Potassium is normally present in commercial
sports drinks in concentrations similar to those
in plasma and in sweat (see Tables 17.1, 17.2), but
there is little evidence to support its inclusion.
Although there is some loss of potassium in
sweat, an increase in the circulating potassium
concentration is the normal response to exercise:
increasing this further by ingestion of potassium
does not seem useful. Compared with the total
daily intake of potassium (about 80 mmol for
men and 60 mmol for women; Gregory et al.
1990), the amounts present in sports drinks are
small. Replacement of losses will normally be
achieved after exercise: 1 litre of orange juice will
provide about 30 mmol of potassium, and tomato
juice contains about twice this amount. A similar
situation applies with magnesium replacement,
and there seems to be no good reason for its addi-
tion to drinks consumed during exercise.
Choice of rehydration fluids
The aim of ingesting drinks during exercise is to
enhance performance, and the choice of drinks
will therefore be dictated by the need to address
the potential causes of fatigue. Provision of sub-
strate, usually in the form of carbohydrate, to
supplement the body’s endogenous stores, and
replacement of water lost in sweat are the
primary concerns. In some situations, replace-
ment of the electrolytes lost in sweat also
becomes important. Because of the interactions
among the different components of a drink,
however, it is difficult to analyse these require-
ments separately. There are also many different
situations in sport which will dictate the compo-
sition of drinks to be taken. The final formulation
must also take account of the taste characteristics
and palatability of the drink: not only will this
influence the amount of fluid that the athlete con-
sumes, but it will also have a major effect on how
he or she feels.
The duration and intensity of exercise will be
the main determinant of the extent of depletion
of the body’s carbohydrate reserves, and the
same factors, together with the climatic condi-
tions, will determine the extent of sweat loss.
234 nutrition and exercise
There will however, always be a large variability
between individuals in their response and there-
fore in their requirements. The requirements for
rehydration and substrate provision will also be
influenced by activity in the preceding hours and
days. In a tournament competition in soccer,
hockey or rugby, which may involve more than
one game in a single day, or in a multistage cycle
race with events on successive days, there is
unlikely to be complete recovery from the previ-
ous round, and the requirements will be different
from those in a single event for which proper
preparation has been possible.
These difficulties are immediately apparent
when any of the published guidelines for fluid
intake during exercise is examined. Guidelines
are generally formulated to include the needs of
most individuals in most situations, with the
results that the outer limits become so wide as to
be, at best meaningless, and at worst positively
harmful. The American College of Sports
Medicine published a Position Statement in 1984
on the prevention of heat illness in distance
running: the recommendations for fluid replace-
ment during running events were more specific
than an earlier (1975) version of these guidelines.
It was suggested that marathon runners should
aim for an intake of 100–200 ml of fluid every 2–
3 km, giving a total intake of 1400–4200 ml at
the extremes. For the elite runner, who takes only
a little over 2 h to complete the distance, this
could mean an intake of about 2 l · h–1, which
would not be well tolerated; it is equally unlikely
that an intake of 300 ml · h–1would be adequate
for the slowest competitors, except perhaps
when the ambient temperature was low. These
same guidelines also recommended that the best
fluid to drink during prolonged exercise is cool
water: in view of the accumulated evidence on
the performance-enhancing effects of adding
glucose and electrolytes, this recommendation
seems even less acceptable than it was in 1984.
This has now been recognized and a further
updated version of the Guidelines (American
College of Sports Medicine 1996) is in accord
with the current mainstream thinking: for events
lasting more than 60 min, the use of drinks con-