Amino acid metabolism by the splanchnic tissues, which include the liver and
the portal-drained viscera (PDV), determines amino acid availability for protein
deposition in peripheral tissues and thus whole-body growth. Threonine, lysine
and methionine are the first limiting amino acids in milk-based and cereal diets
of growing pigs. Studies have shown that both dietary essential and nonessential
amino acids are extensively utilized by intestinal tissues for the
generation of energy, protein synthesis, and other biosynthetic purposes.
Threonine, with 60-80% extraction in first-pass of the dietary intake, is the
single most utilized essential amino acid by the PDV. This can most likely be
attributed to the incorporation of threonine into membrane-associated and
secretory mucosal glycoproteins, the so-called mucins, which are an important
component of the protective mucus covering the gut epithelium. In pigs, mucin
represents 11% of the basal endogenous ileal losses of protein with threonine
contributing approximately 30% to the total amino acid content. Thus, the
secretion, recycling, and loss of intestinal mucins have a substantial impact on
the maintenance requirement for threonine. Factors that increase the
production of mucin will increase threonine requirements and consequently,
decrease the availability of threonine and energy for growth and production.
Furthermore, the availability of threonine may limit intestinal mucin synthesis
and therefore reduce gut barrier function. This highlights the importance of
threonine for maintaining intestinal integrity, which is essential for regulating
dietary amino acid supply for the whole body