Water losses
The turnover rate of water exceeds that of most
other body components: for the individual who
lives in a temperate climate and takes no exer-
cise, daily water losses are about 2–4 l, or 5–10%
of the total body water content. Urine, faeces,
sweat, expired air and through the skin are the
major avenues of water loss, and the approxi-
mate size of these different routes of water loss
are shown in Table 15.1. In spite of its relative
abundance, however, there is a need to maintain
the body water content within narrow limits, and
the body is much less able to cope with restric-
tion of water intake than with restriction of food
intake. A few days of total fasting has relatively
little impact on functional capacity, provided
fluids are allowed, and even longer periods of
abstinence from food are well tolerated. In con-
trast, cessation of water intake results in serious
debilitation after times ranging from as little as
an hour or two to a few days at most.
Environmental conditions will affect the basal
water requirement by altering the losses that
occur by the various routes. Water requirements
for sedentary individuals living in a hot climate
may be two- or threefold higher than the require-
ment when living in a temperate climate, even
when this is not accompanied by obvious sweat-
ing (Adolph & Associates 1947). Transcutaneous
and respiratory losses will be markedly influ-
enced by the humidity of the ambient air, and
208 nutrition and exercise
this may be a more important factor than the
ambient temperature. Respiratory water losses
are incurred because of the humidification of the
inspired air. These losses are relatively small in
the resting individual in a warm, moist environ-
ment (amounting to about 200 ml · day–1), but
will be increased approximately twofold in
regions of low humidity, and may be as high as
1500 ml · day–1during periods of hard work in the
cold, dry air at altitude (Ladell 1965).
The nature of the diet has some effect on
water requirements because of the requirement
for excretion of excess electrolytes and the prod-
ucts of metabolism. An intake of electrolytes in
excess of the amounts lost in sweat and faeces
must be corrected by excretion in the urine, with
a corresponding increase in the volume and
osmolality of urine formed. The daily intake of
electrolytes varies widely between individuals,
and there are also regional variations. Daily
dietary sodium chloride intakes for 95% of the
young male UK population fall between 3.8 and
14.3 g, with a mean of 8.4 g; the corresponding
values for young women are 2.8–9.4 g, with a
mean value of 6.0 g (Gregory et al.1990). A high-
protein diet requires a greater urine output to
allow excretion of water-soluble nitrogenous
waste (LeMagnen & Tallon 1967). Although this
effect is relatively small compared with other
losses, it can become meaningful when water
availability is limited, and may be a factor to be
considered in some athletes who habitually
consume diets with a very high protein content.
When a high-protein diet is used in combination
with fluid restriction and dehydration practices
as part of the making-weight process in weight
category sports, there are real dangers. The water
content of the food ingested will also be influ-
enced greatly by the nature of the diet, and water
associated with food will make some contribu-
tion to the total fluid intake.Electrolyte losses in sweat
The sweat which is secreted onto the skin con-
tains a wide variety of organic and inorganic
solutes, and significant losses of some of theseTable 15.1Avenues of water loss from the body for
sedentary adult men and women. From Bender &
Bender (1997).
Water loss
(ml · day-^1 )Men WomenUrine 1400 1000
Expired air 320 320
Transcutaneous loss 530 280
Sweat loss 650 420
Faecal water 100 90
Total 3000 2100