64 Introduction to Human Nutrition
age and for various physiological groups. Subsequent
sections cover the estimation of the requirements for
the indispensable amino acids.
Nitrogen balance and defi nition
of requirement
The starting point for estimating total protein needs
has been, in most studies, the measurement of the
amount of dietary nitrogen needed for zero nitrogen
balance, or equilibrium, in adults. In the growing
infant and child and in women during pregnancy and
lactation, or when repletion is necessary following
trauma and infection, for example, there will be an
additional requirement associated with the net depo-
sition of protein in new tissue and that due to secre-
tion of milk. Thus, a United Nations (UN) Expert
Consultation in 1985 defi ned the dietary need for
protein as follows.
The protein requirement of an individual
is defi ned as the lowest level of dietary
protein intake that will balance the losses
from the body in persons maintaining energy
balance at modest levels of physical activity. In
children and pregnant or lactating women, the
protein requirement is taken to also include the
needs associated with the deposition of tissues
or the secretion of milk at rates consistent with
good health.
Most estimates of human protein requirements
have been obtained directly, or indirectly, from
measurements of nitrogen excretion and balance
(Nitrogen balance = Nitrogen intake – Nitrogen
excretion via urine, feces, skin, and other minor routes
of nitrogen loss). It must be recognized that the nitro-
gen balance technique has serious technical and inter-
pretative limitations and so it cannot serve as an
entirely secure or suffi cient basis for establishing the
protein and amino acid needs for human subjects.
Thus, there are:
● a number of inherent sources of error in nitrogen
balance measurements that should be considered
● a number of experimental requirements that must
be met if reliable nitrogen balance data are to be
obtained.
These include
● the need to match closely energy intake with energy
need, for the various reasons discussed earlier
● an appropriate stabilization period to the experi-
mental diet and periods long enough to establish
reliably the full response to a dietary change
● timing and completeness of urine collections
● absence of mild infections and of other sources of
stress.
Reference to detailed reviews of the concepts behind
and techniques involved in the nitrogen balance
approach is given in the reading list at the end of this
chapter.
When direct nitrogen balance determinations of
the protein requirement data are lacking, as is the
case for a number of age groups, an interpolation of
requirements between two age groups is usually made
simply on the basis of body weight considerations. A
factorial approach may also be applied; here, the so-
called obligatory urine and fecal nitrogen losses are
determined (after about 4–6 days of adaptation to
a protein-free diet in adults), summated together
with other obligatory losses, including those via
sweat and the integument. For children, estimates of
nitrogen deposition or retention are also included. In
the case of very young infants the recommendations
for meeting protein requirements are usually based
on estimated protein intakes by fully breast-fed
infants.Protein requirements for various age and
physiological groups
The protein requirements for young adult men and
women have been based on both short- and long-
term nitrogen balance studies. This also applies to
healthy elderly people, whose protein requirements
have been judged not to be different from those of
younger adults. In order to make practical recom-
mendations to cover the requirements for most indi-
viduals, it is necessary to adjust the average or mean
requirement for a group by a factor that accounts for
the variation in protein requirements among appar-
ently similar individuals in that group. This factor is
usually taken to be the coeffi cient of variation (CV)
around the mean requirement and traditionally a
value of 2 × CV (SD/mean) is added to the mean
physiological requirement, so that the needs of all but
2.5% of individuals within the population would be
covered. This adjusted requirement value is taken to
be the safe practical protein intake for the healthy
adult (Table 4.8). Most individuals would require less