RecruitingDietary supplementSustained Oral Fiber Supplementation for Patients Undergoing Donor Stem Cell Transplantation for Hematological MalignanciesThis clinical trial examines sustained oral fiber supplementation for patients undergoing donor stem cell transplantation for hematological malignancies. Patients undergoing donor stem cell transplantation often develop oral and gastrointestinal damage from chemotherapy, radiotherapy, or graft-versus-host disease. Oral fiber nutrition support may improve overall nutrition, support a normal gut microbiome (bacteria that live in the gut) and/or improve gut function in patients undergoing stem cell transplants.