The purpose of this study is to determine if adding dietary fiber, such as inulin, to a diet that does not have enough fiber would raise the levels of potentially beneficial bacteria, such as Bifidobacterium, in the gut. There is evidence to suggest that these microbes can affect gut health and immune response, including to vaccines. The investigators will examine how inulin in the diet (compared to the maltodextrin control) (1) causes changes in the composition and function of the gut microbes, (2) reduces gut inflammation and gut leakiness caused by the vaccine, (3) increases immune response to vaccination, and (4) changes the expression of important adhesion molecules on the surface of white blood cells. Intestinal and whole-body responses will be measured in all participants.