Abstract
The progress in our understanding of the endogenous protein concept over the past century is reviewed. Non-dietary proteins found in the digesta at the terminal ileum of poultry, known as endogenous protein loss, are comprised of digestive secretions, mucus and sloughed gut epithelial cells. The measurement of this loss is of fundamental importance because it is an indicator of gut metabolism and is essential to adjust apparent estimates of ileal amino acid digestibility. The ileal endogenous amino acid losses comprise of two components, namely basal and specific losses. The basal losses are fixed and associated with feed dry matter intake, whereas the specific losses are variable and induced by the presence of dietary components such as fibre and anti-nutrients. Currently there is no methodology available to directly measure the specific endogenous losses and these losses are calculated by determining the basal and total (basal plus specific) losses and, then subtracting the basal losses from total losses. The seminal features, specific applications and shortcomings of available methodologies are briefly outlined as well as the practical challenges faced in using the published endogenous amino acid loss values for true digestibility corrections. The relevance of taurine as a component of endogenous protein flow in poultry is identified for the first time.
Highlights
Endogenous protein loss, which links the gut physiology and protein nutrition, has proved to be an alluring subject of research for many years
Bragg et al [5] are credited with the first report of the quantification of endogenous amino acid losses in poultry and using them as correction factors in digestibility calculations
The use of ileal digestibility values in feed formulation is the norm in the poultry industry and the current review will focus on ileal endogenous losses instead of early research on metabolic and endogenous flow measurements at the excreta level
Summary
Endogenous protein loss, which links the gut physiology and protein nutrition, has proved to be an alluring subject of research for many years. The data from isotope dilution demonstrated that the recovery of endogenous proteins in the ileal digesta was higher than those determined by feeding a protein-free diet Most of these isotopic studies have been conducted with pigs and only two with poultry [36, 37]. Homoarginine technique An innovative approach, using homoarginine (HA) as a marker, to determine EAA losses was proposed by Hagemeister and Erbersdobler [44] In this method, ε-amino group of lysine in dietary proteins are converted into HA by the guanidination reaction, involving treatment with O-methylisourea under alkaline conditions. Homoarginine is not found in normal feedstuffs It is digested and absorbed in a manner like other amino acids [31], but does not reappear in endogenous secretions into the gut and this unique feature enables the calculation of total EAA losses. The guanidinated protein is homogeneously and randomly labelled with HA, a premise proven to be true for purified proteins [31]
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