Introduction to Human Nutrition

Nutrition and Metabolism of Proteins 63

major biochemical level the qualitative pattern of
the needs of individual amino acids to support
protein deposition would be expected to be generally
similar.
In humans, in contrast to rapidly growing mammals
such as the rat and pig, the obligatory amino acid
needs for the purposes of net protein deposition are
for most stages in life a relatively minor portion of
the total amino acid requirement. Hence, most of the
requirement for nitrogen and amino acids is associ-
ated with the maintenance of body protein stores (or
body nitrogen equilibrium). A major portion of the
maintenance nitrogen and amino acids needs is
directly associated with protein metabolism and
refl ects two related factors.

● Amino acids released from tissue protein degrada-
tion are not recycled with 100% effi ciency.
● Amino acid catabolism is a close function of the
free amino acid concentration in tissues, and so the
presence of the fi nite concentrations of free amino
acids necessary to promote protein synthesis inevi-
tably leads to some degree of amino acid catabolism
and irreversible loss.
The other metabolic component of the require-
ment for nitrogen and amino acids, as mentioned
above, is due to the turnover of functionally impor-
tant products of amino acid metabolism, which are
also necessary to maintain health. Although this may
not necessarily be a major quantitative component of

the daily requirement, it is qualitatively and function-

ally of considerable importance; health depends on

the maintenance of this component of the protein

need.

Finally, four physiological systems appear to be

critical for health: the intestine, to maintain absorp-

tive and protective function; the immune and repair

system and other aspects of defense; the skeletal

musculature system; and the central nervous system.

Within each system it is possible to identify critical

metabolic roles for some specifi c amino acids (Table

4.7). Also of note is that, with certain exceptions (the

involvement of phenylalanine and tryptophan in the

maintenance of the adrenergic and serotinergic neu-

rotransmitter systems, and methionine as a methyl

group donor for the synthesis of creatine, as well as

the branched-chain amino acids as nitrogen precur-

sors for cerebral glutamate synthesis), the necessary

precursors shown here are the dispensable and con-

ditionally indispensable amino acids.

4.6 Estimation of protein and amino

acid requirements

Having considered the biology of protein and protein

requirements, this section now considers how these

factors may be used to estimate the requirement for

protein and for amino acids. The fi rst section dis-

cusses nitrogen balance and the defi nition of protein

requirements, before discussing how these vary with

Table 4.7 The involvement of amino acids in physiological systems and metabolic function

System Function Product Precursor

Intestine Energy generation ATP Glu, Asp, Glutamine

Proliferation Nucleic acids Glutamine, Gly, Asp

Protection Glutathione Cys, Glu, Gly

Nitric oxide Arg

Mucins Thr, Cys, Ser, Pro

Skeletal muscle Energy generation Creatine Gly, Arg, Met

Peroxidative protection Taurine (?) Cys

Nervous system Transmitter synthesis Adrenergic Phe

Serotinergic Try

Glutaminergic Glu

Glycinergic Gly

Nitric oxide Arg

Peroxidative protection Taurine (?) Cys

Immune system Lymphocyte proliferation (?) Glutamine, Arg, Asp

Peroxidative protection Glutathione Cys, Glu, Gly

Cardiovascular Blood pressure regulation Nitric oxide Arg

Peroxidative protection (?) Red cell glutathione Cys, Glu, Gly