The diurnal cycle of feeding and fasting is accompanied by concurrent changes in protein turnover. Protein feeding is necessary to replenish the body protein stores that would be wasted during fasting [21-24]. Because of this, nitrogen retention calculated on a daily basis is lower than that derived just from the postprandial phase , and, conversely, dietary protein utilisation calculated as the daily gain is lower than the postprandial gain.
Dietary proteins, once ingested, are digested in the gut and thereafter absorbed as either free amino acids or dipeptides . The absorbed amino acids are subjected to a variable first-pass extraction by splanchnic organs (mainly the liver) [26-28] and then they travel as such through the extracellular spaces before being used by the cells, either for catabolism or for protein synthesis. A minor fraction of amino acids are excreted unmodified into the urine .
The acute nitrogen deposition during the postprandial phase is likely to be the most critical in terms of the net deposition of proteins in the tissues, more than the rate of protein synthesis occurring in the postabsorptive periods. Therefore, the assessment of the postprandial utilisation of dietary proteins is a key step to understand net body protein deposition. It also represents an important conditioning factor of the rate of whole-body protein turnover .
The key steps of the fate of dietary nitrogen are: (1) the amount of nitrogen that is actually absorbed; (2) the amount that is deaminated and then recovered mainly in the form of urea; and (3) the amount that is retained in the body.
As regards point (1), nitrogen digestibility within the ileum and the short-term retention of dietary protein nitrogen can be measured by the use of 15N-labelled proteins. By this technique, therefore, it is possible to assess the metabolic utilisation of dietary nitrogen in humans, i.e. the amount that is effectively absorbed [31-35].
As concerns point (2), assuming that whole-body protein turnover is »300 g, and that daily protein intake is »100-110 g/day, it has been calculated that »80 g of the total proteins turned over
(i.e. »27% of total) are lost through the oxida-tive/urea-producing pathways, and »14 g within the ileum [21, 22]. The amounts of dietary nitrogen entering the anabolic (i.e. protein synthesis) and oxidative pathways are 70-80 and 13-20 g/day, respectively, i.e. contributing by 30-40% to total anabolism and by 15-25% to total oxidation (Fig. 1).
This indicates that dietary nitrogen (and proteins) is preferentially directed toward anabolic pathways. Such a preferential orientation of dietary nitrogen toward body protein synthesis is strictly linked to the adequacy (i.e. quality) of the dietary protein amino acid composition with respect to that of body protein.
The maintenance of nitrogen homeostasis involves a complex series of changes in whole-body protein turnover, amino acid oxidation, urea production and nitrogen excretion, during the fasting, fed, postprandial and postabsorptive periods of the day. Whole-body processes also represent the additive result of the metabolism of individual organs and tissues, which may be differently affected during physiological and pathological conditions. Therefore, whole-body measurements are crude, although comprehensive, estimates of body protein metabolism, but rarely can they provide information on regional protein turnover.
The usual daily protein consumption is normally greater than the theoretical requirement based on nitrogen balance estimates . Since body proteins cannot be stored in the body, mechanisms exist to dispose of the protein ingested in excess. Thus, the effects of increased protein loads on whole-body nitrogen balance and protein
turnover must be determined. These investigations should involve the study of nitrogen pools likely to be modified by the level of nitrogen intake, the effects linked to the type of protein ingested, as well as the effects of the nitrogen loads on the different nitrogen pathways .
An increase in protein intake is followed by adaptive processes: (1) an increase in amino acid oxidation and in the associated nitrogen excretion, mainly as urea, which is especially pronounced in the fed state; (2) a trend toward a disproportionate increase in nitrogen balance when nitrogen intake is increased , possibly linked to an enhanced inhibition of protein breakdown by feeding and to an increase in protein synthesis . This likely occurs because whole-body as well as tissue protein synthesis are sensitive to amino acid availability, whereas degradation may be sensitive to an interactive effect by both the amino acid level and insulin . Thus, high protein intakes are associated with a continuous, positive N balance approaching 1-3 g N/day [38, 39, 41, 42]. However, it is not clear whether this apparent retention is a real one or linked to intrinsic errors in calculating N balance.
Interestingly, the amplitude of diurnal body protein cycling increases with an increase in dietary protein intake, with no clear change in the mean daily protein turnover rate .
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