the volume and composition of fluid consumed,
although there is again a large variability
between individuals. The volume of the stomach
contents is a major factor in regulating the rate of
emptying, and the rate of emptying of any solu-
tion can be increased by increasing the volume
present in the stomach; emptying follows an
exponential time course, and falls rapidly as the
volume remaining in the stomach decreases
(Leiper & Maughan 1988). The presence of large
volumes in the stomach may cause discomfort
during exercise, and is not well tolerated by some
individuals, but there is a strong learning process
and the athlete can increase the amount that can
be consumed with practice. The effects of exer-
cise on gastrointestinal function are described in
detail in the following chapter.
Dilute solutions of glucose will leave the
stomach almost, but not quite, as fast as plain
water; the rate of emptying is slowed in propor-
tion to the glucose content (Fig. 17.3) and concen-
trated sugar solutions will remain in the stomach
for long periods (Vist & Maughan 1994). There
has been some debate as to the concentration of
carbohydrate at which an inhibitory effect on
gastric emptying is first observed: these studies
have been reviewed by Maughan (1994). The
230 nutrition and exercise
conflicting results reported in the literature are
caused at least in part by deficiencies in the
methodology employed in some studies. It
appears that glucose concentrations as low as
40 g Ā· lā1will have some slowing effect on the rate
of gastric emptying (Vist & Maughan 1994),
but increasing the concentration will increase the
carbohydrate delivery. Where a high rate of emp-
tying is desirable, fluid delivery can be promoted
by keeping the volume high by repeated drink-
ing (Rehrer 1990), although repeated ingestion of
concentrated carbohydrate solutions is likely to
result in a progressive increase in the volume of
fluid in the stomach (Noakes et al. 1991).
An increasing osmolality of the gastric con-
tents will tend to delay emptying, and there is
some evidence that substitution of glucose poly-
mers for free glucose, which will result in a
decreased osmolality for the same carbohydrate
content, may be effective in achieving a higher
rate of delivery of both fluid and substrate to the
intestine. This has led to the inclusion of glucose
polymers of varying chain length in the formula-
tion of sports drinks. Vist and Maughan (1995)
have shown that there is an acceleration of emp-
tying when glucose polymer solutions are substi-
tuted for free glucose solutions with the same7006005004003002001000
0 102030405060
Time (min)Residual gastric volume (ml)Fig. 17.3Increasing the glucose
concentration in ingested
solutions slows the rate of gastric
emptying in proportion to the
glucose concentration. This figure
shows the total volume in the
stomach after ingestion of 600 ml
of water () or of drinks
containing 2% (), 4% () or 6%
glucose (). A significant slowing
is observed at a concentration of
4%. From Vist and Maughan
(1994).