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

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 hormones.

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.