see Hawley et al. 1997b). Further, when an athlete
requires rapid recovery from training, a CHO
intake of 8–10 g · kg–1 body mass · day–1 is
recommended.
Muscle glycogen stores and the effect
of CHO loading on metabolism
The glycogen content of skeletal muscle of
untrained individuals consuming a mixed diet is
around 80 mmol · kg–1wet weight muscle. For
individuals involved in regular endurance
training and consuming a similar diet, muscle
glycogen content is somewhat higher, at ap-
proximately 125 mmol · kg–1wet weight muscle,
although this figure will obviously depend on
when the measurement was taken in relation to
the last training session. After several days
of a high (8 g · kg–1body mass) CHO diet and a
reduction in training, the muscle glycogen
content may be elevated to values around
175–200 mmol · kg–1wet weight. There is some
evidence that trained athletes who habitually
consume a moderate- to high-CHO diet (ª6g
CHO · kg–1 body mass · day–1) do not increase
their muscle glycogen contents to the same
extent as untrained individuals (Hawley et al.
1997a). Indeed, if well-trained athletes consume
a moderate- to high-CHO diet, muscle glycogen
‘supercompensation’ can occur on a day-to-day
basis. In this respect, Costill et al. (1981) have pre-
viously reported that muscle glycogen content
was not significantly different when trained
runners consumed either 525 or 650 g CHO ·
day–1, suggesting that the extent of muscle glyco-
gen supercompensation is not further increased
by the ingestion of very large (>600 g · day–1)
quantities of dietary CHO.
The mechanism(s) explaining the ergogenic
effect of CHO loading still needs to be estab-
lished. One possibility is that the higher muscle
glycogen content may delay the onset of fatigue
resulting from muscle glycogen depletion during
exercise. Alternatively, the increased availability
of muscle glycogen could slow the rate of liver
glycogen depletion because it would reduce the
muscle’s demand for blood glucose. Liver glyco-
552 sport-specific nutrition
gen sparing would depend on the rate of hepatic
glycogenolysis, which seems to be accelerated by
a high liver glycogen content after CHO loading.Effect of carbohydrate loading on
running performance
The results of studies which have examined the
effects of CHO loading and CHO restriction on
running performances, are summarized in Tables
42.3 and 42.4. Although there are many labora-
tory studies which demonstrate a positive rela-
tionship between pre-exercise muscle glycogen
stores and endurance performance for both
cycling and running (for review, see Hawley et al.
1997b), to the best of our knowledge only one
study has evaluated this effect in the field.
Shermanet al. (1981) examined the effects of
either low-, moderate- or high-CHO diets on
muscle glycogen content and utilization during a
half-marathon event (20.9 km) in six trained
runners. They found that large differences in pre-
exercise muscle glycogen contents of the runners
had no influence on subsequent performance. In
fact, running times were generally a bit slower
when athletes started the trials with higher levels
of muscle glycogen. Perhaps of physiological
interest was that the absolute amount of muscle
glycogen left at the end of the three runs was
similar regardless of the initial muscle glycogen
content.
The results of Sherman et al. (1981) were subse-
quently confirmed in the laboratory by Madsen
et al. (1990). They reported that 25% higher start-
ing muscle glycogen contents did not improve
treadmill run time to exhaustion at 75–80% of
maximal oxygen uptake (V.
o2max.). In agreement
with the data of Sherman et al. (1981), the total
amount of muscle glycogen utilized during the
two treadmill runs was similar. Perhaps the
most important finding was that at the point of
‘exhaustion’, muscle glycogen content was still
relatively high in all subjects. These studies
strongly suggest that CHO loading has no
benefit to performance for athletes who partici-
pate in moderate-intensity events lasting up to
90 min.