Understanding Sodium, Sweat Rate & Hydration for Runners

Disclaimer

This one has been percolating for a while, I wanted to gather as much info as I could before writing a blog on it, however I'd also like to start with a caveat:

Everyone is different: and what works for one person may well ruin another person's day. I know folks who put extra Precision Hydration salt tablets in their Tailwind and have the best races of their lives, and I know folks who wouldn't go a mile before their body rejected that solution all over the path.

This blog is very much based on my own experiences, and researching around the subject. I hope it helps folks, but please always do your own testing too ❤️.

Background

I suck at hydration. My biggest weakness in any race is a hot day, a high sweat rate and a soul crushing death march.

Due to this, it's something I've spent a lot of time researching. When I was on Will's Adventure Athletes podcast all the Punks joked about the amount of times I'd say "sodium" during the interview (and they weren't wrong 😂).

I'm at a stage now where I understand my body better, understand how much water and salt I need, and due to that feel like I'm better able to tackle the hotter or longer races where keeping your fluid balance in check is essential.

Here's what I've learned.

Why Salt Matters

When you exercise you sweat. Your sweat consists of two things: water, and electrolytes. Electrolytes are minerals in your body such as sodium, potassium, calcium & magnesium.

For the majority of exercise under ~2 hours such as a game of tennis, football, a 5-10k run, it's important to drink water, however electrolyte loss is less of a concern, as you will regain those lost minerals in the food you eat that day.

When you start pushing past 2 hours (I'd say half marathon onwards), that's when we need to start thinking about replacing those lost electrolytes, sodium specifically.

You see sodium helps keep the volume of water in your body stable. Too little sodium can lead to your cells swelling with too much water, eventually, if left out of control leading to a condition known as hyponatremia, one of the more dangerous risks of endurance running.

Drinking too much plain water whilst also sweating dilutes the remaining blood sodium volume in your system. From a performance perspective a low concentration can lead to cramping, reduced blood volume, hindered muscle contraction and even brain fog.

Taking on too much salt can lead to high blood pressure, though this is less of an issue during exercise. The main impact is too much sodium can reduce the effectiveness of the water you are hydrating with (basically you'll be thirstier)

Finding Your Numbers

OK so we've established getting the right mix of water and salt in during prolonged exercise is important. Now the big question: how much do we need?

There are two factors here all runners should be aware of:

Sweat Rate

This is the amount of fluid you lose through sweat during an activity. This is highly individual and is influenced by:

• Gender
• Body weight
• Fitness level
• Intensity of activity
• Environmental conditions (heat, humidity)

For example; on a hot day at an easy pace, you might lose 1L/hour; whilst others may lose as little as 300 mL/hour

This is an important metric, as knowing it allows you to estimate how much water you should be getting into to your system in certain conditions.

To find this out is surprisingly straightforward:

1. Just before you go on a run, strip off and weigh yourself
2. Go for a run (ideally with your clothes back on) for an hour without drinking, eating or peeing in a bush
3. When you get home, strip off, towel all the excess sweat off and weigh yourself again

The difference in weight = water lost during that run. Now you can write down your fluid loss for an hours exercise, noting the conditions (temperature, humidity, terrain, intensity).

If you repeat this in a variety of conditions, you'll begin to build a profile up of how much you tend to sweat.

Sodium Loss Rate

This is the amount of sodium you lose per ml of water you sweat out. Unlike your sweat rate this is not dependent on conditions, the amount you sweat out per ml of water is more static.

It is however highly individual and mostly genetic. Each person has a different rate, with the average begin around 800-1000mg sodium lost for each litre of sweat, but it can range a crazy amount from 200mg to 2000mg.

Unfortunately this is harder to test at home, with no real way to get a more concrete value. If it's something that you feel would be especially useful to you, and often struggle with hydration during races, I'd recommend looking at getting a Sweat Test (Precision Hydration have a service run by local coaches, link here). The test involves putting a sensor on your arm that makes you sweat (no needles!) then tests the sodium concentration in that sweat.

