Quantifying the Digestibility of Dietary Protein

Abstract

The current recommendation, when calculating a protein digestibility-corrected amino acid score, is to determine the digestibility of a dietary protein across the entire digestive tract, using the rat as a model animal for humans. This fecal digestibility value is subsequently corrected for endogenous contributions of protein using a metabolic nitrogen value determined by feeding rats a protein-free diet. The limitations inherent with this method are well recognized, however, and determining the digestibility of a dietary protein to the end of the small intestine is the preferred alternative. Unlike the fecal digestibility assay, which has only one basic methodology, ileal digestibility values can be determined in a number of ways. We discuss the various methods available for determining ileal digestibility values and compare results obtained for dietary proteins using both fecal and ileal digestibility assays. The relative value of using individual amino acid digestibility values as opposed to nitrogen digestibility values is reviewed. In addition, we address issues surrounding measurement of endogenous nitrogen flows, and in particular, the relative merits of determining "true" versus "real" digestibility values.

Behaviour influences cholesterol plasma levels in a pig model

“…Pigs have been used extensively in human nutrition research because they are similar in several key areas (Cooper et al, 1997;Darragh and Hodgkinson, 2000). They are monogastric, meal-eating, omnivorous mammals.…”

Influence of dietary protein content and source on fecal quality, electrolyte concentrations, and osmolarity, and digestibility in dogs differing in body size1

“…Apparent digestibility is affected by protein flow into the colon, which can be of dietary and endogenous origins (digestive secretions and exfoliated cells). Given that endogenous sources account for 30 to 60% of total luminal protein in humans (Meunier et al, 1976), during reduced protein flow in the colon (which could be affected by both protein source and content), endogenous protein loss becomes proportionally greater (Darragh and Hodgkinson, 2000). It is also known that increasing dietary protein intake results in an increase in endogenous N loss.…”

Methods of Protein Digestive Stability Assay - State of the Art

“…The methods based on protein nitrogen utilization by an organism are subject of several great reviews, and we refer a curious reader to them for more details (Bender 1958;Darragh & Hodgkinson 2000;von der Decken 1983). Below, a short description of method families will be given.…”

“…Digestibility is defined as the difference between the amount of N ingested and excreted, expressed as a proportion of N ingested. Although accepted as the recommended procedure, the use of fecal digestibility coefficients to evaluate amino acid availability is thought to be inherently inaccurate due to the metabolism of both dietary and endogenous proteins by the hindgut microbial population (Darragh & Hodgkinson 2000;Kies 1981). …”

“…The problem is that they measure protein assimilation and do not provide any data on how exactly a particular protein was or was not digested and transported. This hindrance is circumvented in the second group of in vivo approaches, which include swallowed probe utilization, anastomosis and cannula incision (Darragh & Hodgkinson 2000;Faber et al 2010). …”