1994). The rate of glucose uptake is dependent
on the luminal concentrations of glucose and
sodium, and dilute glucose electrolyte solutions
with an osmolality which is slightly hypotonic
with respect to plasma will maximize the rate of
water uptake (Wapnir & Lifshitz 1985). Solutions
with a very high glucose concentration will not
necessarily promote an increased glucose uptake
relative to more dilute solutions, but, because of
their high osmolality, will cause a net movement
of fluid into the intestinal lumen (Fig. 17.5). This
results in an effective loss of body water and will
exacerbate any pre-existing dehydration. This
effect is sufficiently marked to be apparent
during exercise in laboratory conditions (Fig.
17.6): ingestion of a dilute glucose–electrolyte
solution can be shown to be more effetive than
an equal volume of concentrated glucose solu-
tion in reversing the exercise-induced decrease
in plasma volume that normally occurs. Other
sugars, such as sucrose or glucose polymers, can
be substituted for glucose without impairing
glucose or water uptake. In contrast, the absorp-
232 nutrition and exercise
tion of fructose is not an active process in man: it
is absorbed less rapidly than glucose, is not asso-
ciated with sodium cotransport, and promotes
less water uptake.
Several studies have shown that exercise at
intensities of less than about 70% of V.
o2max.has
little or no effect on intestinal function, although
both gastric emptying and intestinal absorption
may be reduced when the exercise intensity
exceeds this level. Some more recent results,
using an isotopic tracer technique to follow
ingested fluids, have suggested that there may be
a decreased availability of ingested fluids even
during low intensity exercise: a decreased rate
of appearance in the blood of a tracer for water
added to the ingested drinks indicated a
decreased rate of appearance of the tracer at an
exercise intensity of 40% of V.
o2max.(Maughanet
al.1990). These studies have been reviewed and
summarized by Brouns et al. (1987) and Schedl et
al.(1994). The results generally imply that the
absorptive capacity of the intestinal tract is not
seriously compromised by exercise at an inten-600–4002000–300Sports drink
(305 mosmol.kg–1
)–200–100100300400500AbsorptionSecretionWater
(10 mosmol.kg–1)Hypotonic ORS(236 mosmol.kg–1)Apple juice
(789 mosmol.kg–1
)Net water flux (% of value for water)Fig. 17.5Dilute glucose electrolyte
solutions stimulate water
absorption in the small intestine,
with hypotonic solutions being
more effective than sports drinks,
although the latter will be more
effective in supplying energy in
the form of carbohydrate.
Concentrated solutions—such as
fruit juices—will reverse the
movement of water because of the
high intraluminal osmotic
pressure and will exacerbate any
dehydration in the short term. The
initial osmolality of each drink is
given in brackets.