Originally Answered: Irritable Bowel Syndrome ?
Irritable bowel syndrome (IBS), also called spastic colon, is a functional bowel disorder characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits in the absence of any organic cause. In some cases, the symptoms are relieved by bowel movements. Diarrhea or constipation may predominate, or they may alternate (classified as IBS-D, IBS-C or IBS-A, respectively). IBS may begin after an infection (post-infectious, IBS-PI) or a stressful life event. Other functional or pain disorders and certain psychological conditions are more common in those with IBS.
Although there is no cure for IBS, there are treatments which attempt to relieve symptoms, including dietary adjustments, medication and psychological interventions. Patient education and a good doctor-patient relationship are also important.
Several conditions may present as IBS including celiac disease, mild infections, parasitic infections like giardiasis, several inflammatory bowel diseases, functional chronic constipation, and chronic functional abdominal pain. In IBS, routine clinical tests yield no abnormalities, though the bowels may be more sensitive to certain stimuli, such as balloon insufflation testing. The exact cause of IBS is unknown. The most common theory is that IBS is a disorder of the interaction between the brain and the gastrointestinal tract, although there may also be abnormalities in the gut flora or the immune system. 
IBS does not lead to more serious conditions in most patients. But it is a source of chronic pain, fatigue and other symptoms, and it increases a patient's medical costs, and contributes to work absenteeism.
Many different dietary modifications have been attempted to improve the symptoms of IBS. Some are effective in certain sub populations. As lactose intolerance and IBS have such similar symptoms a trial of a lactose free diet is often recommended. Fiber supplements have not been found to be effective in the general IBS population. They however might be beneficial in those who have a predominance of constipation.
Definitive determination of dietary issues can be accomplished by testing for the physiological effects of specific foods. The ELISA food allergy panel can identify specific foods to which a patient has a reaction. Other testing can determine if there are nutritional deficiencies secondary to diet that may also play a role. Removal of foods causing IgG immune response as measured using the ELISA food panel has been shown to substantially decrease symptoms of IBS in several studies.
There is no evidence that digestion of food or absorption of nutrients is problematic for those with IBS at rates different from those without IBS. However, the very act of eating or drinking can provoke an overreaction of the gastrocolic response in some patients with IBS due to their heightened visceral sensitivity, and this can lead to abdominal pain, diarrhea, and/or constipation.
Several of the most common dietary triggers are well-established by clinical studies at this point; research has shown that IBS patients are hypersensitive to fats and fructose.