required for nitrogen balance. Finally, inclusion
of the sedentary group in this study is note-
worthy because any methodological errors
would be similar across all three groups and
therefore the differences in protein intake neces-
sary to elicit nitrogen balance (0.73, 0.82 and
1.37 g · kg–1· day–1for sedentary, strength athlete
and endurance athlete groups, respectively)
should reflect true differences in the dietary
protein need of these groups.
Shortly thereafter, Meredith et al. (1989) used
both the traditional nitrogen status (balance)
technique and protein turnover measures (oral
doses of^15 N-glycine every 3 h for 60 h) to assess
dietary protein needs in young (26.8±1.2 years)
and middle-aged (52.0±1.9 years) endurance-
trained men (>11 years’ training). These nitrogen
status data indicate that protein needs were
elevated similarly in both age groups (by 37%)
relative to the data of a previously published
study on sedentary individuals from the same
laboratory. When these data were used to calcu-
late a recommended dietary allowance for
protein based on regression procedures (as
described above; except here, twice the sample
SD was added because the protein intakes used
were near the requirement, i.e. 0.61, 0.91 and
1.21 g protein · kg–1· day–1) the obtained value
was 1.26 g protein · kg–1· day–1 (157% of the
current RDA in the United States). In addition,
further support for the advantage of the higher
protein intake was found in the protein turnover
data which showed that the protein synthetic
rate was higher in both age groups when 1.21 vs.
0.61 g protein · kg–1· day–1was consumed.
The subsequent data of Phillips et al. (1993),
who found a negative nitrogen status (balance)
in endurance runners (>5 years’ training experi-
ence, 43–50 km · week–1, V.
o2max.=66–68 ml · kg–1
fat free mass · min–1), adapted to a protein intake
of 0.8–0.94 g · kg–1· day–1provide further support
that protein needs are elevated in trained
endurance athletes. In addition, a greater nega-
tive nitrogen status (balance) in the male vs. the
female subjects was noted in this study and this
apparent gender difference in protein use was
confirmed by greater leucine oxidation rates (Fig.
10.10) in the men both at rest and during exercise
(Phillipset al. 1993). Apparently, this gender dif-
ference is related to reduced glycogen and/or
enhanced fat use in women, perhaps as a result of
differing hormonal responses (Tarnopolsky et al.
1995). These observations, if confirmed with sub-
sequent work, provide another example where
data derived on male subjects may not be directly
applicable to women.
At least two groups (Lemon et al. 1992;
Tarnopolsky et al. 1992) have observed even
higher protein needs in strength athletes (Fig.
10.11) and based on nitrogen balance data haveeffects of exercise on protein metabolism 141
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0.0Protein intake (g.kg–1.day–1)Nitrogen balance (g.day–1)–2 0 2 4 6 8 10 12 14 16 18 20RunnersBody buildersSedentaryFig. 10.9Estimated dietary
requirements (protein intake
necessary to elicit nitrogen
balance) in endurance athletes
(), strength athletes () and
sedentary men (). Note that
both athlete groups have protein
requirements (yintercepts)
greater than those of their
sedentary counterparts. Adapted
from Tarnopolsky et al. (1988).