NUTRITION IN SPORT

which all amino acids must pass) is indicated by
the size and central location of its sphere in Fig.
10.2. Physiologically, there are only three ways
amino acids can enter the free pool(s) (from
dietary protein during digestion, from tissue
protein breakdown, or as dispensable—that is,
non-essential—amino acids formed in the body
from NH 3 and a carbon source; numbers 1, 2 and
3, respectively, in Fig. 10.2). Of course, some con-
sumed amino acids are never absorbed (lost in
faeces) and a fourth method of input is possible,
at least in the laboratory (via intravenous infu-
sion of amino acids). When studying indispen-
sible (essential) amino acids, route 3 is
eliminated, as these amino acids cannot be
formed in the body. Once in the free pool(s), there
are also four ways amino acids can leave (secre-
tion into the gut, incorporation into tissue
protein, oxidation—amino acid nitrogen lost in
urine or sweat; carbon in breath—or incorpora-
tion into carbohydrate or fat for storage energy—
amino nitrogen lost in urine; letters a, b, c and d,
respectively, in Fig. 10.2). During exercise, routes
a (due to blood redistribution) and d (due to the
overall catabolic stimulus) are considered unim-
portant. Over time, following constant infusion
or repeated ingestion of a labelled representative
indicator amino acid (tracer), an isotopic equilib-
rium can be obtained, i.e. input into the free
pool(s) equals output, and movement of the
tracer amino acid through the system (turnover

or flux) can be measured. This requires only

minimal invasiveness because tissue values

(enrichment) can be estimated from blood (recip-

rocal pool model; Matthews et al. 1982; Horber

et al. 1989) or urine samples (assumption is that

the urinary enrichment is representative of the

end product of protein breakdown). By combin-

ing these data with dietary intake (and infusion

rate, if applicable), and/or measures of oxidation

(requires breath sampling), it is possible to

estimate whole-body protein degradation rates

(Picou & Taylor-Roberts 1969):

turnover (or flux) – intake+infusion

=degradation

or whole-body protein synthetic rates (i.e. non-

oxidative loss):

turnover – oxidation or urinary excretion

=synthesis

Traditionally, whole-body nitrogen status has

been evaluated by a technique known as nitro-

gen balance. This involves measuring duplicate

meals to those consumed by the experimental

subjects in order to accurately quantify nitrogen

intake (protein intake is estimated by assuming

that the average nitrogen content of food protein

is 16%, i.e. multiplying the nitrogen intake by

6.25), all routes of nitrogen excretion (typically

only urine and faeces are measured and miscella-

neous losses, including through the skin, are

effects of exercise on protein metabolism 135

Dietary

protein

(amino

acids)

Infusion

(amino acids)

CHO or fat C

+NH 3 C oxidizedto CO

2

Tissue

protein

C converted to

Urine (N) Sweat (N) CHO or fat

Faeces

(C and N)

Gut

1

(^43)
(^12)
a
c,d c
d
b
c
Synthesis
Degradation
Free amino
acid pool(s)
Fig. 10.2Simplified diagram of
protein metabolism. Amino acid
entry into the free pool is shown by
numbers and exit from the free
pool by letters. Nitrogen status
(balance) measures involve
quantifying the difference between
all nitrogen intake and excretion
while protein turnover measures
allow estimates of the component
process involved, i.e. whole-body
protein synthesis and degradation.
Adapted from Lemon (1996).