William C. Shiel Jr., MD, FACP, FACR
William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
In this Article
What are common causes of chronic diarrhea?
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Irritable bowel syndrome. The irritable bowel syndrome (IBS) is a functional cause of diarrhea or constipation. Inflammation does not typically exist in the affected bowel. (Nevertheless, recent information suggests that there MAY be a component of inflammation in IBS.) It may be caused by several different underlying problems, but it is believed that the most common cause is rapid passage of the intestinal contents through the colon.
Infectious diseases. There are a few infectious diseases that can cause chronic diarrhea, for example, Giardia lamblia. Patients with AIDS often have chronic infections of their intestines that cause diarrhea.
Bacterial overgrowth of the small intestine. Because of small intestinal problems, normal colonic bacteria may spread from the colon and into the small intestine. When they do, they are in a position to digest food that the small intestine has not had time to digest and absorb. The mechanism that leads to the development of diarrhea in bacterial overgrowth is not clear.
Post-infectious. Following acute viral, bacterial or parasitic infections, some individuals develop chronic diarrhea. The cause of this type of diarrhea is not clear, but some of the individuals probably have bacterial overgrowth of the small intestine. They also may have abnormalities, either microscopic or biochemical, in biopsies of the intestines. This condition also is referred to as post-infectious IBS.
Colon cancer. Colon cancer can cause either diarrhea or constipation. If the cancer blocks the passage of stool, it usually causes constipation. Sometimes, however, there is secretion of water behind the blockage, and liquid stool from behind the blockage leaks around the cancer and results in diarrhea. Cancer, particularly in the distal part of the colon, can lead to thin stools. The diarrhea or constipation caused by cancer usually is progressive, that it becomes progressively worse. Cancer in the rectum can lead to a sense of incomplete evacuation.
Severe constipation. By blocking the colon, hardened stool can lead to the same problems as colon cancer, as discussed previously.
Carbohydrate (sugar) malabsorption. Carbohydrate or sugar malabsorption is an inability to digest and absorb sugars. The most well-recognized malabsorption of sugar occurs with lactase deficiency (also known as lactose or milk intolerance) in which milk products containing the milk sugar, lactose, lead to diarrhea. The lactose is not broken up in the intestine because of the absence of an intestinal enzyme, lactase, that normally breaks up lactose into its component sugars, galactose and glucose. Without being broken up, lactose cannot be absorbed into the body. The undigested lactose reaches the colon and pulls water (by osmosis) into the colon. This leads to diarrhea. Although lactose is the most common form of sugar malabsorption, other sugars in the diet also may cause diarrhea, including fructose and sorbitol.
Fat malabsorption. Malabsorption of fat is the inability to digest or absorb fat. Fat malabsorption may occur because of reduced pancreatic secretions that are necessary for normal digestion of fat (for example, due to pancreatitis or pancreatic cancer) or by diseases of the lining of the small intestine that prevent the absorption of digested fat (for example, celiac disease). Undigested fat enters the last part of the small intestine and colon where bacteria turn it into substances (chemicals) that cause water to be secreted by the small intestine and colon. Passage through the small intestine and colon also may be more rapid when there is malabsorption of fat.
Endocrine diseases. Several endocrine diseases (imbalances of hormones) may cause diarrhea, for example, an over-active thyroid gland (hyperthyroidism) and an under-active pituitary or adrenal gland (Addison's disease).
Reviewed by William C. Shiel Jr., MD, FACP, FACR on 2/14/2013
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