Nitrogen balance data measured after adaptation to different protein levels over periods of several days is the usual approach to measure nitrogen retention [2, 44]. Diets containing poor quality proteins are associated with an increase in nitrogen losses, due to the inefficient utilisation of indispensable amino acids in turn linked to unbalanced amino acid composition. The (relative) lack of essential amino acids generates the ineffective utilisation of dietary nitrogen. Furthermore, besides such an insufficient utilisation, it is important to assess the amount of dietary and intestinal nitrogen that is absorbed as free amino acids or dipeptides, or excreted in the faeces, urine or other routes. Finally, the assessment of the anabolic utilisation for protein synthesis is a key step to measure amino acid retention in the body.
As stated above, classic nitrogen balance studies reflect the integrated net result of the diurnal cycling between the fasted and fed states (i.e. phases of nitrogen accretion postprandially and of nitrogen losses postabsorptively).
Other factors may affect nitrogen retention. Differences in the gastric emptying rate of dietary proteins may result in highly variable rates of amino acid absorption in the small intestine . Also, differences in the rate of protein digestion and/or absorption result in relevant differences in amino acid oxidation and postprandial nitrogen accretion . In this regard, the concept of net postprandial protein utilisation (NPPU) has been proposed, which is calculated using true ileal digestibility and true 15N-labelled protein deami-nation parameters, adding the dietary nitrogen collected in the urine [22, 47] and that retained in the body in the form of urea.
Using this approach, the NPPU values for milk protein and soy protein, measured over 8 h after the ingestion of a standard meal by healthy human subjects, were reported between 80 and 72%, respectively . These data strongly suggest the existence of differences between the nutritional value of proteins and their utilisation for anabolic purposes. These differences are valuable and should be taken into account when calculating amino acid scores. Finally, differences in interorgan amino acid metabolism may be due to the protein source-dependent difference, as shown in pigs after the administration of either soy or casein .
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