Fermentable Oligo-, Di and Mono-saccharides And Polyols (FODMAPs) are carbohydrates that are poorly digested in intestines. The undigested carbohydrates are fermented in the colon by gut bacteria. Fermentation of these carbohydrates can lead to diarrhea, gas and distension of the colon. Low FODMAP diet effect may be mediated by changing the gut bacteria and/or by production of chemicals that influence Veteran's intestines which then result in reduced disease symptoms.

The goal of this study is to compare a low FODMAP (modified healthy) diet to a high FODMAP (typical healthy) diet for effect on Veterans with IBS and symptoms of Gulf War illness.

Study Design This will be a prospective, blinded randomized clinical trial with two parallel groups. There will be 1-week screening period, 3-week diet treatment and, follow-up one month later while Veterans choose their own diet.

Veterans who were deployed and served in Operation Desert Storm and Desert Shield will be recruited to participate.

The two study diets to be provided to the Veterans and compared are:

Low FODMAP (modified healthy) diet:

High FODMAP (typical healthy) diet:

The FODMAP content of this diet will be higher than low FODMAP diet.

All questionnaires will be answered and measurements performed at baseline, end of treatment and at follow-up visits.

Inclusion Criteria:

• Gulf War Veterans
• Men and women age 25-90 years
• Gulf War Illness: Veterans will have Rome III criteria for IBS and two or more of the non-intestinal symptom groups (chronic-once a week or more often-fatigue, insomnia, joint pains, general stiffness, and headache, neurological an mood, respirator and skin symptoms)
• Symptoms of > 6 months duration
• No significant findings on physical examination, complete blood count and clinical chemistry panel
• Normal gross appearance of the colonic mucosa (small colon polyps not an exclusion criteria)
• Veterans with psychological disorders will not be excluded but will be identified for sub-group analysis.

Exclusion Criteria:

• Current evidence of any lower gastrointestinal disorder such as celiac disease or inflammatory bowel disease
• Clinically significant chronic disease: HIV, cardiac, pulmonary, hepatic or renal dysfunction
• Presence of Giardia antigen, and Clostridium difficile toxin in stool
• Current history of drug or alcohol abuse
• Investigator perception of patient's inability to comply with study protocol
• Recent change in gastrointestinal medications
• Subject is currently participating in another research protocol. These subjects will be allowed to enroll after a washout period of one month.