A certain amount of protein is always lost during the digestion process: this is called endogenous protein. Endogenous protein comes from the animal and consists of digestive enzymes, bile, intestinal mucus and dead intestinal epithelial cells. It is excreted in manure.
Although the endogenous protein originates from the animal, loss of endogenous protein is mainly determined by the ration. The lower the digestibility, the more endogenous protein will be lost. In the current intestinal digestible protein system (2007), this is taken into account via the fraction of intestinal digestible metabolic fecal protein (MFP).