This study will look at the effect of the prebiotic inulin compared to placebo on children undergoing stem cell transplant.

Children enrolled in the study will receive either the prebiotic inulin or a placebo for 21 days during the study period. They will start taking the product seven days before transplant starts until 14 days after transplant. Stool will be collected twice weekly until thirty days after transplant or discharge, whichever occurs first. Stool samples will be sampled for metagenomic sequencing to identify the diversity of bacteria within the stool. They will also be analyzed for amount of short-chain fatty acid content (a breakdown product of inulin) as well as for presence of genes that confer antibiotic resistance. From 30 days after transplant until 100 days after transplant, two stool samples will be collected at regularly scheduled follow up appointments (near day 60 and day 100). No product (inulin or placebo) will be given during this time frame. The study period ends 100 days after transplant.

Inclusion Criteria:

1. Age 2-18 years
2. Allogeneic Stem Cell transplant
3. Myeloablative therapy
4. Inpatient at Lurie Children's Hospital

Exclusion Criteria:

1. Previous Hematopoietic Stem Cell Transplant
2. Autologous Stem Cell Transplant
3. Reduced Intensity Conditioning used for transplant
4. History of ulcerative colitis, Crohn's disease, irritable bowel syndrome, celiac disease
5. History of Type 1 or Type 2 Diabetes Mellitus
6. Previous abdominal surgery necessitating the use of an ostomy
7. G-tube dependence
8. Nasal gastric/oral gastric tube dependence prior to starting the conditioning process
9. Graft vs host disease prior to enrollment at any site
10. Children under 2 years of age - literature shows that the microbiome is still developing in children under two years of age, and this development may skew results

Participants will be randomized by an independent statistician based on patient enrollment number (LCH 001- LCH 040). Randomization will be 1:1. The statistician will randomly assort LCH 001-020 into Inulin/Placebo and LCH 021- LCH 040 into Inulin/placebo. This will ensure that an equal number of enrolled participants will be in each arm at 20 patients, and then again at anticipated full 40 patients