Introduction to Human Nutrition

Nutrition and Metabolism of Proteins 59

rates and effi ciencies of one or more of these systems
lead to an adjustment in whole body nitrogen
(protein) balance and retention, with the net direc-
tion and the extent of the balance depending upon
the sum of the interactions occurring among the pre-
vailing factor(s).
In effect, there are two endogenous nitrogen cycles
that determine the status of balance in body
protein:

● the balance between intake and excretion
● the balance between protein synthesis and break-
down (Figure 4.6).

In the adult these two cycles operate so that they are
effectively in balance (nitrogen intake = nitrogen
excretion and protein synthesis = protein break-
down), but the intensity of the two cycles differs, the
fl ow of nitrogen (and amino acids) being about three
times greater for the protein synthesis/breakdown
component than for nitrogen intake/excretion cycle.
Protein synthesis rates are high in the premature
newborn, possibly about 11–14 g protein synthesized
per kilogram of body weight per day, and these rates
decline with growth and development so that in term
babies and young adults these rates are about 7 g and

4–5 g protein/kg per day, respectively. Three points

relevant to nutritional requirements may be drawn

from these data. First, the higher rate of protein syn-

thesis in the very young, compared with that in the

adult, is related not only to the fact that a net deposi-

tion of protein occurs during growth, which may

account for about 30% of the total amount of protein

synthesized in the 6 month old infant, but also to a

high rate of protein turnover (synthesis and break-

down) associated with tissue remodeling and repair,

as well as to removal of abnormal proteins. In the

adult the protein turnover is associated with cell and

f

b

A

Figure 4.5 The major systems in

amino acid uptake, utilization, and

catabolism, with an indication of the

processes involved and some factors

that can affect them. TNF, tumor necro-

sis factor, IL, interleukin.

Figure 4.6 The two endogenous nitrogen cycles that determine the

status of body protein (nitrogen) balance. (Adapted from Waterlow JC.

The mysteries of nitrogen balance. Nutr Res Rev 1999; 12 : 25–54,

with permission of Cambridge University Press.)