Ph D Thesis Amelie Deglaire - TEL

Introduction
Nutrition research has focused on the capacity of food to meet the metabolic demand for
nitrogen (N) and indispensable amino acids (IAA) (FAO/WHO/UNU, 2007). The
evaluation of protein quality aims to determine how well dietary protein is digested and
absorbed and how well absorbed amino acids (AA) are utilized by the body. The protein
digestibility-corrected AA score (PDCAAS) is the current recommended protein quality
index (FAO/WHO/UNU, 2007) despite some important limitations. This index takes into
account the digestibility of dietary protein and its AA content relative to the requirement
pattern (FAO/WHO/UNU, 2007). To date, a level of consensus within the scientific
community has been reached for the protein and IAA requirements of man
(FAO/WHO/UNU, 2007), although further re-evaluation can still be expected. In
addition, whereas the measurement of AA content in dietary protein has significantly
improved over the last decades, the accurate determination of digestibility is still critical.
Protein digestibility is a determining factor for protein bioavailability (Fuller & Tomé,
2005; FAO/WHO/UNU, 2007), as it indicates the extent of intestinal digestion and
absorption of food protein as AA that can subsequently be utilized by the body. Protein
digestibility has been reported to differ substantially among diets, in particular between
those from developing and developed countries (Gilani et al., 2005). Protein digestibility
is indirectly determined from the amount of N flowing at the end of the digestive tract.
Gut N flows arise both from undigested dietary N and from non-dietary (endogenous)
nitrogenous compounds that have escaped digestion and/or (re)absorption, and are
referred to as N losses when determined at the end of the digestive tract. Endogenous N
losses account for a substantial proportion of total N losses and thus need to be
distinguished from undigested dietary N to correct apparent digestibility coefficients
(derived from total N losses) to true digestibility coefficients. True digestibility represents
the specific fate of dietary N in the digestive tract (Fuller & Tomé, 2005). Measurement
of digestibility at the ileal level is now recognized as being more accurate than
measurement of digestibility at the faecal level (Darragh & Hodgkinson, 2000; Moughan,
2003; Fuller & Tomé, 2005), especially for individual AA for which the profile can be
modified by the prolific microbial metabolism in the hindgut. In addition, most dietary
AA are absorbed in the small intestine (Krawielitzki et al., 1990; Fuller & Reeds, 1998).
It remains unclear if the colon can absorb AA to a nutritionally significant extent
(Blachier et al., 2007).
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