NUTRITION IN SPORT

sity that can be sustained for long enough (about
40 min or more) for fluid intake to be seriously
considered.

Electrolyte replacement

during exercise

Commercially available sports drinks intended
for use by athletes in training and competition
are generally rather similar in their electrolyte
content, suggesting a consensus, at least among
the manufacturers, as to the requirements for
electrolyte replacement (Table 17.2). It is clear
that the major requirement is for addition of
sodium, which is important in improving palat-
ability and maintaining the drive to drink
(Hubbard et al. 1990), for the absorption of
glucose and of water in the small intestine
(Maughan 1994), and for the maintenance of the
extracellular volume (Hubbard et al. 1990).

In spite of the need to replace sodium, the main

requirement is for replacement after exercise (see

Chapter 19). During exercise, the plasma sodium

concentration normally rises as water is lost in

excess of sodium. When the exercise duration is

likely to exceed 3–4 h, there may be advantages

in adding sodium to drinks to avoid the danger

of hyponatraemia, which has been reported to

occur when excessively large volumes of drinks

with a low sodium content are taken (Noakes et

al.1990). This condition, however, is rather rare,

and does not in itself justify the inclusion of

sodium in drinks intended for use in exercise

where sweat losses do not exceed a few litres.

The optimum sodium concentration for use in

sports drinks intended for consumption during

exercise has not been established, as this will

vary depending on the conditions and on the

individual, but is likely to be between about 20

and 40 mmol · l–1.

water and electrolyte loss and replacement 233

Exercise

0 15 30 45 60 75 90 105 120

Time (min)

10

5

0

–5

–10

–15

–20

Change in plasma volume (%)

** *

*

*

* **

*

Fig. 17.6Because of the faster
gastric emptying and faster
intestinal absorption of water,
ingestion of dilute carbohydrate–
electrolyte solutions (
) is more
effective in restoring plasma volume
during and after exercise compared
with ingestion of an equal volume of
concentrated glucose solution ().
Values are mean±SEM; *, P<0.05;
**,P<0.01. Adapted from Maughan
et al. (1987).

Table 17.2Composition of some of the most widely used commercial sports drinks.

Carbohydrate Sodium Potassium Osmolality

(g · 100 ml-^1 ) (mmol · l-^1 ) (mmol · l-^1 ) (mosmol · kg-^1 )

Allsport 8.0 10 6
Gatorade 6.0 18 3 330–340
Isostar 6.5 17 5
Lucozade Sport 6.9 23 5 260–280
Powerade 8.0 10 4