Conclusions and recommendations
for optimal fluid replacement
during distance running
The principal aims of fluid ingestion during dis-
tance running are to improve performance by:
- limiting any dehydration-induced decreases
in plasma volume and skin blood flow; - limiting any rise in serum sodium osmolality
or serum osmolality; - diminishing progressive rises in rectal
temperature; - decreasing the subjective perception of effort;
and - supplementing endogenous CHO stores.
Although it has been assumed that the
optimum rate of fluid ingestion is the rate that
closely tracks the rate of fluid loss, the exact com-
position of the solution that will optimize elec-
trolyte and fluid replacement of the extracellular
space has not been established. Furthermore, the
rates of fluid ingestion needed to replace the high
(>1l·h–1) sweat rates typically induced during
prolonged exercise probably exceed the maximal
intestinal absorptive capacity for water. Most
runners will not be able to achieve such fluid
intakes without great difficulty. However, fluid
consumption can be maximized during distance
running by paying careful attention to the tem-
perature and palatability of the drink and the
addition of electrolytes, particularly sodium, to
the beverage.
CHO ingestion during distance running is rec-
ommended whenever the exercise is of sufficient
duration or intensity to deplete endogenous
CHO stores. If CHOs are ingested frequently
enough and in appropriate volumes, it appears
that, with the exception of fructose: - the type of CHO consumed does not greatly
influence the rate of gastric emptying of isoener-
getic solutions; - there are no physiologically important
differences in the rates of CHO oxidation re-
sulting from repeated ingestion of a variety of
mono-, di- and oligosaccharides during exercise;
and - all ingested CHOs are oxidized at a maximum
rate of approximately 1 g · min–1after the first
70–90 min of exercise.
The reason for similar peak rates of ingested
CHO oxidation from different CHOs is because,
in all likelihood, it is the prevailing concentra-
tions of glucose and insulin normally present
during prolonged, moderate-intensity exercise
that set the upper limit for the rate of glucose
uptake and oxidation by skeletal muscle
(Hawleyet al. 1995b).
The following practical guidelines are sug-
gested for runners participating in prolonged,
moderate-intensity exercise of up to 6 h duration:- Immediately before exercise or during the
warm-up, the athlete should ingest up to
5ml·kg–1of body mass of cool, flavoured water. - For the first 60–75 min of exercise, the athlete
should ingest 100–150 ml of a cool, dilute (3.0–
5.0 g per 100 ml) glucose polymer solution at
regular intervals (10–15 min). It seems unwar-
ranted to consume CHO in amounts much
greater than 30 g during this period, as only 20 g
of ingested CHO are oxidized in the first hour of
moderate-intensity exercise, irrespective of the
type of CHO consumed or the drinking regimen. - After about 90 min of exercise, the concentra-
tion of the ingested solution should be increased
to 7–10 g per 100 ml, to which 20 mEq · l–1of
sodium should be added. Higher sodium con-
centrations may not be palatable to most athletes,
although they may be beneficial. Potassium,
which may facilitate rehydration of the intracel-
lular fluid compartment, could also be included
in the replacement beverage in small amounts
(2–4 mEq · l–1). For the remainder of the race, the
athlete should consume 100–150 ml of this
solution at regular (10–15 min) intervals. Such
a drinking regimen will ensure optimal rates
of both fluidandenergy delivery, thereby limiting
any dehydration-induced decreases in plasma
volume, and maintaining the rate of ingested
CHO oxidation at approximately 1 g · min–1late
in exercise.
References
Chryssanthopoulos, C., Williams, C., Wilson, W.,