What is in your sweat

Hydration & the Importance of Electrolytes

When you run you will sweat - more so when it's hot and humid.

Everybody knows that - it's no secret. But not many people understand what's in your sweat and the importance of maintaining a balance of those vital ingredients in your body.

Sweating is our bodies main way of cooling off the heat generated by our muscles when we exercise. We all know that sweating excessively will make us thirsty, as our bodies will seek to replenish the lost fluid. But there's more to being well hydrated than just gulping down water. Unfortunately it doesn't involve glugging large quantities of beer or prosecco either...

Water is vital for all life on earth. It's what makes our planet different from every other known to man. We are also about 70% water. So making sure we keep our internal water supplies are topped up during exercise is a good idea.

The dangers of dehydration

Dehydration is defined as excessive fluid loss and it can lead to:

  • Impaired performance
  • Gastro-intestinal problems
  • Heat exhaustion
  • Hallucinations
  • Impaired circulation (because the blood thickens)
  • Collapse
  • Death

All of the above are things most folks will want to avoid - whether they're strapping on a pair of running shoes or not.

So how do you play it safe when running and make sure you...

  • Make sure you're well hydrated before you start a run. Even if this means delaying the start of the run, so that you can have a drink and give it a few minutes to percolate into your body.
  • Aim to replace at least 80% of sweat loss during a run (you'll need to calculate your sweat rate for this).
  • Aim to top-up the other 20% after finishing your run (preferably gradually - not all in one gulp).

How to calculate your sweat rate.

I suggest you do this test before a run of about 1 hours duration, because it will make calculating your sweat loss/hour a lot easier. Trying to gauge your sweat rate on much longer runs (>90mins) is not recommended, as glycogen depletion will become a factor in the weights lost and could skew the figures significantly. On longer runs you will also need to make detailed notes of how much you drink and urinate. For a run of one hour - you should be able to complete that without needing to drink or have a loo stop.

  • Weigh yourself before you run. Ideally naked or with minimal clothing/underwear. Use your discretion if you're using a 'public' weighing scale - unless it's located in a nudist camp.
  • Run for about 1 hour
  • Weigh yourself after the run (again ideally naked or with minimal clothing/underwear). You should also towel off any sweat, so your skin and hair are reasonably dry.
  • Calculate your weight loss, then use that figure to calculate your sweat loss.
1L of water (sweat) weighs 1kg or 2.205 lbs

Example Calculation:

In imperial and metric units of measure

Weight before run

Weight after run

Weight loss

Sweat Rate / Hour

170 lb

169 lb

1 lb

16 fl oz

77.11 kg

76.66 kg

0.45kg

450 ml

1 lb or 0.45 kg is about 16 fluid ounces or 450 ml of water.

As you get fitter - your sweat rate will also increase.

So it's worth bearing that in mind and maybe re-calculate your sweat rate as your training progresses and when the seasons change.

Winter sweat rates can be 30% less than in the Summer.

How much should you drink?

This will depend on a number of factors including your own individual sweat rate, the temperature, humidity and your rate of exertion - because the harder you work - the more you sweat.

But I do have a few general rules of thumb:
  • For runs of less than 1 hour - drink before and after the run - no need to take a drink with you.
  • For runs between 1 and 1 and a half hours - take a drink with you - just water should be ok. Unless it's very hot/humid &/or you're a heavy sweater - in which case a sports drink with electrolytes in it would be better.
  • For runs over 1 and a half hours - definitely take a drink with you and get some electrolytes on board too.

Why isn't water always enough?

If your sweat has dripped into your eyes when you're running - you know it stings and if you've ever tasted it - you know that it can be very salty.

That's because sweat is not just water. Sweat also contains electrolytes.

Electrolytes are minerals that carry an electric charge (+ve or -ve). They're essential for many bodily functions including creating strong nerve signals, muscle contractions and maintaining correct PH balance. 

The table below shows the 4 most important electrolytes in our bodies, what they do and approximately how much is lost in 100ml (3.5 fl oz) of sweat.

Electrolyte

Key Function

mg/100ml of sweat

Sodium

maintain fluid volume and balance, muscle & nerve function

55 - 100

Potassium

regulation of heart beat/rate

7 - 18

Calcium

nerve impulses, muscle contractions, blood clotting

4 - 6

Magnesium

muscle function/repair & heart rate

1 - 1.5

As you can see electrolytes are very important for exercise/running because they are critical for making sure that your muscles keep contracting and that your heart keeps a regular and appropriate rhythm. Excessive losses of electrolytes over prolonged periods will have a negative impact on your ability to keep running and to perform generally.

Electrolyte replacement

This is why it's important to replace the electrolytes lost through sweat as we run. 

Failing to do so will potentially result in a drop in performance and you'll most likely feel like crap too.

It was believed that lack of electrolytes caused cramp. More recent studies have pointed to other more likely causes for runners.

