estimated), and then calculating the difference
between the two. Estimating the miscellaneous
nitrogen losses is usually appropriate because in
sedentary individuals they are small, quite con-
sistent and extremely difficult to measure com-
pletely. However, with exercise, dermal nitrogen
loss via sweating should be quantified, as it can
be substantial (Consolazio et al. 1963; Lemon &
Mullin 1980). When intake of nitrogen exceeds
the total excreted, one is in positive nitrogen
balance (negative nitrogen balance if excretion
exceeds intake). This latter situation cannot con-
tinue for very long without losses of essential
body components because, unlike carbohydrate
and fat, the body does not contain an energy
reserve as protein (all body protein has a struc-
tural or functional role). Although ‘negative’ and
‘positive’ as descriptors of balance are common-
place in the literature, it is recommended that
‘status’ be used instead of ‘balance’, to avoid the
terms ‘positive balance’ or ‘negative balance’,
which seem nonsensical.
Nitrogen balance (status) is a classic technique
which has been used in the vast majority of
studies considered by the expert committees in
many countries when determining the recom-
mended dietary allowance for protein (US Food
and Nutrition Board 1989). However, it should
be understood that this method has a number of
limitations (inconvenient for the subjects, labour
intensive for the investigators, tends to over-
estimate the nitrogen that is actually retained,
i.e. generally overestimates intake and under-
estimates excretion), and due to its ‘black box’
nature cannot provide specific information about
the various component parts of protein metabo-
lism (Lemon et al. 1992; Fuller & Garlick 1994).
Also, nitrogen status (balance) is affected by
energy balance (Munro 1951), which can con-
found the data, especially in exercise studies
where this is not always tightly controlled.
Further, a number of potential confounders fre-
quently exist, including: inadequate adaptation
time to changing experimental diets (Scrimshaw
et al. 1972), exercise-induced changes in the time
course and/or relative importance of the various
136 nutrition and exercise
routes of nitrogen excretion (Austin et al. 1921;
Lemon & Mullin 1980; Dolan et al. 1987), techni-
cal problems making complete collections of
nitrogen excretion difficult (Lutwak & Burton
1964; Bingham & Cummings 1983; Lemon et al.
1986; Dolny & Lemon 1988), and the inappropri-
ate use of linear regression to estimate protein
need with either very high or very low protein
diets, i.e. when the response is curvilinear
(Rennieet al. 1994). As a result, the literature
must be examined very critically.
More recently, investigators have utilized the
metabolic tracer technique, where the compo-
nent parts of the protein metabolism ‘black box’
can be investigated (Waterlow 1995). As alluded
to above, this means one can estimate whole-
body protein synthetic rates, if oxidation rates or
urinary excretion are measured, and whole-body
protein degradation rates, if dietary/infusion
rates are measured. Although this technique has
great promise to help elucidate how exercise af-
fects protein metabolism, it too has several limi-
tations, including expense, invasiveness and the
validity of its various assumptions (Young et al.
1989; Wolfe 1992; Garlick et al. 1994; Rennie et al.
1994; Tessari et al. 1996). Although technically
more difficult, muscle protein synthesis, which
represents about 25–30% of whole-body protein
synthesis, can also be measured by quantifying
isotope enrichment in muscle samples obtained
via the needle biopsy technique (Nair et al. 1988;
Chesleyet al. 1992; Biolo et al. 1995; MacDougall
et al. 1995).Evidence that protein needs are
increased with physical exercise
In recent years, a variety of experimental data
which suggest that exercise has dramatic effects
on protein metabolism have begun to accu-
mulate. For example, several investigators have
measured losses in rodent muscle (Varrik et al.
1992) and/or liver protein (Dohm et al. 1978;
Kasperek et al. 1980) following exercise, espe-
cially with prolonged endurance exercise (Fig.
10.3). Consistent with these observations, we