energy density: at low (about 4%) concentra-
tions, this effect is small, but it becomes apprecia-
ble at higher (18%) concentrations; where the
osmolality is the same (as in the 4% glucose solu-
tion and 18% polymer solution), the energy
density is shown to be of far greater significance
in determining the rate of gastric emptying (Fig.
17.4). This effect may therefore be important
when large amounts of energy must be replaced
after exercise, but is unlikely to be a major factor
during exercise where more dilute drinks are
taken. There may be benefits in including a
number of different carbohydrates, including
free glucose, sucrose and maltodextrin: this has
taste implications, which may influence the
amount consumed, and may maximize the rate
of sugar and water absorption in the small intes-
tine (Shi et al. 1995).
The temperature of ingested drinks has been
reported to have an influence on the rate of emp-
tying, and it has been recommended that drinks
should be chilled to promote gastric emptying
(American College of Sports Medicine 1984). The
balance of the available evidence, however, indi-
cates that there is not a large effect of tempera-
ture on the rate of gastric emptying of ingested
liquids (Maughan 1994). Lambert and Maughan
(1992) used a deuterium tracer technique to show
that water ingested at high temperature (50°C)
appears in the circulation slightly faster than if
the drink is chilled (4°C) before ingestion. The
temperature will, of course, affect palatability,
and drinks that are chilled are likely to be
preferred and therefore consumed in greater
volumes (Hubbard et al. 1990). Other factors,
such as pH, may have a minor role to play.
Although there is some evidence that emptying
is hastened if drinks are carbonated, more recent
results suggest that carbonation has no effect
(Lambertet al. 1993): it is probable that light car-
bonation as used in most sports drinks does not
influence the gastric emptying rate, but a greater
degree of carbonation, as used in many soft
drinks, may promote emptying of the gastric
contents by raising the intragastric pressure.
Zachwiejaet al. (1992) have shown that carbon-
ated and non-carbonated carbohydrate (10%)
solutions were equally effective in improving
cycling performance relative to water adminis-
tration: there was no effect of carbonation on the
rate of gastric emptying or on the reported preva-
lence of gastrointestinal symptoms. Lambert
et al. (1993) did report a greater sensation of
stomach fullness in exercising subjects drinking
a carbonated 6% carbohydrate solution relative
to the same drink without carbonation, but there
was no apparent effect on physiological function.
No net absorption of carbohydrate, water or
electrolytes occurs in the stomach, but rapid
absorption of glucose occurs in the small intes-
tine, and is an active, energy-consuming process
linked to the transport of sodium. There is no
active transport mechanism for water, which will
cross the intestinal mucosa in either direction
depending on the local osmotic gradients. The
factors which govern sugar and water absorp-
tion have been extensively reviewed (Schedl et al.water and electrolyte loss and replacement 231
7006005004003002001000
0 1020304050 60
Time (min)Residual gastric volume (ml)Fig. 17.4Substituting glucose polymers for free
glucose reduces the inhibitory effect on gastric
emptying. This figure shows the total volume in the
stomach after ingestion of 600 ml of drinks containing
glucose at concentrations of 4% () or 18.8% () or of
glucose polymer at concentrations of 4% () or 18.8%
(). The difference between isoenergetic solutions is
small at low concentrations but becomes meaningful at
high carbohydrate concentrations. Adapted from Vist
and Maughan (1995).