To evaluate the safety and effectiveness of oral nutritional therapy Serum-Derived Bovine Immunoglobulin (SBI) on nutritional status, epithelial barrier function, and mucosal expression of pivotal genes including tight junction, secretory mechanisms, tissue repair proteins and chemokines in subjects with IBS-D.

This is an open label study evaluating the impact of SBI 5.0 g twice daily on

1. nutritional status (plasma amino acid profile and kynurenine to tryptophan ratio),
2. intestinal permeability (in vivo) and
3. mucosal expression of pivotal genes including tight junction, secretory mechanisms, tissue repair proteins and chemokines status in patients with IBS-D.

Plasma, duodenal and stool samples will be collected. Fifteen eligible subjects will be enrolled to receive SBI for 8 weeks (SBI 5.0g BID for 8 weeks).

Intestinal permeability will be measured in vivo by two sugar urine excretion(s) after oral ingestion.

Biopsies from the distal second or third portion of the duodenum will be obtained endoscopically, to measure mRNA expression of tight junction proteins and markers of immune function,

Inclusion criteria:

1. Age 18-65y
2. Male or non-pregnant female
3. IBS by Rome III criteria with predominant symptom of diarrhea
4. Baseline 14 day diary showing average of 2 days per week with >3 bowel movements per day

Exclusion criteria:

1. Intake of medications that interfere with the study
2. Antibiotic within prior 2 weeks and throughout study
3. Prior abdominal surgery except appendectomy
4. Active gastrointestinal diagnosis other than IBS
5. History of allergy or intolerance to beef or to any ingredient in the investigational product
6. Uncontrolled psychiatric disorders (includes significant depression or suicidal ideation), in investigator's judgment
7. Use of tobacco products within the past six months or nonsteroidal antiinflammatory drugs or aspirin within the past week (since they all may affect intestinal permeability)
8. Bleeding disorders or medications that increase risk of bleeding from mucosal biopsies.
9. For two days prior to studies, patients are instructed to avoid ingestion of artificial sweeteners such as Splenda trademark (TM) (sucralose), Nutrasweet TM (aspartame), foods containing lactulose or mannitol.