tion beverages can be justified on two accounts.
Firstly, sodium stimulates glucose absorption in
the small intestine (Olsen & Ingelfinger 1968):
water absorption from the intestinal lumen is a
purely passive process that is determined largely
by local osmotic gradients (Parsons & Wingate
1961). The active cotransport of glucose and
sodium creates an osmotic gradient that acts to
promote net water absorption (Sladen 1972),
and the rate of rehydration is therefore greater
when glucose–sodium chloride solutions are
consumed than when plain water is ingested.
This was discussed in detail in Chapter 18. Sec-
ondly, replacement of sweat losses with plain
water will, if the volume ingested is sufficiently
large, lead to haemodilution: the fall in plasma
osmolality and sodium concentration that occurs
in this situation will reduce the drive to drink
and will stimulate urine output (Nose et al.
1988b) and has potentially more serious conse-
quences such as hyponatraemia (Noakes et al.
1985).
It has been proposed that drinks used for post-
exercise rehydration should have a sodium con-
centration similar to that of sweat (Maughan
1991), but as the electrolyte content of sweat itself
shows considerable variation between individu-
als and over time (see Chapter 17), it would seem
impossible to prescribe a single formulation for
every individual or every situation. However, a
study to investigate the relation between whole-
body sweat sodium losses and the rehydration
effectiveness of beverages with different sodium
concentrations seems to confirm that optimum
rehydration is achieved with a drink with a
sodium concentration similar to that of sweat
(Shirreffs & Maughan 1997b).
Sodium is the major ion in the extracellular
fluid but potassium is the major ion in the intra-
cellular fluid (see Table 17.1). It has been sug-
gested therefore that potassium may also be to
some degree important in achieving rehydration
by aiding the retention of water in the intracellu-
lar space. Yawata (1990) undertook experimental
work on rats subjected to thermal dehydration of
approximately 9% of body mass and then given
free access to either tap water, a 150 mmol · l–1
260 nutrition and exercise
NaCl solution or a 154 mmol · l–1KCl solution.
The results indicated that despite ingestion of a
smaller volume of the KCl solution compared
to the NaCl solution, there was a tendency for
a greater restoration of the intracellular fluid
space in the KCl group than in the NaCl group.
Maughan et al. (1994) undertook a study in
which men were dehydrated by approxima-
tely 2% of body mass by exercising in the
heat, and then ingested a glucose beverage
(90 mmol · l–1), a sodium-containing beverage
(NaCl 60 mmol · l–1), a potassium-containing
beverage (KCl 25 mmol · l–1) or a beverage con-
sisting of the addition of all three. A smaller
volume of urine was excreted following rehydra-
tion when the electrolyte-containing beverages
were ingested than when the electrolyte-free
beverage was consumed (Fig. 19.2). An esti-
mated plasma volume decrease of 4.4% was
observed with dehydration over all trials but the
rate of recovery was slowest when the KCl bever-
age was consumed. Although there were differ-
ences in the total amount of electrolyte replaced
as well as differences in the type of electrolytes
present in the drinks, there was no difference in
the fraction of ingested fluid retained 6 h after
finishing drinking the drinks which contained
electrolytes. This may be because the beverage
volume consumed was equivalent to the volume
of sweat lost and subjects were dehydrated,
because of the ongoing urine losses, throughout
the entire study, even following the drinking
period. The volumes of urine excreted were close
to basal levels and significant further reductions
in output may not have been possible when both
sodium and potassium were ingested, over and
above the reductions already induced when the
sodium and potassium were ingested separately.
The importance of potassium in enhancing rehy-
dration by aiding intracellular rehydration over
and above that with sodium seems therefore to
be realistic but further investigation is required
to provide conclusive evidence.Drink volume
Obligatory urine losses persist even in the dehy-