58 Introduction to Human Nutrition
gests that, even in the presence of abundant quantities
of the appropriate precursors, the quantities of condi-
tionally essential amino acids that can be synthesized
may be quite limited. Thus, there are circumstances,
for example in immaturity and during stress, under
which the metabolic demands for the amino acids rise
to values that are beyond the biosynthetic capacity of
the organism. This appears to be the case with regard
to the proline and arginine nutrition of severely
burned individuals, and cysteine and perhaps glycine
in the nutrition of prematurely delivered infants.
4.5 Biology of protein and amino
acid requirements
Body protein mass
A major and fundamental quantitative function of
the dietary α-amino acid nitrogen and of the indis-
pensable amino acids is to furnish substrate required
for the support of organ protein synthesis and the
maintenance of cell and organ protein content.
Therefore, in the fi rst instance the body protein mass
is a factor that will infl uence the total daily require-
ment for protein. Adult individuals of differing size
but who are otherwise similar in age, body composi-
tion, gender, and physiological state would be
expected to require proportionately differing amounts
of nitrogen and indispensable amino acids. Changes
in the distribution and amount of body protein that
occur during growth and development and later on
during aging may be considered, therefore, as an
initial approach for understanding the metabolic
basis of the dietary protein and amino acid needs.
(For more detailed considerations of body composi-
tion please refer to Chapter 2.)
Direct measures of total body protein cannot yet
be made in living subjects, although there are various
indirect measures from which it is possible to obtain
a picture of the body nitrogen (protein) content at
various stages of life. From these approaches it is clear
that body nitrogen increases rapidly from birth during
childhood and early maturity, reaching a maximum
by about the third decade. Thereafter, body nitrogen
decreases gradually during the later years, with the
decline occurring more rapidly in men than in
women. A major contributor to this age-related
erosion of body nitrogen is the skeletal musculature.
Strength training during later life can attenuate or
partially reverse this decline in the amount of protein
in skeletal muscles and improve overall function.
The protein requirement of adults is usually con-
sidered to be the continuing dietary intake that is just
suffi cient to achieve a “maintenance” of body nitro-
gen, often measured only over relatively short experi-
mental periods. For infants and growing children
and pregnant women an additional requirement is
needed for protein deposition in tissues. However,
this concept is oversimplifi ed since the chemical com-
position of the body is in a dynamic state and changes
occur in the nitrogen content of individual tissues
and organs in response to factors such as diet, hor-
monal balance, activity patterns, and disease. Thus,
proteins are being continually synthesized and
degraded in an overall process referred to as turnover.
The rate of turnover and the balance of synthesis and
degradation of proteins, in addition to the mass of
protein, are also important determinants of the
requirements for nitrogen and amino acids, and these
aspects will be discussed in the following section.Turnover of proteins and amino
acid metabolism
Protein synthesis, degradation, and turnover
The principal metabolic systems responsible for the
maintenance of body protein and amino acid homeo-
stasis are shown in Figure 4.5. They are:
● protein synthesis
● protein breakdown or degradation
● amino acid interconversions, transformation, and
eventually oxidation, with elimination of carbon
dioxide and urea production
● amino acid synthesis, in the case of the nutritionally
dispensable or conditionally indispensable amino
acids.
Dietary and nutritional factors determine, in part,
the dynamic status of these systems; such factors
include the dietary intake levels relative to the host’s
protein and amino acid requirements, the form and
route of delivery of nutrients, i.e., parenteral (venous)
and enteral (oral) nutritional support, and timing of
intake during the day, especially in relation to the
intake of the major energy-yielding substrates, which
are the carbohydrates and fats in foods. Other factors,
including hormones and immune system products,
also regulate these systems. This will be a topic for
discussion in the following volume. Changes in the