Prebiotic Galacto-oligosaccharide and Acute GVHD

Inclusion Criteria: * Plan to undergo allogeneic HCT for any cancer or non-cancer illness * Age 18-80 years * Karnofsky Performance Status \>70 Exclusion Criteria: * Pregnant/lactating * Malabsorption syndrome, short bowel or chologenic diarrhea * At time of enrollment, Grade 2 or higher GI symptoms per NCI-CTCAE * Active treatment with other prebiotics, probiotics, or herbal supplementation (ok if stops before enrollment) * Active treatment with antibiotics (with the exception of prophylactic antibiotics) * Concurrent enrollment on the Duke HCT Home Transplant study or another clinical trial targeting GVHD; patients who are enrolled in observational or non-pharmacologic intervention trials (for example, the Duke HCT Research-POP Pre and Peri-HCT Optimization Program aka "R-POP") or pharmacologic or cellular therapy trials with other targets (for example, NK DLI) are NOT excluded