The starting points of metabolism are the substances absorbed after the digestion of food. For all practical purposes we may regard the end products of carbohydrate digestion in the simple stomached animal as glucose, together with very small amounts of galactose and fructose. These are absorbed into the portal blood and carried to the liver.

In ruminant animals, the major part of the carbohydrate is broken down in the rumen to acetic, propionic and butyric acids, together with small amounts of branched-chain and higher volatile acids. Digestion of proteins results in the production of amino acids and small peptides, which are absorbed via the intestinal villi into the portal blood and are carried to the liver, where they join the amino acid pool. They may then be used for protein synthesis in situ or may pass into the systemic blood, where they join the amino acids produced as a result of tissue catabolism in providing the raw material for the synthesis of proteins and other biologically important nitrogenous compounds.