Application of linear regression to elucidate ileal inevitable flow and digestibility of amino acids and consequences for standardised digestibility system

Several techniques are available solely applicable to the study of inevitable endogenous flow or the digestibility of amino acids (AAs) at the terminal ileum. But the linear regression technique combines these tasks in one simple and flexible platform extendable to multifactorial evaluation of feedstuffs, for example under alternating dietary and physiological conditions. There is evidence that the amount and composition of inevitable endogenous flows are not constant. Also their estimates are dependent on assay protocols and other laboratory conditions. This variability is a drawback to methodological precision of the current conventional standardised digestibility (cSID) system, where inevitable endogenous flow is applied to correct apparent digestibility. Notably cSID may suffer systematic errors when used to assay low AA ingredients or those high in AAs and in anti-nutrients, leading to underestimation or overestimation of standardised digestibility, respectively. Thus a standardised system not encumbered by the need to determine inevitable flow should offer more latitude for precision. The latter approach is followed in linear regression. The linear regression approach is based on validated hypotheses both mathematical and physiological, which enable its ordinate intercept and slope respectively to be considered as inevitable endogenous flow and standardised digestibility. The literature shows that this method does not penalise ingredients low in AAs as its estimates are independent of dietary crude protein level; and it is efficient to determine standardised digestibility in feedstuffs which elevate the production of specific endogenous ileal AA flow. Nevertheless work remains to be done to standardise its protocols to improve comparability of data across investigations. It can be concluded that linear regression is a reliable path for routine determination of inevitable endogenous flow and standardised AA digestibility at the end of the ileum.