In contrast, there is some evidence to indicate
that elevating pre-exercise muscle glycogen con-
tents extend endurance time in events lasting
longer than 90 min (Table 42.4). Evidence for the
important role of muscle glycogen in continuous
endurance exercise also comes from studies of
the effects of high-CHO diets on running times
to fatigue at 70–75% of V
.
o2max.. The largest
increases in running endurance were found in an
investigation by Galbo et al. (1967). In that study,
the subjects ingested extreme diets with either a
low (10%) or a high (77%) CHO content. Com-
pared with the low-CHO diet, the high-CHO
diet increased muscle glycogen content by about
150% and subsequently extended running times
to exhaustion by approximately 66%.
In addition to increasing running times to
fatigue, CHO loading may also improve running
performance during prolonged exercise in which
a set distance must be covered as quickly as pos-
sible (i.e. in a race situation). Karlsson and Saltin
(1971) reported that the consumption of a diet
high in CHO for several days before exercise
resulted in improvements of about 6% in race
times during a 30-km event. Interestingly, the
twofold higher starting muscle glycogen con-
tents did not increase the initial running speed
but, instead, allowed the athletes to maintain a
fast race pace for longer. Williams et al. (1992)
have also reported a similar finding. They
observed that a high-CHO diet before exercise
increased the speed over the last 5 km of a 30-km
treadmill running time-trial and improved
overall performance by approximately 2%.Fluid and energy replacement during
distance running
The pioneering studies showing that CHO
ingested during prolonged exercise could
enhance endurance performance were con-
ducted on runners competing in the 1924 and
1925 Boston Marathon. The results of these inves-
tigations clearly highlighted the importance of
CHO loading before and CHO ingestionduring
prolonged, steady-state running. Unfortunately
for the athletic community, these findings were
completely ignored. So too, it seems, were thedistance running 553
Table 42.3Effects of carbohydrate loading on moderate intensity running lasting 60–90 min. Adapted from
Hawleyet al.(1997b).
Muscle glycogen
(mmol◊kg-^1 wet weight)
Performance
Dietary treatment Pre-exercise Postexercise measure Results
A: 3 days LCHO (1.5 g◊kg-^1 BM, A: 208± 30 A: 102± 39 20.9-km run A: 83±15 min (n=6M)
CHO◊day-^1 , then 3 days
7.7 g◊kg-^1 BM CHO)
B: 3 days HCHO (5.0 g◊kg-^1 BM, B: 203± 28 B: 96± 17 B: 83±9 min
CHO◊day-^1 , then 3 days
7.7 g◊kg-^1 BM CHO)
C: 6 days NORM (5.0 g◊kg-^1 BM, C: 159± 13 C: 96± 28 C: 83±15 min
CHO◊day-^1 )
A: NORM A: 135± 28 A: 101± 32 Run to exhaustion A: 70±20 min (n=3 M, 3 F)
at 75–80%
V
.
o2max.
B: 3 days 50% CHO B: 168± 19 B: 129± 40 B: 77±30 min
3 days 70% HCHO
BM, body mass; F, female; HCHO, high carbohydrate intake; LCHO, low carbohydrate intake; M, male; NORM,
normal diet; 1 mmol◊kg-^1 wet weight=4.3 mmol◊kg-^1 dry weight.
All values are mean±SD.