Carbohydrate digestion, metabolism, and their role in the gut microbiota modulation are the focus of multiple studies. The objective of this weight of evidence systematic review is to investigate the potential relationship between ingested carbohydrates and the gut microbiota composition at different taxonomic levels. Microbial fermentation of complex non-digestible dietary carbohydrates and host–derived glycans in the intestine has important consequences for health ^[1]. Certain dominant species, notably among the Bacteroidetes, are known to possess very large numbers of genes that encode carbohydrate active enzymes and can switch readily between different energy sources in the gut depending on availability. The impact of dietary carbohydrates, including prebiotics, on human health requires understanding of the complex relationship between diet composition, the gut microbiota and metabolic outputs ^[1]. Hay and straw comprise the main course fodder in the ration of ruminants as they are rich in cellulose and contain a relatively small amount of proteins and soluble carbohydrates.