From a DIY perspective your best best is to understand the sodium concentration to liquid of whatever you are currently using (be it a drinks powder, a tablet you drop in your water, salt capsules, chews etc) and keep notes on how it feels during a long run. If you start to feel like getting water in is a struggle, or are overly thirsty, that's a potential sign you've not got the balance right.

Combining Them

Once you know your sweat rate, and have a reasonable understanding of your sodium loss rate (or are happy using the average at ~900mg/1L water), it will help you understand how much water you should be drinking during a race, and what concentration of salt you should be targeting.

I would advise when it comes to salt intake, during a race a little more than you need is better than too little, your body takes time to absorb sodium so it's worth having a bit of a "backlog" in your system for your body to work through.

Also remember your fluid in your bottles during running may be well balanced, but if you drink a big glug of liquid in an aid station it can still dilute your blood sodium levels. It's worth carrying swallowable tablets / chews to take when you take on extra liquids.

Below is some tips and advice on the logistics of this during races.

Available Salt Sources

There's a whole range of options out there for replacing your electrolytes, here's a breakdown of the types and why you may choose one over the other.

• Hydration tables: e.g., SIS Hydro, Precision Hydration Tabs.
  • A great cost effective solution: you pop a tablet in your water, it fizzes up then your water has salts and a bit of flavour
  • Usually taste quite light, a bit like a squash
  • The fizziness of the effervescent tablets can sometimes upset your stomach
• Drinks mixes: e.g, Tailwind, Skratch, Active Root
  • Powder that goes into your water, usually a mix of sugar and salt
  • Can simplify fuelling by reducing what you need to eat due to getting carbs from the drink itself
  • Tend to be quite sweet, dependent on the level of carbs in the drink (Skratch is quite subtle, Tailwind fairly sweet, Active Root extremely sweet)
  • If your body starts rejecting the drink you lose the ability to hydrate, get sodium and fuel all in one, a double edged sword
• Salt chews: e.g, Salt Stick Chews
  • Sweets that are very salty you chew to get your electrolytes
  • A good way to top up your salts if feeling a little low: pro-tip: let them melt in your mouth as salt gets into your system faster through saliva than your stomach
  • Usually quite low sodium, you'd need to eat 18 chews to hit the 900mg/L average sodium loss rate, so more a supplement to your overall strategy then the main driver
• Salt capsules: e.g: Salt Stick Caps, Precision Hydration Caps
  • Swallowable capsules that usually contain ~250mg sodium
  • Easy to carry and very cost effective
  • Slower absorption rate due to your stomach needing to process them

My Sweat Rate Is Too High!

You'll note on my sweat rate log I can go over 1L fluid loss an hour. This is a bit of a problem for 2 reasons:

1. Carrying 1L per hour of running on a long run is challenging!
2. Your stomach can only process a certain amount of water an hour, which is usually about 1L

Thankfully your body does have mechanisms to cope with this. Your body can handle a certain amount of dehydration without any ill effects (subjective but ~2L is a good threshold estimate), so you can factor that into your calculations.

I've resolved to drink 750ml/water every 10k now, which I find easier to reason about from a planning and carrying perspective, then I aim to top up my fluids at aid stations in that rest period before the next section.

Recovering From Low Sodium Mid Race

At some point, you will definitely get this wrong and underestimate your sodium loss during an event. This is a tough state to be in, it is easier to recover from under fuelling then it is to recover from low sodium levels and dehydration.

To start, you'll need to slow down and reduce your sweat rate whilst also helping your stomach recover a bit so it can process your salts and fluids.

Then make sure you get in a bump of sodium: I usually take 2 Salt Stick Caps then also a few chews which I'll keep in my mouth so it absorbs through my saliva.

Salt takes time to process through your body, so you'll need be patient to recover, keep ensuring your getting water in whilst boosting your salt levels, before you start to feel better and can run again.

Conclusion

Hopefully this information is helpful and you can utilise it in your next race.

I'd love to hear any tips or tricks you've found works for you in the comments below 👇👇