The factors which influence gastric emptying
rate to a significant degree have, however, the
same relative effect among both ‘slow’ and ‘fast
emptiers’. Beverage or meal size and composi-
tion, including nutrient concentration, osmolal-
ity and particle size, are all strong modulators of
gastric emptying. In particular, increasing carbo-
hydrate concentration, osmolality and particle
size decrease the gastric emptying rate. For a
more complete review of the factors which influ-
ence gastric emptying, see Murray (1987), Costill
(1990), Maughan (1991) or Rehrer et al. (1994).
Bearing in mind the individual differences and
situations, it is useful to look at the upper limits
to gastric emptying rates during continuous
exercise and to compare these with sweat rates
when one is attempting to balance fluid losses.
Mitchell and Voss (1990) observed increasing
gastric emptying rates with increasing gastric
volume up to approximately 1000 ml. With inges-
tion rates of around 1000 ml · h–1, depending
upon the composition of the beverage, the
volume emptied can reach about 90% of that
ingested (Ryan et al. 1989; Mitchell & Voss 1990;
Rehrer et al. 1990b). Similar high rates of gastric
emptying have also been observed with intermit-
tent exercise (1336 ml · 2 h–1ingested; 1306±76,
1262 ±82, 1288±75, 1278±77 ml ·2 h–1emptied for
water, 5%, 6% and 7.5% carbohydrate, respec-
tively; Beltz 1988). For a review on the effects of
volume on gastric emptying, see Noakes et al.
(1991).
Intestinal absorption
Net absorption of water and carbohydrates
occurs primarily in the small intestine (duo-
denum and jejunum). To a lesser extent, water
absorption also occurs in the large intestine
(colon). A large body of research evidence exists
which describes the functioning of the intestinal
tract and the factors which influence the absorp-
tion of fluids and carbohydrates, at rest (Riklis
& Quastel 1958; Curran 1960; Crane 1962; Schedl
& Clifton 1963; Holdsworth & Dawson 1964;
Fordtran 1975; Leiper & Maughan 1988; Gisolfiet
al.1990). Relatively little has been published
244 nutrition and exercise
regarding intestinal absorption during exercise.
Herewith only research specifically designed to
look at the effects of exercise upon intestinal
function will be discussed.
One of the first controlled studies directly mea-
suring intestinal absorption during exercise was
conducted by Fordtran and Saltin (1967). Intesti-
nal perfusion of the jejunum and ileum with a
triple lumen catheter was done with subjects
at rest and during treadmill running at 70% of
V.
o2max.. A 30-min equilibration period was main-
tained prior to measurement in each condition.
No effect of exercise on glucose absorption
within the jejunum or ileum was observed.
Similarly, no consistent effect of exercise on net
water or electrolyte absorption or secretion was
observed. It should be noted, however, that only
four or five subjects were used in the jejunal per-
fusion experiments and only two to three in the
ileal perfusion experiments. Conflicting results
were found in another jejunal perfusion study
(Barclay & Turnberg 1988), in which cycling was
performed at a constant, absolute exercise inten-
sity (15 km · h–1, 40–50% above resting heart rate).
A significant net decrease in net water and elec-
trolytes was observed. One difference between
this study and that of Fordtran and Saltin (1967)
is that in the earlier study, the perfusate con-
tained glucose and in the latter study it did
not. The stimulatory effect of glucose on water
absorption may have masked any inhibitory
effects of exercise. Further, the amount of glucose
in the perfusate was not constant across subjects.
This may have given added variability in results
and with the small sample size may have pre-
cluded finding a consistent effect.
A more recent study by Maughan et al. (1990)
has been conducted using deuterium accumula-
tion in the plasma after drinking a^2 H 2 O-labelled
beverage to investigate the effects of exercise on
absorption. Subjects performed four separate
trials at rest and cycling at 42%, 61% and 80% of
V.
o2max.. A consistent effect of exercise to reduce
the rate of plasma deuterium accumulation was
observed. One must bear in mind that the rate of
deuterium accumulation in the plasma is not
solely a consequence of the rate of intestinal