Higher gut microbiome diversity has been associated with improved survival following autologous stem cell transplantation in multiple myeloma and lymphoma. This study hypothesises that prebiotic supplementation with resistant starch (RS) will improve gut microbiome diversity at time of stem cell engraftment. To test this, participants will either have RS or a placebo (maltodextrin) mixed into a food item of their choice for approximately 10 days prior to stem cell infusion and continue to the first day of neutrophil engraftment. The study will look at the difference in gut microbiome diversity between the RS and placebo arm collected at the engraftment timepoint, dietary evaluation to assess the impact of subject diet on microbiome response to intervention and serum sample collection to assess differences to gut permeability during transplant.