From Cathy Wong,
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Mood - Anxiety, hostile feelings, sadness, depression, and sleep disturbance are associated with IBS. Adverse life events such as family death, marital stress, financial difficulties, and especially physical and sexual abuse, have also been reported more frequently in IBS patients than in the general population. However, it is possible that IBS patients with this social or psychological background may be more likely to seek medical treatment or participate in research studies.

The impact of stress on bowel motility and pain were explored in one study by administering corticotrophin-releasing factor (CRF), a hormone released in the body during stress. CRF increases motility of the descending colon and can induce abdominal pain. The researchers found that IBS patients had greater colonic motility and more abdominal pain after receiving CRF than controls.

Antidepressants have been shown to be very effective for treating bowel motility and visceral nerve responses, in addition to addressing the emotional component of IBS.

Small Intestine Bacterial Overgrowth - Excess bacteria in the small intestine, an area that is normally relatively free of bacteria, is being recognized as important in the development of IBS. When these bacteria are present in the small intestine, excessive gas, bloating, abdominal distension and pain, and altered gut motility can result.

Causes of small intestine bacterial overgrowth include decreased gastric acid secretion (possibly due to natural aging, stomach ulcer, and colonization by helicobacter pylori bacteria), decreased bile flow, or decreased pancreatic enzymes with poor absorption of carbohydrates, fats, and proteins. The resulting undigested and unabsorbed carbohydrates in the small intestine and colon cause excess fermentation and encourage growth of unwanted bacterial species. An abundance of gas is produced, as well as short-chain organic acids such as lactic acid, which can damage the mucus lining of the intestines and further aggravate carbohydrate malabsorption.

In addition, putrefaction of proteins in the small intestine produces substances called vasoactive amines that can affect intestinal muscles.