Does altitude cause you to dehydrate faster?

By Andy Blow | 8 Minute Read

It was a momentous moment when the 1968 Olympic Games in Mexico took place at an altitude of 2250m (~7400 ft). This was one of the first times that a major international sporting event had been held at significant elevation and, unsurprisingly, the altitude had a pronounced effect on performances in most events.

Notably, athletes went faster in the sprint events and they threw further in the throwing events, with the lower air pressure resulting in less resistance to 'bodies' moving through it at high speed.

As a specific example, a relatively recent paper analysing the 100m results at the 1968 Olympics suggests that it shaved an average of ~0.2 seconds off times for both the men and the women!

But a negative effect was observed in the endurance events, with winning times significantly slower across the board than they had been in Tokyo (elevation 40m / 130 ft) four years earlier, as the table below demonstrates (events shown are men’s track and road running events):

 

 

3,000 m SC

5,000 m

10,000 m

Marathon

Tokyo (1964)

8m 31s

13m 49s

28m 24s

2h 12m 11s

Mexico (1968)

8m 51s

14m 5s

29m 27s

2h 20m 26s

% Change

-3.9

-1.9

-3.7

-6.2

 

A slow-down in times for the endurance events when competing up a mountain would seem obvious nowadays, but far less was known about the effects of altitude on performance in the 1960s. There was even some disagreement at the International Olympic Committee around whether the altitude would make any difference at all in the run-up to the games!

That said, a number of experts with a level of medical knowledge did predict some kind of reduced endurance performance due to the lower partial pressure of oxygen up at 2250m (7400 ft). Extremely little was really known about the magnitude of the effects and how athletes could best prepare for it before the games started.

In the aftermath of the 1968 Games, the resulting analysis of performances stimulated a significant amount of research into the subject area. There has been a great deal of interest in the topic from all manner of perspectives ever since, varying from how to mitigate the performance degradation of having to compete at altitude, to using training at higher elevations to legally boost performances at sea-level.

As an aside, since 1968 no Summer Olympics has been held at a really significant altitude, as demonstrated by the graph below (borrowed from this interesting article on the 1968 games):

 

Summer Olympics Hosts

  

Altitude Training

These days, altitude training is considered a key component of many endurance athletes’ preparation for major competition and there are altitude training bases in numerous locations around the world to cater for this growing market.

Famous examples of altitude training centers include:

  • Flagstaff in Arizona, USA (~2100m / 6890 ft)
  • Font-Romeu in the French Pyrenees (~1800m / 5900 ft)
  • Colorado Springs in Colorado, USA (~1800m / 5900 ft)
  • Iten Village in Kenya (~2400m / 7900 ft)
  • St. Moritz in Switzerland (~1800m / 5900 ft)
  • Sierra Nevada in Spain (~2300m / 7500 ft)

It’s worth noting that these places are clustered either side of the 2000m / 6500 ft elevation mark. This puts them in and around the ‘moderate’ altitude zone and this is probably because athletes have learned that this kind of elevation seems to provide adequate stimulus for the body, without significantly risking high altitude sickness or reducing performance levels to a degree that makes meaningful training impossible.

For context, whilst there aren't any universally recognised definitions to fall back on, ‘low altitude’ is generally said to begin in the 1000-1500m (3300-4900 ft) range (the height at which elevation ‘starts to affect humans’, according to Wikipedia at least).

‘Moderate altitude’ then kicks in above ~1800m (5900 ft). It's up at somewhere between 2400-3000m (7900-9800 ft) that we find ourselves in ‘High Altitude’ territory, which is when the risk of AMS (Acute Mountain Sickness) starts to become more significant.

Beyond 'high', so somewhere north of ~3000m (9800 ft), we get to ‘Very High Altitude’ with the cheerily named ‘Death Zone’ starting at 8000m (26,000 ft).

 

Hydration at altitude

The main challenge faced by the body at altitude is a decrease in the amount of available oxygen in the air, but there are other secondary issues. A few of these issues impact hydration status and they're worth being aware of if you’re heading off to train or compete ‘up there’.

The most significant of these 'secondary issues' for athletes travelling to moderate altitude is an increase in 'insensible water losses'.

Basically, water loss from the body is split into 2 types:

1) ‘Sensible losses’ are things that can easily be perceived by your senses (like sweat and urine production) and can be measured easily.

2) ‘Insensible losses’ on the other hand go largely undetected and are hard to measure. The main routes to insensible fluid loss are evaporation of moisture from the skin and moisture lost from the respiratory tract during breathing.

There are a couple of reasons why insensible water loss can increase quite dramatically when you first go to altitude.

