Nutrition and Metabolism of Proteins 65than this intake to maintain an adequate protein
nutritional status.
It is worth emphasizing two points. First, the
current UN recommendations shown in Table 4.8
apply to healthy individuals of all ages. However, it is
highly likely that the needs of sick or less healthy
patients would differ from and usually exceed those
of healthy subjects. In this case, the values given in
this table can be regarded only as a basis from which
to begin an evaluation of how disease and stress,
including surgery, affect the needs for dietary protein.
Unfortunately, the quantitative needs for protein
(total nitrogen) in sick, hospitalized patients can be
only very crudely approximated at this time.
Second, the values shown in Table 4.8 apply to
high-quality food proteins, such as eggs, milk, meat,
and fi sh. The differing nutritional value of food pro-
teins will be considered below.
Defi nition and determination of
indispensable amino acid requirements
Defi nition
It is possible to modify slightly the earlier defi nition
for the requirements for protein (nitrogen) for a spe-
cifi c, indispensable amino acid, which can be stated,
therefore, as:
.... the lowest level of intake of an indispensable
amino acid that achieves nitrogen balance or
that balances the irreversible oxidative loss
of the amino acid, without requiring major
changes in normal protein turnover and where
there is energy balance with a modest level of
physical activity. For infants, children and preg-
nant and lactating women, the requirements for
the amino acid will include the additional
amount of the amino acid needed for net protein
deposition by the infant, child or fetus and con-
ceptus and for the synthesis and secretion of
milk proteins.
The foregoing is an operational defi nition of
requirement, as in the case of protein. Ideally, a func-
tional defi nition and determination of these require-
ments inherently would be preferable. However, the
choice and nature of the functional index or (indices)
(such as maximum resistance to disease or enhanced
physical performance) and its quantitative defi nition
remain a challenge for future nutrition and health-
related research.
Determination
In general, the approaches and methods that have
been most often used to determine specifi c indis-
pensable amino acid requirements are similar to
those used for estimation of total protein needs,
i.e., nitrogen excretion and balance and factorial
estimation. Thus, amino acid requirements have
been assessed by nitrogen balance in adults, and by
determining the amounts needed for normal growth
and nitrogen balance in infants, preschool children,
and school-aged children. For infants, they have also
been approached by assessment of the intakes pro-
vided by breast milk or those supplied from intakes
of good-quality proteins. In addition, factorial pre-
dictions of the amino acid requirements of infants
and adults have been made. One such factorial
approach for use in adults includes the following
assumptions.
● The total obligatory nitrogen losses (those losses
occurring after about 4–6 days of adjustment to a
protein-free diet) are taken to be approximately
54 mg/kg nitrogen per day in an adult, or equiva-
lent to 0.36 g protein/kg/day.
● The average amino acid composition of body pro-
teins can be used to estimate the contribution madeTable 4.8 The United Nations (1985 FAO/WHO/UNU) and Institute of
Medicine (2002/2005) recommendations for a safe practical protein
intake for selected age groups and physiological states. Reproduced
with permission from WHO
Group Age (years)
Safe protein
level (g/kg/day)
UNU IOMInfants 0.3–0.5 1.47 1.5
0.75–1.0 1.15 1.1
Children 3–4 1.09 0.95
9–10 0.99 0.95
Adolescent 13–14 (girls) 0.94 0.85
13–14 (boys) 0.97 0.85
Young adults 19 + 0.75 0.80
Elderly 0.75 0.80
Women: pregnant 2nd trimester +6 g daily ~1.1
3rd trimester +11 g daily ~1.1
lactating 0.6 months ~+16 g daily ~1.1
6–12 months 12 g daily ~1.1
Values are for proteins such as those of quality equal to a hen’s egg,
cow’s milk, meat, or fi sh.