Irritable bowel syndrome
The alternating bouts of diarrhea and constipation associated with irritable bowel syndrome typically develop in the late teens or early 20s, primarily in women. Many people who have it are never diagnosed, however, because medical tests routinely show nothing out of the ordinary.
There’s plenty of speculation about why the colon misfires. Some suggest that hormonal problems play a role (women with irritable bowel syndrome seem to have more symptoms on premenstrual days) or that a chemical imbalance in the brain may be present. Specific foods seem to trigger IBS flare-ups in some people, including the inability to digest lactose. Overuse of antibiotics could be another cause, as can a bacterial, viral, or parasitic infection.
Stress also appears to be another key factor of irritable bowel syndrome. Even a healthy person experiencing the natural “fight or flight” response that occurs in a very stressful situation will have intestinal spasms and sometimes an involuntary emptying of the bowels. But, people susceptible to IBS experience this intestinal response to stress to an excessive degree.