Fluid needs are increased at high altitude for two reasons.
- Firstly, acute exposure and first phase of acclimatisation
causes an increase in sodium and water excretion (diuresis)
- Secondly by the cold, dry air at altitude.
Diuresis increases the oxygen carrying capacity of your
blood until red blood cells numbers have had time to increase
(by an increase in the concentration of haem in your blood).
The mechanisms underlying this are not well understood but most
of the studies available have found a decrease in sodium retaining
Fluid loss through diuresis is reported to be as much as 1.5
litres day when energy intake is inadequate. However if energy
intake is adequate, fluid loss may only be 500ml. After 7 days
diuresis should be reduced if energy balance is attained.
Air at altitude is cold and dry. Therefore, each inspired
breath needs the body to warm the air before reaching the lungs
and each expired air contains water and heat which is lost to
the environment. This is called insensible water loss. This
fluid loss often leads to moderate dehydration and accompanying
symptoms of dryness of the lips, mouth and throats. Insensible
water losses in men performing only moderate exercise at 4300m
reported a insensible loss of nearly 2 litres per day through
a 3 week stay at altitude. However, women showed less of a loss
(750ml/d). These insensible losses do not include the obligatory
water loss for kidney function, fluid losses during exercise
to remove heat from body and sweat loss.
For the above reasons it has been recommended that fluid requirements
at high altitude may be as high as 3-5 litres per day at rest.
This recommendation should be increased further if physical
activity is performed.
Type of drink
Given that carbohydrate requirements are also increased and
water alone is a poor re- hydrator, it is practical to consume
an isotonic sports drink with electrolytes. Sodium (an electrolyte)
will help maintain the drive to drink, minimise urinary losses
during exercise and maintain the extracellullar fluid space.
Moreover, the taste of the commercial available sports drinks
has been show to enhance voluntary intake, which has a further
practical advantage over water.
However, choice of the wrong drink (very high carbohydrate
concentration, no sodium etc.) or one recommended by a friend
that you don't like is going to counteract its benefits. So,
find an isotonic drink that tastes good to you.