SODIUM makes up a far greater proportion of our sweat than any other electrolyte. This is why your sweat tastes salty. The concentration of sodium and other electrolytes in your sweat is what creates the white tide-marks on your running gear after a long run too. As the salts crystalize after the water in your sweat evaporates. These marks are more noticeable on dark kit - like the picture below of my hydration back-pack after an 18 mile run I did a couple of days ago. Which was also the trigger for writing this blog post...

Your sweat also contains other electrolytes: 

  • chloride 
  • phosphate 
  • bicarbonate

as well as small amounts of:

  • amino acids 
  • lactate  
  • urea.

Yes - the last one is the same substance found in your urine, but in much less concentrated form. 

The amount of electrolytes excreted in sweat also varies from person to person and will in part depend on how much of each mineral you consume . Your body is always striving to maintain the ideal balance. So, if you consume too much salt/sodium, it will attempt to get rid of the excess through your urine &/or sweat.

Sports Drinks

Most modern 'sports drinks' like lucozade sport and gatorade contain a mix of electrolytes and have been designed specifically to aim to replace the electrolytes lost during running. Be aware that they all have a slightly different mix and will also most likely contain some carbohydrate (usually a mixture of fructose, glucose or maltodextrin) for an added energy boost.

There are 3 main types of sports drink available:
  1. Hypotonic - absorbed quicker than water. These are designed to rehydrate you quickly and usually don't have much if any carbohydrate in them. They also have less electrolytes than blood.
  2. Isotonic - absorbed at the same rate as water. These are designed to be as close to the electrolyte balance of blood as possible and usually have added carbohydrates too.
  3. Hypertonic - absorbed slower than water. These generally have a lot more salt and carbohydrate in them and are aimed at use after exercise.
A drink that is close to your electrolyte balance (Isotonic) is best for long runs.

If you don't want or need the added calories of carbohydrate in a sports drink, there are other options available like salt/hydration tablets. These are designed either to be swallowed whole or dissolved in water. The dissolving type usually have flavourings and colouring added to them - to make them look and taste better. You should bear this in mind when choosing, as many include artificial sweetners. Sweat obviously doesn't contain these ingredients, they're totally surplus to requirements and could upset your delicate tummy. I personally find the taste of most artificial sweetners to be revolting. Though probably preferable to drinking my own sweat - just... 

How to make your own sports drink

Buying large quantities of sports drink can prove to be expensive and it's not always available when you travel. But you can make your own relatively quickly and cheaply. Just mix the following ingredients together and you're good to go.

  • 500ml water
  • 500ml fruit juice (any fruit juice you like the taste of)
  • 1/4 teaspoon of salt

There is a lot of guff on the internet about how coconut water is a great sports drink - because it contains carbohydrate and electrolytes.

Whilst this is true. The balance of electrolytes is not a close match to sweat. As noted in the table above around 80%+ of the electrolyte content of sweat is Sodium. This is therefore by far the most important electrolyte to consume when exercising. 

Coconut water contains primarily Potassium - more than 10 times the concentration found in human sweat. It does contain Sodium - but only about one tenth of the amount found in sweat. Coconut water does not contain any Calcium or Magnesium.  

So drinking just coconut water during long runs in the heat - whilst sweating heavily could lead to an electrolyte imbalance - most seriously a lack of Sodium (hyponatremia).

Lets be clear - I'm not saying that coconut water is unhealthy - far from it. It has many heath benefits and is a highly nutritious and tasty beverage. If you enjoy it, please feel free to keep drinking it. But what I am saying is that you should avoid using it as your sole source of hydration/electrolyte replacement for long runs. 

Hyponatremia

Hypo = low.
Natremia = sodium in the blood.

This is an imbalance in the concentration of sodium/salt in the body. It can happen when you sweat out/loose electrolytes over a long period and don't replace them, but do drink water or other fluids. This results in the remaining sodium in the body becoming too diluted. Which interferes with the important jobs that electrolytes perform.

Symptoms include:

  • Muscle weakness
  • Loss of co-ordination
  • Disorientation
  • Incontinence
  • Seizures
  • Coma

Hyponatremia can be fatal, though it is a relatively rare for a runner to die of hyponatremia - it can and does happen. The longer the race and the more extreme the temperature/humidity - the greater the risk. Recent testing of Frankfurt ironman finishers showed that 10% of them were suffering from some degree of hyponatremia. 

So the longer you're out running in the heat and the more you sweat - the more important it is to get some electrolytes (especially sodium) down your kneck.

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About the Author Coach D

Hi, I'm Dave. I'm a UK Athletics qualified and licensed Coach in Running Fitness (CiRF) and Endurance Event Group Coach. I coach groups and individuals of all abilities both online and in person.I particularly enjoy coaching beginner and improver runners in the 40+ age range.I'm also a regular recreational runner and I've been competing in races from 5k to marathon distance for over 30 years.

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