needed to drink ‘at least 900 ml of fluid per hour
during competition in order not to collapse
from heat stroke’ (Wyndham & Strydom 1969).
Modern studies show that runners do not volun-
tary consume much more than 500 ml · h–1during
distance races. In contrast to these moderate
rates of fluid intake, sweat rates are invariably
around 1.0–1.2 l · h–1during events lasting 2 h or
more (for review, see Noakes 1993).
One explanation for the failure of runners to
match their fluid intake to their fluid losses
during exercise is that they develop symptoms
of ‘fullness’ when they attempt to drink fluid at
high rates. Feelings of abdominal fullness may, in
part, be due to limited rates of fluid absorption,
as duodenal and jejunal perfusion studies show
the maximum rate of water absorption occurs
from isotonic solutions containing glucose, and
is limited to about 0.8 l · h–1(Davieset al. 1980).
Similarly, in studies in which sufficient fluid was
ingested to match fluid losses during exercise,
556 sport-specific nutrition
not all of the ingested fluid appeared in the extra-
cellular or intracellular fluid pools. Thus, the
maximum rate of fluid absorption by the small
bowel during exercise may be less than the high
rates of fluid loss incurred by some athletes
during more intensive exercise, leading to pro-
gressive or ‘involuntary’ dehydration (Noakes
1993).
An alternative hypothesis is that man, unlike
other mammals, may develop progressive dehy-
dration during exercise because of the sodium
chloride (NaCl) losses in sweat. Large sodium
losses attenuate the rise in serum osmolality
during exercise-induced dehydration in
humans, and since thirst is regulated by changes
in both serum osmolality and plasma volume,
dipsogenic drive in dehydrated humans ceases
before either fluid or sodium losses are fully
replaced. Ingestion of NaCl solutions also termi-
nates drinking prematurely by restoring plasma
and extracellular volumes before intracellular
fluid losses have been replaced. The practical sig-
nificance of this observation is that whether
dehydrated humans drink plain water or NaCl
solutions, they tend to stop drinking before they
are fully rehydrated. These complex interactions
may explain why some humans are unable to
prevent the development of ‘involuntary’ dehy-
dration during prolonged exercise. Additionally,
the rapid alleviation of the symptoms that indi-
cate drinking, such as dryness of the mouth,
may also cause premature cessation of drinking
before full rehydration has occurred.Carbohydrate ingestion and oxidation
during exercise
Runners are often confused as to the optimum
fluid replacement regimen to enhance their per-
formance. Although the addition of high (>15 g
per 100 ml) concentrations of CHO to fluid
replacement beverages may impair intestinal
fluid absorption, inadequate CHO ingestion
impairs performance by limiting the rates of
CHO oxidation late in exercise. Accordingly,
recent attention has focused on strategies to opti-Table 42.5The rate of fluid loss and fluid ingestion
during various long-distance running races. Adapted
from Noakes et al. (1995).
Estimated
Race distance Fluid intake sweat rate Weight loss
(km) (l · h-^1 ) (l·h-^1 ) (kg)
32 0.15 1.35 2.4
42* 0.4±0.2 1.1±1.1 2.4±0.3
56 0.5 0.9 2.0
67 0.4 0.8 2.4
90 0.5 0.85 3.5
The total fluid intake of runners was determined as the
sum of their individual intakes, as reported at various
recording points during the race. Sweat rate was
estimated from the rate of water loss, minus estimated
respiratory losses. Total weight loss was determined as
the sweat loss, plus metabolic fuel loss plus fluid
intake minus urine output.
- 42-km values are means±SD of the average values
from seven studies on male subjects. Female sweat
rates were lower than those for males over distances
of 42 km (0.6 l · h-^1 vs. 1.1 l · h-^1 ) and 67 km (0.5 l · h-^1 vs.
0.8 l · h-^1 ).