Nutrition Articles -

Ultra Endurance Racing, Hydration & Hyponatremia

We blogged last week about rhabdomyolysis which is a serious and potentially critical concern for ultra endurance athletes. We've noticed that cases are becoming increasingly common and by spreading the word and better informing athletes, we hope that the number of critical cases can be reduced by early diagnosis. In last week's blog we also mentioned a condition known as hyponatremia and thought we should shed some light on this condition also.

We know that exercise leads to a loss of body fluids via sweating and breathing and this loss of fluid can eventually lead to what is commonly termed dehydration. As athletes, we are all in the habit of carrying bottles or bladders in our packs to ensure that we stay hydrated throughout events, to maximise performance. However, drinking too much can create a condition known as hyponatremia, which can can be critical and potentially fatal. The chance of someone being critically ill due to dehydration is much lower than when they over-hydrate, so let's look at the basics.

What happens when we drink?

When you put fluid into your stomach, it passes through the stomach wall into your blood vessels. As your blood stream can pretty much reach any part of your body, any tissue or any cell, this fluid can be transferred from the blood stream into the tissues or cells.

How does fluid actually pass from one place to another?

To get the fluid from your stomach into your blood stream or from your blood stream into tissue cells requires a process termed ‘osmosis’. Salt acts a bit like a magnet drawing fluid towards it. When you take a drink of water it reaches your stomach and waits to pass through the wall into your blood stream. Your blood is saltier than the water in your stomach and due to the higher level of salt in the blood, the water is drawn towards it. This water becomes blood 'plasma' and travels around your body. If it finds muscle tissue, which has a higher salt concentration than the blood, the 'osmotic' pull of the salt within the muscle will draw the fluid from the blood into the muscle. In simple terms, when something is dehydrated, it becomes salty. By becoming salty this magnetic or 'osmotic' pull increases in power and it attracts water towards it. That’s how fluid movement and hydration works within the body, that’s ‘osmosis’.

So how much should I drink?

Most guides will recommend somewhere between 1 – 1.5 litres per hour depending upon individual sweat rates, but it is unlikely that this amount can actually be absorbed when you are exercising. In past years, athletes were always told 'if you get thirsty, it's too late. Keep drinking so you don't get thirsty'. The problem with this approach is that there's no 'off switch'. Athletes would just keep 'panic drinking' in case they dehydrated. Following a number of deaths at international marathons, due to over-drinking, this advice has now changed and athletes are encouraged to 'drink to thirst'. 

Hyponatremia can be explained in this simple manner:

1. Take 1 medium sized bucket and add 1 teaspoon of salt and 1 pint of water to create a salt solution.

2. Add another pint of water to the same bucket and you have now diluted the salt solution (it’s a bit weaker).

3. Add another pint of pure water to the same bucket and dilute the salt even further. Keep going until the salt solution is so weak you can hardly even taste the salt.

We said earlier in this article that salt acts like a magnet and attracts water towards it:

‘When you take a drink of water it reaches your stomach and waits to pass through the wall into your blood stream. Your blood is saltier than the water in your stomach and due to the higher level of salt in the blood, the water is drawn from the stomach, through the wall and into the blood’

What if you added so much water to your body that the blood wasn’t salty at all, it was diluted so much that it lost all its pulling power?

Salt intake:

Salt intake is a big question for many athletes and the basic guidelines tend to be relatively poor. Salt tablets used to be used on a regular basis 20 years ago and then blood pressure scares, related to sodium, meant that they all but disappeared. The more common advice was that 'you can get enough salt from your normal diet and don't need any more'. However, if you are running in warm weather, you will need to get salt from somewhere as water alone is not enough.

Just to clarify, when we talk salt, the key one is sodium. Steady running on a warm day, you can very easily lose 500mg per hour of sodium and that's the one which really impacts upon fluid movement within your body. Just to give you an idea, a product such as High5 Zero Electrolyte Tabs (the kind which can be dissolved in water) have 200mg of sodium per tablet. So you'd need 2.5 tablets per hour. That's probably more that you thought?

There are other salts, not just sodium!

That's right, there are other salts. There are 4 key salts (electrolytes) and they are sodium, calcium, potassium and magnesium. These are important too as they allow electrical impulses and contractions in heart, nerve and muscle tissue. However, the term hyponatremia actually means hypo (low) natremia (sodium), it is sodium which is most important followed by potassium.

Things to look out for:

  1. If you urinate frequently and it's clear, you may be drinking too much. Constantly urinating clear is NOT a sign of good hydration.
  2. Nausea, headaches, dizziness and vomiting are all common signs of hyponatremia.
  3. A bloated stomach is one of the first signs of hyponatremia. If you are vomiting clear liquid, that is another key indicator.
  4. Use electrolyte tablets in hot weather, check your nutrition products to find out what's in them.
  5. People with hyponatremia often don't urinate, don't confuse this with dehydration.
  6. If in doubt, ask advice. Confusion with dehydration is common and as a consequence, people drink more, making it worse!
  7. Be careful post-event. This is often a time when people 'over-hydrate'.
  8. Same as last week's blog post, if you're taking anti-inflammatory and pain killer drugs such as ibuprofen, you are in the higher risk category. Just don't take them, the risk is too high.

The best advice we can give to avoid any of the cases we have discussed is to monitor your condition throughout events and 'look after yourself'. These issues are generally caused by people not paying attention early in events and then trying to resolve the issue at a later time. Pace yourself correctly from the start, eat and drink small amounts regularly. Listen to your body, monitor you condition and act early rather than reacting when it's too late.

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Marc Laithwaite
The Endurance Store