Hydration at altitude is key to maintaining energy levels

Fatigue, headaches, dry mouth, aches, cramps... These symptoms of dehydration are well-known, but often associated with high temperatures. However, mountain climates are not exempt from this risk; on the contrary, it's a very common phenomenon because the sensation of thirst is often less present in cold, dry weather, even though our hydration needs are in no way reduced.

An aggressive mountain climate

In general, temperature depends on atmospheric pressure. The higher the pressure, the higher the temperature.

As one ascends in altitude, pressure decreases: the air must then expand to maintain the same volume, which costs it heat, so it cools. Due to this effect, the temperature decreases by 8°C per kilometer of altitude. Since cold air can contain much less humidity than warm air, air humidity is halved every 2 km of altitude.

The air in a mountain climate is therefore often very cold and dry.

Added to this is an increased exposure to wind because it rushes violently through passes where it can flow freely: this is the Venturi effect, also called the tunnel effect. Wind increases the evaporation of perspiration and dries out the skin, leading to a state of dehydration.

Multiple effects on hydration

Body hydration is the result of a balance between water intake (drinking) and the body's fluid losses. The latter come from four phenomena and are all affected by the mountain climate.

Firstly, contact of the air with the skin produces natural evaporation: a part of the water contained in the epidermis evaporates.

Cold, dry air and strong wind lead to greater water loss through the skin in the mountains. The body, in response to the cold, constricts the blood vessels in the hypodermis. This results in a lower water supply to the epidermis. This leads to dry and dehydrated skin, particularly where it is thin (lips, hands...): these are chapped areas. Good hydration helps prevent the appearance of this skin dryness.

We lose approximately 1 glass of water (~250-300 mL) per day, at rest, through respiration.

During inhalation, the inspired air is warmed and humidified as it passes through the nose and upper respiratory tract. A rapid transfer of heat and water occurs from the mucous membrane of the respiratory tract to the air. Thus, the air that reaches the pulmonary alveoli is always warmed (37°C) and saturated with water vapor (100% relative humidity).

During exhalation, some of the heat and water are recovered by the mucous membrane of the upper respiratory tract.

In a cold and dry climate, despite the fact that the respiratory tract is able to recover a larger percentage of heat and water transferred to the inspired air, the total heat and water loss in the exhaled air is greater.

Furthermore, the higher the altitude, the lower the oxygen pressure. With each breath, we therefore supply less oxygen to our cells: this is hypoxia. In response to this, breathing intensifies – hyperventilation – to bring oxygen to the lungs, which increases water loss through the respiratory tract and promotes dehydration.

In the mountains, it is therefore necessary to compensate for these water losses and remember to hydrate to replace electrolytes and fluids.

Diuresis is the elimination of urine by the body, a product of blood filtration by the kidneys. When cold drives blood from the extremities, this mechanically produces an influx of circulating blood, which passes through the kidneys and leads to urine production. The increase in blood pressure also creates the urge to urinate: thus, more fluids are lost more quickly through urine – this loss must therefore be compensated by more frequent water intake.

Perspiration helps regulate body temperature by eliminating excess heat via the evaporation of body water. While ambient cold helps cool the body better, the warm and waterproof clothing often worn in winter in the mountains (for winter sports) prevents the evacuation of sweat and causes more abundant sweating – and thus a risk of dehydration.

In summer, the wind and dry air give the deceptive sensation of not sweating because perspiration is directly evaporated.

It is noted that fluid losses through these four mechanisms are greater due to the mountain climate, leading to a state of dehydration of the body and its associated symptoms, and consequently, an increased need for hydration.

Added to this is the fact that cold diminishes the sensation of thirst. Although water losses are higher, we are tempted to drink less!

Physical activity in the mountains

A stay in the mountains is often an opportunity for physical activities, such as hiking, mountaineering, climbing, winter sports... Our hydration needs are doubly increased, by physical activity and by the cold and dry climate described above. Fatigue, headaches, muscle aches, and cramps due to dehydration can quickly disrupt a hike.

Dehydration is also an indirect cause of accidents in the mountains and injuries as it can lead to states of weakness or confusion and make the body more susceptible to hypothermia.

To prevent the consequences of dehydration in order to avoid endangering oneself or spoiling one's stay, one must hydrate well, before, during and after effort, and use a drink rich in electrolytes.

Good hydration is one of the keys to a successful ascent!

Some advice

Drinking enough water, even without feeling thirsty, remains the primary advice!

Always have an insulated water bottle with you or in your backpack with fresh or warm water.

It is also recommended to wear breathable clothing, or layers that can be adjusted for effort, to limit perspiration, protect the skin of your face and extremities from cold and wind, and breathe through a scarf to limit fluid loss.

Finally, the hydrolipidic films of moisturizing creams and balms limit the evaporation of water from the skin. They help, for the former, to protect sensitive areas from dehydration and for the latter, to repair them.