Firstly, your breathing rate increases significantly, even at rest, as your body starts to compensate for the lower levels of oxygen in the blood. More breathing = more insensible water loss.

Secondly, transdermal fluid loss (via the skin) can also be increased in the very dry air.

In addition to insensible water loss increasing, most studies of people travelling to altitude highlight the fact that, during the first few days at least, it is normal to pee quite a lot more (called ‘diuresis’, to use the medical term).

This diuresis is stimulated by changes to the release in hormones that act on the kidneys, causing them to excrete more water and this means that maintaining adequate hydration status becomes even harder than normal.

As a result of these two factors, one recent paper looking at dietary recommendations for cyclists at altitude reckoned that an athlete's baseline fluid requirement could go up by anywhere from ~850 millilitres (~29oz) per day to 1,900ml/day (64oz) due to fluid lost through respiration and extra urination.

This range of additional fluid intake seems to agree with advice on the Institute for Altitude Medicine’s website, which suggests that visitors to altitude should try to consume an additional 1-1.5 litres (34-51oz) of fluid per day.

Whilst you will need more drinks when you go up a mountain, it's definitely worth pointing out that a sudden, massive increase in fluid intake is neither necessary, nor likely to be helpful as it can increase the risk of a condition called hyponatremia, which can be detrimental to both health and performance.

So, although upping your consumption is almost certainly a wise idea, an element of moderation is still important.

 

Drinking at altitude

 

Temperature extremes

Sometimes, visits to altitude also come with exposure to more extreme temperatures and this can exert further pressure on hydration levels.

I can remember doing some cross-country ski training at altitude in St. Moritz, Switzerland, during the winter of 2003/2004. After a few days, my lips dried out and started cracking and I also felt fatigued a lot of the time.

I quickly learned that taking hot blackcurrant squash out in an insulated flask encouraged me to drink quite a bit more during training sessions and that I started to feel much better as a direct result.

Hot drinks are definitely a morale-booster in the cold and make drinking much more pleasant, so they're highly recommended if you're planning a trip to altitude somewhere with sub-zero temperatures.

At the other extreme, I’ve been mountain biking up at moderate altitude in Sierra Nevada in Spain during the super-hot, super-dry summer months, and I took to carrying a bladder of electrolyte drink in my backpack, as well as two bottles on my bike frame for longer days out.

The extra weight was definitely counteracted by the benefit of having the fluids available to compensate for the huge additional sweat and insensible water losses, which were accelerated by the hot temperatures, dry climate and the altitude.

The first few days were only a handful of the few occasions in my life where I’ve felt like I simply could not drink enough each hour on the bike to keep up with my thirst!

 

Sierra Nevada

 

What about fluid balance at very high altitudes? (remember we’re talking about <3000m here)

Although it’s rare that many of us will venture to very high altitudes in the course of participating in endurance sports - ok, so the Ladakh Marathon at 5370m / 17,600 ft is one exception - there is an interesting relationship between fluid and electrolyte intake and altitude sickness when you get up very high.

Basically, when altitude sickness gets really bad it can result in either:

  • HAPE (High Altitude Pulmonary Edema) - the fluid builds up in your lungs 
  • HACE (High Altitude Cerebral Edema) - the build-up of fluid in the brain

Both are potentially life-threatening and there is some speculation that excessive fluid and salt intake could worsen them by encouraging fluid retention in the body. 

Whether HAPE and HACE can be caused by increased fluid and salt intake, or they're more the result of decreased excretion of fluid and salt, doesn't seem completely clear from the scientific literature that I've managed to wade through.

Some sources commenting on appropriate steps to take to minimise the risk of HAPE/HACE cite a ‘low-salt diet’ or ‘low-salt intake’ - along with an increase in fluids - as sensible when ascending to great heights. For a couple of examples, see this paper from 1995 and this 'cheat sheet' for doctors working with those going to altitude.

 

The bottom line

Overall, what's clear is that hydration needs are going to be higher for most athletes ascending to moderate altitude to train or compete than they are at sea level.

This is mostly due to increased 'insensible' fluid loss via breathing and the skin, plus an increase in the amount you pee in the first few days up high.

Compensating for these additional losses with something in the region of 1-1.5 additional litres of fluid per day is generally recommended as a sensible step to take. But overdoing fluid intake can be as much of an issue as it is at sea level, if taken to the extreme, so be sensible.

If you’re exposed to the cold at altitude, then finding ways to get access to warm drinks during training or competition can be helpful in encouraging the consumption of enough fluid.

On the flip side, if it’s hot and dry, making sure you’ve got extra electrolyte-based drinks on hand to help balance out increased sweat losses is a good idea. Additional electrolytes in your drink also help prevent fluid freezing in colder temperatures up at altitude

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