain), a doctor should be consulted. If, on the doctor's examination, the person does not appear to be dehydrated or seriously ill, and the diarrhea is not severe and has lasted for less than a week, testing is usually not needed. Other people may need blood tests for electrolyte abnormalities or stool tests for blood, white blood cells, and the presence of infectious organisms (for example, bacteria such as Campylobacter and Yersinia and parasites such as amebas, Giardia, and Cryptosporidium). Some causes of infection are detected by looking under the microscope, whereas others require a culture (growing the organism in the laboratory) or special enzyme tests (for example, Shigella or Giardia). If the person has taken antibiotics recently, the doctor may test the stool for Clostridium difficile toxin. A colonoscopy is usually not necessary.

For chronic diarrhea, similar tests are performed. In addition, the doctor may test the stool for fat (indicating malabsorption) and perform a sigmoidoscopy or colonoscopy to examine the lining of the rectum and colon. Sometimes a biopsy (removal of a tissue specimen for examination under a microscope) of the rectal lining is performed. Sometimes the volume of stool over a 24-hour period is determined. Secret (surreptitious) use of a laxative also can be identified in the stool sample.


Diarrhea is a symptom, and its treatment depends on its cause. For most people, treating diarrhea involves only removing the cause to suppress the diarrhea until the body heals itself. A viral cause usually resolves by itself in 24 to 48 hours. Extra fluids containing a balance of water, sugars, and salts are needed for people who are dehydrated. As long as the person is not vomiting excessively, these fluids can be given by mouth. Seriously ill people and those with significant electrolyte abnormalities require intravenous fluid and sometimes hospitalization.

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