The purposes of this study are to prospectively determine the effect of a very low carbohydrate diet on quality of life and gastrointestinal symptoms in patients with diarrhea-predominant irritable bowel syndrome (IBS-D); and to determine possible physiological correlates of symptom improvement, as related to post-prandial 5-hydroxytryptamine (5-HT) release, weight loss and fiber content.

Approximately 10-15% of individuals in the United States have symptoms consistent with irritable bowel syndrome (IBS) which is a costly disorder and negatively impacts patient quality of life. The pathogenesis of this heterogeneous disorder is still not well understood. Patients frequently identify worsening of symptoms after meals and often cite particular foods as triggers of their IBS symptoms. Unfortunately, there is insufficient randomized clinical trial data to allow for specific dietary recommendations. Previous research has suggested a role for carbohydrate ingestion in IBS. There is also evidence for the role of the post-prandial release of 5-hydroxytryptamine (5-HT) and its turnover (as represented by the ratio of its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), to 5-HT in response to a carbohydrate-rich meal, especially in those with IBS-D. As the prevalence of overweight (body mass index [BMI] > 25 kg/m2) and obesity (BMI > 30 kg/m2) has risen in recent years, very low carbohydrate diets have become popular for those attempting to lose weight. Patients with IBS, especially IBS-D, anecdotally report improvement in their gastrointestinal symptoms after initiating a very low carbohydrate diet. However, no study has investigated the effect of a very low carbohydrate diet on symptoms and quality of life in patients with IBS-D.

Inclusion Criteria:

1. Age 18-70 years old, male or female
2. Meet Rome II Criteria for IBS-D
3. Body mass index > 25 kg/m^2
4. Desire to use a very low carbohydrate diet for weight loss
5. Score of > 36 on the FBDSI
6. Ability to understand consent form
7. In stable health by screening history, physical examination performed by a study physician, laboratory tests (normal blood counts, kidney function tests, liver tests, TSH).

Exclusion Criteria:

1. Age < 18 years or age > 70 years

2. History of inflammatory bowel disease

3. History of any gastrointestinal surgery that preceded the onset of IBS symptoms

4. Pregnancy or breastfeeding

5. FBDSI symptom score of ≤ 36

6. Inability to understand consent form

7. Diabetes requiring medications (must be controlled with diet and exercise alone).

8. Chronic narcotic use for any reason

9. Use of serotonin-selective reuptake inhibitors unless patient has been on a stable dose for at least 4 weeks.

10. Use of any over-the-counter or prescription weight loss medications.

11. Any chronic or unstable diseases (e.g., kidney disease, heart disease, or cancer) that may put the subject at increased risk from the intervention

12. Any of the following baseline abnormalities of laboratory tests or physical exam findings:

    1. Serum creatinine > 1.5 mg/dL in men, > 1.3 mg/dL in women.
    2. Liver disease (AST or ALT > 2 times the upper limit of normal or total bilirubin > 1.6mg/dL).
    3. Blood pressure > 160/100 mm Hg.
    4. Fasting triglycerides > 600 mg/dL.
    5. Fasting serum low-density lipoprotein (LDL) cholesterol > 190 mg/dL.

Low N, lack of standard control group and specific overweight/obese female predominant population.