Nutrient metabolism by gut tissues.

The gut uses a disproportionate amount of energy (about 25% of total oxygen consumption) for the size of the tissue (about 6% of body weight). Therefore, knowledge of metabolites used in gut tissues is important to assess nutrient needs. The VFA produced intraruminally are used to varying degrees by rumen epithelial tissue. A review of net portal absorption data essentially confirms earlier studies that show acetate, propionate, and butyrate metabolism by stomach tissue to be 30, 50, and 90%, respectively, of ruminant production. Glucose also is used by rumen epithelial tissue; however, glutamine appears to be a poor substrate for energy production. Tissue of the small intestine uses glutamine, glucose, and ketones but does not use VFA very well for energy. Cecal and colonic tissues can use glucose and glutamine for energy, but butyrate is the preferred substrate in both tissues.