Alongside pacing and foot care, hydration is one of the most important factors behind success at the MdS. Luckily, it’s something you can get ahead of before you arrive in the desert, simply by gaining a better understanding of the issues you’re likely to face and by having a plan for how you’ll manage them.

Why is hydration so important?

Your blood has multiple roles to play when you’re exercising hard…

  1. It has to be directed to active muscles to supply them with oxygen and energy; and to remove toxic byproducts that would otherwise accumulate.
  2. More of it than usual has to be sent to the skin to help dissipate heat and keep your core body temperature in check.
  3. You need to sweat to increase the effectiveness of this heat transfer from your skin to the environment, and sweat is drawn directly from your blood plasma.

Dehydration increases the stress your body is under during these processes and so hampers performance. That’s why maintaining body fluid and electrolyte levels during exercise is critical. This is especially true in hot desert conditions, where heat dissipation is more difficult and fluid loss dramatically increases.

How worried should I be about being dehydrated at MdS?

Whilst hydration is clearly important for maintaining performance, there are a few things to bear in mind...

  1. Getting somewhat dehydrated is almost inevitable for most people on the move under the midday desert sun. Your sweat rate can be as high as 2-3 litres per hour, whereas your maximal fluid absorption rate is likely to be only 800ml-1 litre per hour. You don’t need a maths degree to realise you’re likely to be fighting a losing battle when it comes to fluid loss.
  2. A certain degree of acute dehydration during stages is not necessarily as bad as you might think. Whilst some widely reported studies in the past suggested that losing as little as 2% bodyweight via dehydration can hamper performance significantly, more recent analysis of athletes in competition settings has shown that many can continue to perform well at 4-6% dehydration, with some elite athletes (such as marathoner Haile Gebresallassie) finishing races in world class times whilst nearly 10% dehydrated! This is not to say that dehydration won’t derail your event if you don’t pay attention to the risk, it absolutely has the potential to do so. Just that, when dehydration occurs acutely (and at relatively moderate levels) during exercise it might not be quite as detrimental to performance as was once thought.

So, should I just drink as much as possible during the event?

Forcing down large amounts of fluid even if you really don’t feel like it is not going to help. So, keep in mind that, just because you are issued a large amount of water each day, this doesn’t mean you have to drink absolutely all of it.

So, how much is enough? It’s reassuring to remember that you have a pretty powerful, innate ally in the fight against dehydration; your thirst instinct. If you pay attention to your body’s own signals and respond to the early signs of thirst you’re unlikely to get horribly dehydrated in a hurry. Yes, you’ll need to be aware of the need to drink more than you normally would when training in the UK, but that doesn’t mean you should stop listening to your body altogether.

The reason to avoid mindless drinking is that excessive fluid consumption can have some pretty nasty effects on your performance and health. You may well have heard of ‘hyponatremia’ as the condition has got more and more press coverage in the last few years. The word literally means “low (‘hypo’) blood sodium (‘natremia’)” and most often results from consumption of fluids to an extent that you actually dilute the concentration of sodium in your blood stream to dangerously low levels. Sodium is an electrolyte critical for a variety of functions in the body, including maintenance of:

  • Cell membrane potentials
  • Nerve impulses
  • Muscle contractions
  • Fluid balance

If you drink far more than you need (without replacing sodium adequately through the foods you eat, or the fluids you take in) it can have very serious side effects. In an effort to maintain blood sodium levels in the face of water overload, the body shifts fluid from the blood stream into its own cells, causing them to swell up. At first this might just result in some slightly swollen fingers, ankles and a general feeling of malaise and lethargy. But, if allowed to progress, can also result in swelling of the brain, headaches, coma and even death in extreme cases. This is a partly why you will be issued with salt tablets with your ration of water at MdS; taking in adequate sodium along with the fluids you consume is helpful in maintaining levels in your blood stream.

The key is to…

  1. Keep your fluid levels reasonably topped up during each stage, whilst…
  2. Being realistic that much of your re-hydration will have to be done between stages

What about electrolytes?

  • Electrolytes are a big part of the hydration equation simply because you lose a lot of them in your sweat.
  • The main electrolyte you lose in your sweat is sodium. It accounts for around 90% of the ions lost because it’s prevalent in your blood plasma, the pool from which sweat is drawn.
  • It is true that you do also lose some calcium, magnesium and potassium, but in relatively trivial amounts, so sodium replacement should be your main focus when it comes to electrolyte replenishment.
  • Sodium replacement helps you maintain blood volume, which in turn helps with management of your core temperature and delivery of blood to working muscles and the skin.
  • Because you lose a relatively large amount of sodium in your sweat, when you’re going at it hard for several hours at a time total sodium loss can be really high. Therefore a reasonable level of supplementation is usually required alongside the fluids you consume in order to keep the body balanced.
  • The interesting thing about sodium loss in sweat is that it varies dramatically from person to person, with some people losing as little as 0.2g per litre of sweat and others losing near 2.0g per litre! This means that different athletes need to take in very different amounts of sodium when sweat output is high. This variance in sweat sodium loss is largely down to genetics. Whatever the cause, it’s clear that a ‘one-size-fits-all’ approach to electrolyte supplementation doesn’t work.

How can I understand my personal hydration needs?

This is something you can do either through trial and error, or by having a Sweat Test.

The ‘trial and error’ route

Key signs that you might be in the ‘high’ or ‘very high’ sweat sodium loss category are things like…

  • Seeing large salty deposits on your skin or clothing after exercise
  • Your sweat tasting salty or stinging your eyes
  • Regularly getting muscle cramps during long exercise sessions
  • Craving salty food during or after sweating a lot.

If you tick any of those boxes, then it’s worth experimenting with more aggressive sodium intake strategies before, during and immediately after exercise to see what effect it has on your performance and recovery.

The ‘Sweat Test’ route

If you prefer a more comprehensive, scientific approach then we offer an Advanced Sweat Test (available at a discount at the MdS Expo again this year, or through one of our test centres - see below for more details).

How can I maintain electrolyte levels during the MdS?

You can replace the sodium you lose during the event in a variety of ways… 

  1. Through your food. Re-hydrated processed foods contain lots of sodium, so things like ration packs are a great way to replenish electrolytes between stages, as is snacking on salty foods during the day.
  2. Taking some of the salt tablets issued at the event to top you up whilst on the course.

3. Taking your own hydration tabs or capsules with your water. The advantage of this over taking the standard issue salt tablets is that you can more accurately measure the amount of sodium you’re taking in, meaning you can tailor your intake to your personal needs. Basing your intake on the levels that have worked for you in key training sessions and build up events is likely to be highly effective. This is why devising and optimising your hydration strategy should be high on your priority list over the next few months.

I hope this post helps you get started with your hydration plan. We’re also giving a free hydration workshop and have a stand at the MdS Expo in London, so if you have any questions about hydrating at the MdS, come and see us. Or email us at

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