Every time you run you will sweat - when it's hot and humid you will sweat a lot more.
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.
The dangers of dehydration
Dehydration is defined as excessive fluid loss and it can lead to:
- Impaired performance
- Gastro-intestinal problems
- Heat exhaustion
- Impaired circulation (because the blood thickens)
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.
Weight before run
Weight after run
Sweat Rate / Hour
16 fl oz
As you get fitter - your sweat rate will also increase.
How much should you drink?
- 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?
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.
mg/100ml of sweat
maintain fluid volume and balance, muscle & nerve function
55 - 100
regulation of heart beat/rate
7 - 18
nerve impulses, muscle contractions, blood clotting
4 - 6
muscle function/repair & heart rate
1 - 1.5
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.
Your sweat also contains other electrolytes:
as well as small amounts of:
- amino acids
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.
- 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.
- 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.
- Hypertonic - absorbed slower than water. These generally have a lot more salt and carbohydrate in them and are aimed at use after exercise.
How to make your own sports drink
- 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.
HyponatremiaHypo = 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.
- Muscle weakness
- Loss of co-ordination
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.