**1. Introduction**

There is increasing recognition of the beneficial role of prebiotics in modulating the microbiome of humans and animals, with the opportunity to beneficially impact health. While the initial focus has been the digestive tract, there has been increasing attention on modification of the microbiome on the skin, in the mouth, and in the urogenital tract. The efficacy of prebiotics is predicated upon selectively feeding beneficial microbes within the target region, with key metabolites, inflammatory and immune markers, etc. driving "distal" health benefits. Within this chapter, we focus on prebiotics that modulate the microbial community in the digestive tract, with a specific focus on novel prebiotics that are based upon 5-carbon sugars as well as 6-carbon sugars with less common beta bonds. When prebiotics are consumed, they are feeding a mixed community of microbes in a dynamic environment in the gastrointestinal tract, where carbohydrates (polymers, oligomers, dimers, monomers) of various types are broken down by digestive enzymes, absorbed into the bloodstream, and utilized by microbes via specific transport and enzyme systems. We elaborate on these concepts in sections 2 and 3, below.

We thus start by describing the environment within the digestive tract – digestive enzymes, systems for carbohydrate absorption, and the microbial communities therein. We then discuss various types of prebiotics, with a particular emphasis on differences in subunits and bond structure. These differences, coupled with differences in microbial enzymes and transport systems, contribute to differences in efficacy, selectivity, and dose between prebiotics. We then focus on prebiotics derived from xylan, arabinan, and mannan, differentiating them from "conventional" prebiotics that rely on subunits of common 6-carbon sugars. Finally, we discuss results from clinical and animal trials with these novel prebiotics, discussing impacts on human and animal health.
