Sodium plays a key role in how your body functions as it helps maintain fluid balance and cognitive function, so it's important to replace the sodium you lose to some extent when your sweat losses really begin to mount up...

Why sodium is important

A 2015 study found that athletes who adequately replaced the sodium lost in their sweat finished a middle distance triathlon an average of 26 minutes faster than those who didn’t.

Whilst that sort of performance gain isn't going to be possible for everyone, it does highlight the potential impact of getting your hydration strategy right.

Your body contains lots of water - 50-70% of it is made up of the stuff in fact, depending on the amount of muscle and fat that you have. Around a third of that water exists outside your cells, in extracellular fluids like your blood.

What does sodium do?

The main electrolyte in this extracellular fluid is sodium and much of your body’s total sodium reserves are found here. This makes it rather ‘salty’ and the total volume of extracellular fluid in your body is directly related to the amount of sodium you have on board at a given time. So, more sodium equals more fluid; less sodium means less fluid.

As well as maintaining fluid balance, sodium plays an important role in the absorption of nutrients in the gut, maintaining cognitive function, nerve impulse transmission and in muscle contraction. Basically, it's pretty darn important.

Most of the sodium we consume is in the form of sodium chloride, or the common table salt found in food and drinks.

We take salt for granted these days as we've developed ways to make it widely available. But in the past wars were fought over access to and the control of salt, which gives you a pretty big clue as to its importance to life!

Because your body can’t produce or store it beyond a certain point, you need to consume sodium every day to keep your levels topped up.

Individual differences in sweat sodium losses

Sweating is the main way athletes lose sodium and fluids during exercise. That's basically why those of us who train regularly have different needs when it comes to replacing sodium than those who don’t.

Everyone loses a different amount of sodium in their sweat. At Precision Fuel & Hydration, we see athletes who lose from as little as 200mg of sodium per litre of sweat to as much as 2,000mg/l. I personally lose ~1,800 mg/l and I often suffered from hydration issues in hot climates as a result. It was my personal search for a solution that led to me founding the company.

Sweat rates also vary from person to person of course; and from situation to situation for any given person (from almost nothing in cooler conditions and at low intensities, to several litres per hour during intense exercise in the heat).

When you combine differences in sodium concentration with those in sweat rates, the potential variance in the total net sodium losses experienced from one athlete to another can be really significant, especially over a middle or long distance triathlon.

And, in a lot of cases, those losses are many times higher than someone who’s not sweating on a regular basis. This is why the standard government guidelines for sodium consumption should be viewed cautiously by athletes. It's more than possible to lose the daily 2,325mg of sodium recommended by the existing government guidelines in just 1 hour of exercise, if you’re sweating heavily and you're sweating out lots of sodium. Your loses during a longer period of exercise really can be massive.  

What happens when sodium losses mount up?

It's impossible to nail down the exact point at which sodium (and fluid) loss through sweating becomes a problem for an athlete. But, it's clear that when losses reach a certain point, the effects can be detrimental to your performance.

Your blood volume is gradually reduced as your sweat losses increase. That’s because sweat is drawn from your blood plasma. This increases the strain on your cardiovascular system, making it harder to pump blood to your skin to cool you down and to your working muscles.

Other issues such as a general feeling of fatigue and muscle cramps can also be experienced if losses are allowed to go uncorrected for long enough, or if significant imbalances between fluid and sodium are allowed to occur.

Up to a certain point, taking in plain water is enough to mitigate sweat losses. But, as those losses start to mount up, you need to replace sodium too in order to avoid your blood becoming diluted.

This is a potentially disastrous condition called hyponatremia, which can certainly ruin your race and, tragically, has even been fatal on occasion.

How much sodium should you replace when sweating? 

Because sweat/sodium losses are so individual, any generic guidelines on the replacement of sodium and fluid should always viewed with suspicion.

Having said that, figuring out whether your net losses are likely to be low, moderate, or high can be a great starting point for honing in on the level of sodium and fluid replacement that'll work best for you in different circumstances.

The two main inputs that drive your personal net sodium losses are...

  1. The total amount you sweat. This is a factor of your sweat rate and the number of hours you spend sweating during a given timeframe.
  2. Your sweat sodium concentration. i.e. how much salt you lose in your sweat.

Figuring out approximately what these are is a sensible place to start.

Calculating your sweat rate can be a bit awkward, but here's a guide to help you get you to a reasonable estimate of how much you're sweating per hour.

Your sweat sodium concentration is largely genetically determined and doesn’t vary much at all which means that, whilst you can only find it out by getting your sweat tested, you only really need to get tested once in most cases.

We offer an at-rest, non-invasive Sweat Test at Sweat Test Centers around the world for anyone wanting to find out exactly how much they're losing.

We also offer a free Fuel & Hydration Planner that'll help you get started with understanding your losses and refining your hydration plan through some good old fashioned trial and error in training.

Further Reading