What is a sweat test?

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A closer look at my sodium levels revealed some *interesting* results

Twenty-six kilometres into this year’s London Marathon, I had to stop to be sick on the sidelines. I’ll spare the unpleasant details, but let’s just say it resulted in a very long (and tearful) 16-kilometre run/walk to the finish.

At the time, I wasn’t sure what had caused my nausea – I’d had a good night’s sleep, my training had (for the most part) been strong, and I thought I had my fuelling strategy down to a tee.

Afterwards, a friend suggested that perhaps it had been due to a lack of electrolytes – essential minerals, including sodium, that the body needs to stay hydrated. Sometimes, if sodium levels drop too low, it can cause symptoms such as gastrointestinal distress, nausea, bloating and fatigue.

Since I’m running the New York City Marathon with New Balance this November, I decided to reevaluate my hydration strategy in order to prevent a repeat of London. So, I went for a sweat test at Precision Hydration and it turns out, the results have been pretty game-changing for my training and race day strategy.

'Everyone loses a different amount of sodium in their sweat,' Emily Arrell, sports scientist at Precision Hydration, tells me. 'From as little as 200mg per litre up to a massive 2000mg per litre. This is largely genetically determined and stable over a lifetime.

'A sweat test is useful to find out where an individual falls on this large scale to help them establish the best personalised hydration strategy for them to replace a good proportion of their sweat losses come race day and enable optimal performance.'

'For any event over 1 hour 30 minutes [90 minutes], you need to be replacing the sodium you’re losing,’ she adds.

There’s no need to get your running kit on because Precision Hydration’s sweat test is actually done at rest. First, they get your forearm sweating using a process called pilocarpine iontophoresis – a light electrical stimulation to get your sweat glands going.

Next, they take a sample of your sweat using a watch-like sweat collector strapped to your forearm. Finally, this sample is put through an analyser to find out exactly how much sodium you’re losing.

My sweat test revealed that I lose 1,044mg of sodium per litre of sweat, making me a moderately salty sweater. For context, the average person’s sodium loss is 949mg, so that puts me just above average in terms of sweatiness.

But knowing the saltiness of your sweat is only one part of the equation. Depending on the temperature and humidity where you’re running, your sweat rate will also change.

Why is this important? Well, if it’s a cooler, milder day in NYC, I may only be sweating 0.5 litres per hour, in which case I’d be looking at consuming 500mg of sodium, based on my average sodium loss. If it’s hot and humid, I could be losing upwards of 1.5 litres of sweat per hour, in which case I’d be needing to consume more than 1,044mg sodium per hour.

Arrell explains that it’s pretty easy to measure your own sweat rate:

1. Go for a pee and then record your body weight, ideally with no clothes on.

2. Perform your session (or event) and record exactly how much you drank. If you drink from bottles, weigh the bottles before and after, and record the difference (Z).

3. After exercise, towel yourself dry and then record your weight. Again no clothes on is best.

4. Now subtract your post-exercise weight (B) from your pre-exercise weight (A) to get the weight you lost during the session (C).

5. Now you can calculate your sweat rate, based on your weight loss (C) and the amount of fluid you consumed (Z).

Sweat rate = (C+Z) / time

And hey presto, I have my very own New York Marathon hydration strategy! Here’s how (weather dependent) I’ll be keeping my sodium levels topped up...

1 x PH 1500 with 500 ml of water.

1 x PH 1500 with 500 ml of water, 90 minutes before the start. The aim is to try not to have too much other fluid, especially plain water on top of that 'preload'.

2-3 x 250mg capsules for every 500 ml of water. The volume of fluid/cups will be weather dependent, and based off my millilitres per hour sweat losses.

While accurate sweat testing doesn’t come cheap (you’ll be looking at around £100), it can be a good investment if you’re training for a big event like a marathon, an ultra-marathon, or if you consistently find yourself struggling with nausea, muscle cramping or GI upset towards the later stages of a race.

Alternatively, there is a way you can determine a ballpark figure for how much sodium you’re losing, without taking a proper sweat test. According to Precision Hydration, these are the six things to look out for:

1) Salty marks on your kit/skin

2) Your sweat tastes salty and/or stings your eyes

3) You feel faint or suffer head rushes when standing up quickly after exercise

4) You suffer from muscle cramp during/after long periods of sweating

5) You often feel terrible after exercising in the heat

6) You crave salty foods during or after exercise

The more of these you resonate with, the more likely you are to be a saltier sweater. You can then experiment with taking on slightly more sodium during your training runs to see if it helps your performance. Happy hydrating!

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