Ph D Thesis Amelie Deglaire - TEL

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Bos, 2000; Lacroix et al., 2006b; Bos et al., 2007; Humayun et al., 2007). Net
postprandial protein utilization has been proposed as a better indicator of protein quality.
This method has been fully developed and applied over recent years, as discussed below.
Recently, Humayun et al. (2007) proposed an approach based on the “indicator AA
oxidation” method as a potentially useful tool for the determination of the metabolic
availability of AA in human foodstuffs. However, this method needs substantial
refinement before being widely applied.
2.2. Net postprandial protein utilization
Net protein utilization can be directly determined using the classical N balance method;
however, its major limitation is in the estimation of the net protein retention as a daily
gain. Protein metabolism is subject to a diurnal cycle (fed and fasting states), resulting in
alternate periods of postprandial accretion and postabsorptive losses of body proteins
(Tomé & Bos, 2000). Acute N deposition during the postprandial phase is critical for the
deposition of dietary N in tissues (Millward & Pacy, 1995; Tomé & Bos, 2000). Dietary
N utilization directly for protein synthesis can be estimated when determined in the
postprandial period, and is assumed to be a good indicator of the dietary protein
nutritional value (Mariotti et al., 1999; Tomé & Bos, 2000). Net postprandial protein
utilization was thus proposed as a more sensitive approach for assessing dietary protein
quality (Gaudichon et al., 1999; Tomé & Bos, 2000; Reeds et al., 2000b). It can be
assessed by measuring total dietary N losses usually for 8 h following ingestion of a test
meal containing 15 N-labelled dietary protein (Tomé & Bos, 2000) and using the following
equation:
15 15
Net postprandial protein utilization = [ Nintake − ( Nileal + 15 15 15
Nbody urea + Nurine)]/ Nintake.
When combined with true ileal digestibility, the net postprandial protein utilization
allows for the measure of postprandial biological value, i.e. the proportion of absorbed N
effectively used in the anabolic pathway (Mariotti et al., 1999).
Unlike the net protein utilization index, which requires long-term studies due to the
adaptation period (Millward & Pacy, 1995), the net postprandial protein utilization can be
determined acutely because postabsorptive losses, depending on the previous level of
protein intake, are not taken into account. The net postprandial protein utilization has thus
been suggested as a better criterion for protein quality evaluation (Gaudichon et al., 1999;
Bos et al., 2002; Bos et al., 2005a). In particular, it penalizes protein sources deficient in