Introduction to Human Nutrition

60 Introduction to Human Nutrition

organ protein maintenance since there is no net tissue
growth under most circumstances. Second, as will be
seen later, at all ages in healthy subjects the rates of
whole body protein synthesis and breakdown are
considerably greater than usual intakes (the latter are
about 1–1.5 g protein/kg per day in adults) or those
levels of dietary protein thought to be just necessary
to meet the body’s needs for nitrogen and amino
acids (about 0.8 g protein/kg per day). It follows,
therefore, that there is an extensive reutilization
within the body of the amino acids liberated during
the course of protein breakdown. If this were not the
case it might be predicted that we would be obligate
carnivores and this, undoubtedly, would have changed
the course of human evolution. Third, although not
evident from this discussion alone, there is a general
as well as functional relationship between the basal
energy metabolism or resting metabolic rate and the
rate of whole body protein turnover. Protein synthe-
sis and protein degradation are energy-requiring pro-
cesses, as will be described elsewhere in these volumes,
and from various studies, including interspecies com-
ponents, it can be estimated that about 15–20 kJ (4–
5 kcal) of basal energy expenditure is expended in
association with the formation of each gram of new
protein synthesis and turnover. In other words,
protein and amino acid metabolism may be respon-
sible for about 20% of total basal energy metabolism.
Because basal metabolic rate accounts for a signifi cant
proportion of total daily energy expenditure, it should
be clear from this discussion that there are signifi cant,
quantitative interrelationships between energy and
protein metabolism and their nutritional require-
ments. For these reasons it would not be diffi cult to
appreciate that both the level of dietary protein and
the level of dietary energy can infl uence the balance
between rates of protein synthesis and protein break-
down and so affect body nitrogen balance. Their
effects are interdependent and their interactions can
be complex. This can be illustrated by the changes in
body nitrogen balance that occur for different protein
and energy intakes (Figure 4.7); as seen here, the level
of energy intake, whether above or below require-
ments, determines the degree of change in the nitro-
gen balance that occurs in response to a change in
nitrogen intake. Conversely, the level of nitrogen
intake determines the quantitative effect of energy
intake on nitrogen balance. Therefore, optimum
body protein nutrition is achieved when protein and

energy intakes (from sources such as carbohydrates

and lipids) are suffi cient to meet or balance the needs

for amino acids, nitrogen, and the daily energy expen-

diture or, in the case of growth, the additional energy

deposited in new tissues.

Amino acids as precursors of

physiologically important

nitrogen compounds

As already pointed out, amino acids are also used for

the synthesis of important nitrogen-containing com-

pounds that, in turn, play critical roles in cell, organ,

and system function. In carrying out these particular

roles the amino acid-derived metabolites also turn

over and they need to be replaced ultimately by the

nitrogen and indispensable amino acids supplied by

protein intake. Estimates on the quantitative utiliza-

tion of these precursor and nonproteinogenic roles of

amino acids in human subjects are limited but it is

possible to give some examples.

● Arginine is the precursor of nitric oxide (NO); the

total amount of NO synthesized (and degraded)

per day represents less than 1% of whole body

arginine fl ux and less than 1% of the daily arginine

intake.

Nitrogen balance

Energy intake

Diet adequate

in protein

Diet low

in protein

AB

Figure 4.7 Relationship between nitrogen balance and energy intake

with diets of different protein levels. Between energy intake A (low)

and B (higher) the two lines are parallel. (Reproduced from Munro HN,

Allison JB, eds. Mammalian Protein Metabolism, vol. I. New York:

Academic Press, 1964: 381 with permission.)