In this reviewarticle we present an overall summary of the role that high-grain/low foragediets have on rumen composition of microbiota and how changes in the diet affectthe release of bacterial cell wall components that are toxic to the host. Oneof these toxic compounds is lipopolysaccharide or endotoxin, a component ofthe outer membrane of all Gram-negative bacteria. Moreover, data are providedthat support the concept that endotoxin translocates into the blood circulationand show that rumen endotoxin is associated with multiple perturbations of bloodvariables related to carbohydrate, lipid, and mineral metabolism. In addition,endotoxin induces a general, nonspecific immune response known as acute phaseresponse. We also pinpoint the fact that high-grain diets are associated withdistinct clusters of plasma metabolites and immune variables suggesting thatchanging cereal grain to forage ratio in the diet is very important for thehealth of dairy cattle. Furthermore, we provide information that support theconcept that endotoxin is involved in multiple metabolic diseases such as fattyliver, milk fever, laminitis, retained placenta, displaced abomasum, and downercow syndrome. More research is warranted to clarify the mechanisms by whichnutrition, microbiota, and endotoxin contribute to development of metabolicdiseases in dairy cattle. It is concluded that besides the aforementioned causalagents other compounds generated in the gastrointestinal tract such as lipoteichoicacid or methylated amines might be involved in the etiology of several metabolicdiseases.
