Introduction to Human Nutrition

66 Introduction to Human Nutrition

by each amino acid to this obligatory nitrogen
output (equivalent, therefore, to the obligatory
amino acid losses).
● At requirement intake levels, an absorbed amino
acid is used to balance its obligatory oxidative loss
with an assumed effi ciency of about 70%.
This predictive or factorial approach is analogous
to the factorial method for estimating the total nitro-
gen (protein) requirement of individuals at various
ages (where various routes of nitrogen excretion and
nitrogen gains are summated and an effi ciency factor
is used to estimate the intake needed to balance this
summation).

Tracer techniques

With advances in the routine measurement of stable
isotope enrichment in biological matrices and the
expanded use of tracers enriched with these isotopes
in human metabolic research, a series of tracer studies
was begun at the Massachusetts Institute of Tech-
nology, USA, in the early 1980s to determine amino
acid requirements in adults. Since that time several
research groups have used different paradigms in
tracer-based studies of human amino acid require-
ments. These can be distinguished according to the
choice of tracer and protocol design applied:

● studies involving the use of a labeled tracer of the
dietary amino acid being tested and with its rate
of oxidation (O) at various test intake levels [the
direct amino acid oxidation (DAAO) technique,
e.g., [^13 C]-lysine as a tracer to determine the lysine
requirement]; this technique has been used to assess
the requirements in adults for leucine, valine, lysine,
threonine, and phenylalanine
● studies involving use of an “indicator” tracer to
assess the status of indicator amino acid oxidation
(IAAO) or indicator amino acid balance (IAAB)
with varying levels of a test amino acid; examples
of the IAAO and IAAB approaches are where the
rate of [^13 C]-phenylalanine oxidation (Figure 4.9)
is measured or a [^13 C]-leucine balance determined
at varying levels of lysine intake to estimate the
lysine requirement
● kinetic studies designed to assess the retention of
protein during the postprandial phase of amino
acid metabolism, using [^13 C]-leucine as a tracer: the
postprandial protein utilization (PPU) approach;
this last and promising approach has not yet found

an extensive use in studies of human amino acid

requirements.

None of these methods is without its limitations, but

at present the IAAO and IAAB approaches, involving

tracer studies lasting for a continuous 24 hour day,

would appear to be the “reference method” for esti-

mating amino acid requirements in adults.

Indispensable amino acid

requirement values

There is still debate and uncertainty about the precise

requirements for amino acids in humans of all ages.

Three major sets of proposed amino acid require-

ment values for healthy subjects should be noted in

this text. First, there are the requirements proposed

by the UN in 1985 for the various age groups,

which are presented in Table 4.9. Second, another

expert group in 1994 (International Dietary Energy

Consultancy Group; IDECG) also assessed the amino

acid needs of infants by using a factorial method and

these turned out to be much lower than those shown

in Table 4.9 for infants. It should be noted, however,

that the 1994 IDECG values approximate the aver-

age requirements, whereas the requirement intakes

derived from estimates of breast milk intake (shown

in Table 4.9) would be expected to be well above the

requirement for virtually all infants and certainly well

Test amino acid intake

(leucine or lysine)

Requirement intake

[^13 C]-Phenylalanine (indicator) oxidation

Indicator amino acid oxidation

Figure 4.9 Outline of the concept of the indicator amino acid oxida-

tion technique for estimation of indispensable amino acid require-

ments. Here the indicator is [^13 C]-phenylalanine and the dietary

requirement is being estimated for either leucine or lysine.