Electrolyte composition of sweat:
implications for electrolyte balance
Electrolyte losses in sweat are a function of
sweating rate and sweat composition, and both
of these vary over time as well as being substan-
tially influenced by the exercise conditions and
the physiology of the individual. Added to this
variability is the difficulty in obtaining a reliable
estimate of sweat composition (Shirreffs &
Maughan 1997): as well as problems of contami-
nation of the sample and of ensuring complete-
ness of collection, there are regional variations in
electrolyte content, so measurements made at a
single site may not reflect whole body losses.
In spite of the variability in the composition
of sweat, it is invariably isotonic with respect
to plasma, although the major electrolytes are
sodium and chloride, as in the extracellular space
(Table 17.1). It is usual to present the composition
in mmol · l–1, and the extent of the sodium losses
in relation to daily dietary intake, which is
usually expressed in grams, is not widely appre-
ciated. Loss of 1 litre of sweat with a sodium
content of 50 mmol · l–1represents a loss of 2.9 g of
sodium chloride: the athlete who sweats 5 l in a
daily training session will therefore lose almost
15 g of salt. Daily dietary intakes for the 95% of
the young male UK population fall between 3.8
228 nutrition and exercise
and 14.3 g with a mean of 8.4 g: the correspond-
ing values for young women are 2.8–9.4 g, with
a mean value of 6.0 g (Gregory et al. 1990). For
the same population, mean urinary sodium
losses were reported to account for about 175
mmol · day–1(Gregory et al. 1990), which is equiv-
alent to about 10.2 g of sodium chloride. Even
allowing for a decreased urinary output when
sodium losses in sweat are large, it is clear that
the salt balance of individuals exercising in the
heat is likely to be precarious. The possible need
for supplementary salt intake in extreme condi-
tions will be discussed below.
The potassium concentration of sweat is high2520151050
150 180 210 240 270 300 330
Finishing time (min)Sweat rate (g.min–1
)Fig. 17.2Sweat rates for subjects
who competed in a marathon race
held in cool (about 12°C)
conditions. The sweat rate was
closely related to the running
speed, but there was a large
variation between individuals, even
at the same speed. Total sweat loss
was unrelated to finishing time.
r= – 0.629;P<0.001. From
Maughan (1985).Table 17.1Concentration (mmol · l-^1 ) of the major
electrolytes in sweat, plasma and intracellular water.
Values are taken from a variety of sources identified in
Maughan (1994).Sweat Plasma IntracellularSodium 20–80 130–155 10
Potassium 4–8 3.2–5.5 150
Calcium 0–1 2.1–2.9 0
Magnesium <0.2 0.7–1.5 15
Chloride 20–60 96–110 8
Bicarbonate 0–35 23–28 10
Phosphate 0.1–0.2 0.7–1.6 65
Sulphate 0.1–2.0 0.3–0.9 10