irritable bowel syndrome. irritable bowel syndrome 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 known. 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 have bacterial overgrowth of the small intestine. this condition also is referred to as post-infectious ibs. inflammatory bowel disease. crohn's disease and ulcerative colitis diseases causing inflammation of the small intestine and/or colon commonly cause chronic diarrhea. 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. 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. 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. 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 overactive thyroid gland (hyperthyroidism) and an underactive pituitary or adrenal gland (addison's disease). laxative abuse. the abuse of laxatives by individuals who want attention or to lose weight is an occasional cause of chronic diarrhea.
what are common causes of chronic diarrhea?