Whether you come back from your ride as dry as a bone or covered in white crystals, you’re guaranteed to have sweated, regardless of the duration or intensity.
But could your perspiration, and its concentration of electrolytes – namely sodium (aka salt) – help to guide your hydration strategy, just as continuous glucose monitors such as those made by Supersapiens have done for nutrition and carbohydrate intake?
That’s the sales pitch of companies that offer sweat tests, such as Precision Fuel & Hydration and the wearable hydration sensor Flowbio.
They claim that, because people don’t sweat at the same rate or intensity – as evidenced by the varying array of patches and salt lines post-ride – everyone has a unique sweat profile, and therefore different requirements to stave off dehydration, cramping and the subsequent drops in performance.
But do you really need to fork out hundreds of pounds to find out your sweat rate and sodium concentration, or does the tried-and-tested 500ml of liquids per hour with a run-of-the-mill electrolyte tablet suffice for everyone? I headed down to Flowbio’s south London headquarters to investigate.
Sweat the small stuff
For my test, I rode on a Wahoo Kickr smart bike in zone 2 for 60 minutes in a room at 20°C. I strapped one of Flowbio’s S1 hydration sensors on my upper right arm, and the device continuously tracked my sweat rate and sodium concentration to produce lab-grade data for analysis post-ride.
I know that I’m something of a salty sweater – it can sometimes sting my eyes and taste salty on my top lip – and my 0.76 litres per hour sweat rate and 1,818mg/l sodium concentration confirmed this. But what does that mean in practice?
“[The sweat rate] is within a typical range that we’d see in cyclists,” says performance nutrition consultant Dr Sophie Killer. “[But] there’s such individual variability in sweat rates. Even within a person, sweat rates will vary day by day.
“The reason you lose sweat during exercise is it’s your body’s way of cooling down (through evaporation off the skin) after it’s generated heat during exercise (from working muscles).
"The major influences are environmental temperature and exercise intensity – the longer you work, and the harder the intensity, the more sweat you’ll produce.
"Heat acclimation also causes thermoregulatory adaptations, which include lower core body temperature, lower heart rate, and increased onset and rate of sweating.”
Now I know how much sweat I’m losing, she recommends multiplying total fluid loss by 1.5 to get the amount required for optimal hydration – which in my case is 1.14 litres.
A third of this (380ml) should be consumed per hour of exercise to limit overall dehydration, while the rest (760ml) is required during recovery afterwards.
“The aim of minimising dehydration is to optimise performance, because this can be negatively impacted by relatively low levels of dehydration,” she explains.
Research suggests that just a 2% loss in body mass from sweating can reduce aerobic performance by up to 10%, which would only take two hours for someone who’s my weight (70kg) and has my sweat rate.
Because sweat rate is so variable, Killer warns that it’s important to perform a test in the environment you’re going to be riding in, to limit dehydration and aid recovery.
She adds that if testing isn’t possible, then the 500ml per hour guideline is “sensible” but suggests “listening to your body and not just taking such recommendations at face value – if the race day is particularly hot then adjustments should be made; if gastrointestinal comfort is an issue then you should listen to your body”.
The salt path
With my sweat rate sorted, what does my sodium concentration mean? And could this be the marginal gain that various electrolyte product manufacturers would have you believe – and that would justify the financial outlay of a sweat test?
“Sweat contains lots of different electrolytes, of which sodium (salt) is most abundant,” explains Killer.
“Sodium plays an important role in controlling the total amount of water in the body. After sodium has been lost in sweat during exercise, its replacement is essential for re-establishing blood volume and total body water.
“Many athletes believe that taking on sodium during exercise can help prevent cramping, although the research in this area is inconclusive.
"What is known, however, is that very high sweat sodium losses, which are not replaced over prolonged endurance exercise can lead to dangerously low sodium levels in the blood, known as exercise-associated hyponatremia (EAH),” she says.
Symptoms include weakness, dizziness and headaches, and in the extreme can lead to seizures and even death.
“This is rare but usually occurs when large amounts of water are consumed during exercise and sodium isn’t replaced. Measuring your individual fluid and sodium losses across a range of different environments – particularly in those that you’re planning on racing in and especially if it’s of long duration and in a warm environment – can therefore be hugely beneficial for both performance and health.”
She states that, like sweat rates, sodium losses are highly individual and variable depending on the nature of the exercise and environmental conditions. That said, Killer adds that based on a large study of sweat sodium losses in athletes, I’m above average. So what is her salt-based solution?
Remedial action
“It’s recommended that sodium should be included in fluids consumed during endurance exercise lasting more than two hours. It should also be included in fluids consumed by athletes in any event who lose more than 3g to 4g of sodium in their sweat,” says Killer.
Doing some quick sums based on my sweat test, both are effectively the same for me. My go-to SiS Hydro tablets and their 345mg of sodium are leaving me short by almost 1g per hour, meaning it might be time to up the intake – particularly on longer rides – to minimise the risks of EAH.
“Like any bespoke strategy, knowing your exact needs means that you’re not having too much sodium (potentially bad for health in the long-term) or too little sodium (resulting in delayed restoration of fluid balance in recovery),” Killer concludes.
While that’s the ultimate benefit of having a sweat test, the variability in both sweat rate and sodium concentration means that to get a truly custom hydration plan, a one-off test isn’t going to cut it.
Instead, if you want more data, a sweat-sensing wearable such as Flowbio will provide better insight long-term and enable you to refine your strategy whatever the conditions.
Its price – £329 – will soon pay for itself compared to a standalone sweat test. And if you’re searching for marginal gains, that could be a relatively inexpensive way to keep your performance and recovery at their peak.
Top tip
It’s essential to replace the salts lost through sweating, in particular sodium, so always consume an electrolyte drink during exercise that lasts over two hours
