Dehydration and sodium: why replacing salt is crucial

Your body contains lots of water, 50 to 70% of it is made up of the stuff in fact, depending on the amount of muscle and fat that you have.

Around two-thirds of that water is inside your cells (as intracellular fluid - ICF) and the remaining third outside your cells (in the extracellular fluid - ECF).

Your blood accounts for 15 to 20% of this extracellular fluid (about 5 litres in the average sized adult) with interstitial fluid, lymph, saliva and gastric juices making up the rest.

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The main electrolyte in this extracellular fluid is sodium; and much of the 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; i.e. more sodium equals more ECF, less sodium equals less ECF.

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 really darn important.

How and why do we lose sodium?

Most of the sodium we consume is in the form of sodium chloride (NaCl), or the common table salt found in food and drinks. When you lose extracellular fluids, when you go to the toilet and when you sweat, you lose sodium from your body.

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

Sweating is the main way athletes lose sodium during exercise. (The same applies for fluids too). That's basically why those of us who train regularly have different needs when it comes to replacing sodium than those who don't.

The amount of sodium in sweat, and the total volume of sweat lost, can vary dramatically from athlete to athlete. At Precision Hydration, we see up a wide variance in the sweat sodium concentrations (i.e. the amount of sodium in sweat) of the individuals we test.

We see athletes who lose as little as 200mg of sodium per litre of sweat to those who lose up to 2000mg/l. I personally lose 1842 mg/l and I often suffered from hydration issues in hot climates as a result.

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The sweat rates from individual to individual can also vary of course; as it can 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 net sodium losses experienced from one athlete to another can be really significant.

And, in a lot of cases, those losses are many times higher than someone who is not sweating on a regular basis. This is why the standard government guidelines for sodium consumption should be viewed cautiously by athletes who train a lot.

It's more than possible to lose the 2,300mg 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.

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.

Blood volume is gradually reduced as sweat losses increase, 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 like a general feeling of fatigue or malaise 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, 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 salt should you be taking in?

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.

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

Calculating the volume of sweat you lose can be a bit awkward and hit and miss, but there are plenty of online calculators that get you to a reasonable estimate.

Your sweat sodium concentration is genetically determined and doesn’t vary much at all (we’ve tested pro athletes playing in frozen Michigan, then retested them years later in the humidity of Florida and seen consistent results, for example), which means that, whilst you can only find it out by getting sweat tested, you only need to get tested once. (For full disclosure, my company offers an exercise-free, non-invasive sweat test).

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