prior to exercise, show similar effects. It is tempt-
ing to speculate that the 15-min time period is not
sufficient to give a full picture of the gastric emp-
tying rate of the total volume of fluid ingested
and that the chance of error would be greater.
However, Costill and Saltin (1974) also only
looked at 15 min of exercise and found no sig-
nificant effect of exercise up to around 70% of
V
.
o2max.. Furthermore, in the experiments in
which complete emptying curves were moni-
tored, the 15-min measuring point also showed
no significant effect.
The previously cited studies all examined the
emptying of water or carbohydrate-containing
liquids. One study also investigated the effect of
mild exercise (walking at speeds of 3.2 km · h–1
and 6.4 km · h–1) on the emptying of a solid meal
(Moore et al. 1990). An increased emptying was
observed during exercise as compared with
rest, based upon half-emptying times of radio-
labelled meals and gamma camera monitoring.
Another fairly recent study, by Brown et al.
(1994), demonstrated with ultrasound imaging
that the emptying of a semisolid meal was
delayed with cycling at 85% of the predicted
maximum heart rate compared to rest. In this
study, postexercise contraction frequencies and
antral areas were also monitored. Both were
significantly decreased when compared with
measurements made in experiments without
exercise. Further, there was closure of the pylorus
and a narrowing of the antrum. These changes in
gastric function might explain the decreased
gastric emptying observed. Another study, per-
formed with dogs, showed that with 2 h of
exercise, at 60–70% of the maximum heart rate,
the gastric emptying of a mixed (23.5% protein,
3.5% fat, 66.5% carbohydrate) liquid meal was
delayed and the migrating motor complex
ceased, again indicating that exercise inhibits
gastric motility. Gastric acid and pepsin secretion
were also inhibited during exercise.
In summary, there appear to be no clear effects
on gastric emptying at moderate intensities.
High-intensity exercise (>70% of V
.
o2max.) does
appear to increase emptying time. The few
studies that were done at very low intensities
(<42% of V.
o2max.), however, do indicate that
gastric emptying may be enhanced by mild
exercise.
One may speculate that some of the disparity
in results of different studies was due to the train-
ing status of the subjects or the mode of exercise.
However, when two groups, one trained and
competitive in bicycling and the other untrained
for all endurance activity, were compared, no
difference in gastric emptying rate at rest or
during cycling was observed (Rehrer et al. 1989).
Another study, however, was conducted in
which emptying of a radio-labelled egg omelette
was compared in distance runners and sedentary
subjects (Carrio et al. 1989). The trained runners
had an accelerated gastric emptying of the meal
at rest (runners, t1/2=67.7±5.9 min; sedentaries,
t1/2=85.3±4.5 min, P<0.001).
Two studies have also compared gastric emp-
tying rates of the same subjects cycling and
running at similar relative intensities (Rehrer
et al. 1990b; Houmard et al. 1991). No difference
in gastric emptying rates was observed due to
mode of exercise. These results are somewhat
unexpected, since the magnitude of accelerations
of the body while running are more than double
that experienced during bicycling; therefore
one might expect that this would result in an
increased gastric emptying rate during running
(Rehrer & Meijer 1991).
Other consequences of exercise can have an
indirect effect on gastric function. Exercise can
sometimes result in hypohydration and hyper-
thermia. When hyperthermia and hypohydra-
tion occur during exercise, the rate of gastric
emptying is reduced (Neufer et al. 1989b; Rehrer
et al. 1990a). The emotional stress that competi-
tion can cause may also delay gastric emptying
and thus some of the conclusions drawn from
data collected in a standardized laboratory
setting may not always be applicable to an indi-
vidual athlete during high-level competition.
Further, a large variation in standardized gastric
emptying rates between individuals exists
(Foster & Thompson 1990; Brunner et al. 1991).