Why Cyclists Don't Need A Sweat Test?

For a long time, electrolytes have been a key component of many endurance athletes' competition and training nutrition strategies. As a nutritionist, I speak to many athletes who prioritise their electrolyte intake over potentially more important aspects of their nutrition strategy, such as their total fluid or carbohydrate intake during an event. 

There is a huge market for electrolyte-based products and testing services, but in this blog, we'll take a look at the science and examine whether they're really needed.

A recent study that modelled sodium requirement during exercise (PMID: 35616504) showed that due to the hypotonic nature of sweat (i.e. sweat has less sodium in it than blood plasma) as we sweat, the concentration of sodium within the body actually increases and suggested that unless we are exercising for longer than 4 hours, sweating at a rate that can realistically be replaced with sufficient fluid intake (i.e. less than 1800ml/hr) and drinking over 70% of our losses, there is no inherent need to replacement sodium to maintain sodium concentrations within the body during exercise. 

During exercise, we lose both fluid and the electrolyte sodium through sweat. There are five key electrolytes: sodium, magnesium, potassium, calcium, and chloride, which are within the body, but sodium is the only one we lose significantly during exercise. There can be huge variability in sweat sodium losses. Some of us lose very little sodium, whilst others lose huge amounts, as illustrated in the graph below, of sweat sodium data taken from 157 trained marathon runners during a race (PMID: 27478425).  

Due to the huge variability in individual sodium losses, there is interest in personalising sodium intake. After all, we all like to be told we are special and have nutrition strategies that are completely individualised to our own personal needs. 

A sweat test is one of the only ways to determine your individual sodium losses during exercise. Several companies and methods exist to do this. Despite companies willing to take your hard-earned cash, we have almost no scientific evidence demonstrating that personalising sodium intake is more beneficial than taking on a standardised amount (e.g., what you get in a typical sports drink) or taking on no sodium—until now. 

Before we get into the study, it’s important to understand what sodium actually does. Sodium plays a key role in regulating fluid balance within the body. It is reported to enhance fluid absorption and retention when added to sports drinks, and it drives thirst, which can help support fluid intake during exercise and, therefore, support hydration. It also may play a small role in preventing cramps, although cramps are a highly complex phenomenon, and we don’t have clear evidence that sodium plays a key role in preventing them. 

A recent study from Monash University examined the different responses between consuming zero sodium or the amount of sodium required to replace 100% of losses based on a previous sweat test, and the findings give us some amazing insight (PMID: 37944507). 

In the study, participants had an initial screening, during which their sweat sodium was tested during exercise. They then returned to the lab to complete two experimental trials. Before each trial, they consumed a standardised diet for 24 hours with the same amount of sodium both times. Each trial involved a staggering 5 hours of running on a treadmill at 60% VO2 Max in a hot environment (~30оС & ~32% humidity). 

As this was the first study of its kind, the investigators picked two extremes to try and see if there was any effect. Comparing 0 g of sodium to 100% of losses and also including an exercise task in the heat would hopefully lead to an effect (i.e. a big difference between trials). If ever there was going to be an effect, you'd hope to see in in 5 hours of running in the heat. Only a handful of research studies have ever exercised participants in a lab for anywhere close to 5 hours, so this is extreme, and I am certainly glad I wasn’t a participant! 

Based on the earlier sweat testing, they replaced either 0% of their sodium losses or 100% during the trials. They were also given a standardised amount of a carbohydrate drink and additional water to drink based on thirst. This is what they found...

There were no differences between the placebo and sodium trials for

❌ Heart Rate
❌ Rectal Temperature
❌ Thermal Comfort
❌ Rating Of Perceived Exertion
❌ Thirst
❌ Salt Cravings
❌ Physiological Strain Index

They also saw no difference in terms of the voluntary fluid intake between trials (sodium is reported to help increase thirst and drive fluid intake, supporting hydration) or the athletes' hydration status.

The placebo trial resulted in a sodium deficit 24 hours post-exercise, but the kidneys limited this deficit through sodium conservation. Replacing 100% of sodium would be unnecessary to restore sodium balance by 24 hours of exercise.

The study also showed large variability in total sodium losses from the initial testing to the experimental trials, showing that it is very difficult to achieve precise personalised sodium replacement strategies even with testing. This further raises questions about the benefits of testing in the first place. This is likely due to the impact sodium intake in our diet has on our sweat sodium intake (i.e., if we eat more sodium, we tend to lose more in our sweat) and the lack of dietary control prior to the experimental trials. 

Is there any benefit to sweat sodium testing? Based on the current evidence, there doesn't appear to be.

- Coach Ben