should be ingested during exercise while slightly
higher rates of ingestion during the first hour
may speed up the achievement of these high
levels of oxidation. However, ingesting more
than 1.5 g · min–1 during exercise may not
result in increased exogenous CHO oxidation
(Wagenmakerset al.1993) and increases the risk
of gastrointestinal problems.
Most studies of cyclists have shown that CHO
ingestion does not alter the rate of muscle
glycogen breakdown during exercise, although
during intermittent exercise glycogen may be
resynthesized during the low-intensity cycles
(Hargreaveset al.1984; Kuiperset al.1986). So the
mechanism by which CHO ingestion during
cycling improves performance in road races may
not only be maintaining the plasma glucose con-
centration, but also the resynthesis of muscle
glycogen during periods of low intensity.
Nutrition during high-intensity exercise
of about 1 h
Although previous studies suggested that CHO
feedings can improve exercise performance
during exercise of longer than 90 min duration,
recent evidence shows that CHO feedings can
also be effective during high-intensity exercise of
shorter duration (60 min) (Anantaramanet al.
1995; Belowet al.1995; Jeukendrupet al.1997a).
We recently found improved time-trial perfor-
mance (comparable to a 40-km time trial) in
well-trained cyclists when they ingested a
carbohydrate–electrolyte solution during exer-
cise (75 g of CHO) compared with placebo (Fig.
43.3) (Jeukendrupet al.1997a). Seventeen out of
19 subjects showed improved time-trial perfor-
mance, while two athletes displayed a decreased
performance with the carbohydrate–electrolyte
solution. The average power output during the
time trial when the carbohydrate–electrolyte
solution was ingested was 298±10 W vs. 291±
10 W with placebo. Although the beneficial effect
of the CHO ingestion during high intensity exer-
cise of about 1 h duration has now been con-
firmed by several studies, the mechanism behind
this performance effect remains unclear and
central effects of glucose on the brain cannot be
excluded at this point (Jeukendrupet al.1997a).
Optimally, athletes should ingest a carbohy-
drate–electrolyte solution throughout exercise in
order to maintain a certain volume of fluid in the
stomach which will enhance gastric emptying
(Rehreret al.1990). It has recently been shown
that ingestion of CHO throughout exercise
improves performance more than ingestion of an
identical amount of CHO late in exercise
(McConell et al. 1996). Again, these results
suggest that CHO ingestion improves perfor-
mance through mechanisms other than, or incycling 567
686562605855Time to complete work (min)Placebo CE Placebo CE
5758596061Time to complete work (min)(a) (b)Fig. 43.3Ingestion of a
carbohydrate–electrolyte (CE)
drink reduces time to complete a
set amount of work (analogues to
completion of a 40-km time trial).
(a) Individual data of 17 male ()
and 2 female athletes (); (b) the
means.