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Do I need a sweat sodium test?

Feb 07, 2024

Do I need more salt? Should I take salt capsules? There is a white crust on my cap, I must need more sodium!

Until recently, there has been little guidance on sodium replacement during exercise. It is now understood that ultra endurance sport is the only situation when a sweat sodium test and targeted sodium replacement may be helpful. It's not as simple as popping a salt capsule every hour, like some sports nutrition companies may claim. But getting it right could make a big difference to your performance and avoiding the medical tent!

What is the role of sodium during exercise?

Sodium is an essential mineral that plays a critical role in various bodily functions, including maintaining fluid balance, facilitating water absorption, regulating blood pressure, and aiding muscle contraction. Normally, your body maintains a tight control over blood sodium levels, keeping them within a narrow range of 135-145mmol/L to ensure optimal functioning.

During exercise, however, this balance can be disrupted as you lose both fluid and sodium through sweat. If you compensate for this loss by consuming a large amount of fluid without sufficient sodium, you risk developing hyponatremia, a condition characterised by a drop in blood sodium levels. Symptoms of hyponatremia can range from gastrointestinal issues, headaches, and swelling of extremities to more severe effects like weight gain, disorientation, muscle twitching, and in extreme cases, coma or death. Notably, up to 30% of participants in endurance events like ultramarathons and Ironman races have been found to have hyponatremia via a blood test after racing, with a higher incidence among female, slower, or less experienced athletes.

Conversely, inadequate fluid intake or excessive fluid loss can lead to hypernatremia, marked by symptoms such as nausea, irritability, lethargy, weakness, confusion, fainting, seizures, and even unconsciousness, primarily due to reduced blood volume and decreased blood pressure. The key, therefore, lies in achieving the right sodium-fluid balance.

In addition to its physiological roles, sodium enhances the flavour of foods and drinks consumed during exercise, which is particularly beneficial in longer events for combating flavor fatigue and maintaining fuel intake — crucial for sustained performance.

Research shows that fully replacing your expected sweat sodium losses does not necessarily correlate with increased fluid consumption or improved hydration, as seen in a study with runners exercising for 5 hours in hot conditions. Interestingly, slightly exceeding your sodium needs hasn't been shown to lead to excessive thirst or fluid intake, though the implications for longer-duration exercises are still not entirely clear and require further exploration.

Current research also suggests that sodium intake does not directly prevent muscle cramps or enhance performance. Instead, the primary benefits of sodium during exercise appear to be reducing the risk of hyponatraemia and in supporting fuel intake by helping to fight flavour fatigue.

Do I need a sweat sodium test?

If you are exercising for less than four hours, a sweat sodium test is unnecessary as your blood sodium will usually rise. Including a fluid with a sodium concentration of 400-500mg per litre may assist in hydration and fuelling through improving taste.

If you are exercising for more than four hours, it is often necessary to replace a large percentage of fluid losses to avoid dehydration. If this is combined with high sweat sodium losses and inadequate sodium replacement, you may be at risk of hyponatraemia. This is when sweat testing may be helpful. If you've encountered symptoms indicative of hyponatremia, which cannot be attributed to other causes, undergoing a sweat sodium test could be a beneficial step in identifying and addressing the issue.

What is involved in a sweat sodium test?

During sweat sodium testing, a sample of your sweat will be analysed to determine the sweat sodium concentration. Easily accessible methods for this include sweat patch testing and biometric sensors. The sweat patch technique is more accurate and can be performed by a trained technician (such as myself). Home kits are available if you cannot access a trained technician, though their accuracy is questionable.

Sweat sodium testing should be conducted under similar conditions to your targeted event. This includes similar food and fluids, weather, pace, duration and clothing. If you will be racing in hot conditions, ideally you would do sweat sodium testing after heat adaptation. It is important to note that there can be significant variation in day-day sweat sodium losses of 15-40%.

How do I use sweat sodium test results?

During exercise, it's likely unecessary to replace 100% of your expected sweat sodium losses. Instead, a more appropriate approach involves using a mathematical formula that considers your individual sweat sodium results. This allows you to tailor your sodium intake more effectively by adjusting the sodium content in the foods and drinks you consume as part of your race plan.

In multi-day events, some level of deliberate sodium replacement within 24 hours is likely required to rebalance your body's sodium levels. It's important to recognise that our bodies have mechanisms to conserve sodium when faced with excessive losses, reducing the need for complete replacement.

Given the challenges of achieving precise, personalised sodium replacement during strenuous activities, adopting general strategies that cater to different levels of sweat sodium losses – low, moderate, or high – is likely the most practical approach for athletes. These strategies allow for a more flexible and realistic management of sodium intake, aligning with the body's natural regulation and the demands of endurance sports.


Sodium likely helps in fighting flavour fatigue, therefore supporting fuelling, which is essential for maintaining energy levels and peak performance. More importantly, it aids in keeping your blood sodium concentration within a normal range, crucial for avoiding hyponatremia, which could lead to negative health implications and associated poor performance.

If you're an ultra-endurance athlete committed to optimising your performance and safeguarding your health, consider undergoing a sweat sodium test, particularly if you are participating in events of 4 hours or longer duration and hot environments. This test will empower you with the knowledge to fine-tune your hydration and nutrition plans, minimising health risks and associated performance losses. Consult with an Accredited Sports Dietitian with a special interest in endurance sports to get started on exploring this aspect of your next big event.

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Cairns et al 2014, 'Incidence of Exercise-Associated Hyponatraemia and Its Association with Nonosmotic Stimuli of Arginine Vasopressin in the GNW100s Ultra-endurance Marathon, Clinical Journal of Sports Medicine, vol. 0, n.0.

Knechtle, B et al 2019, 'Exercise-Associated Hyponatraemia in Endurance and Ultra-Endurance Performance – Aspects of Sex, Race Location, Ambient Temperature, Sports Discipline, and Length of Performance: A Narrative Review', Medicina, doi: 10.3390/medicina55090537

McCubbin 2022, ‘Modelling sodium requirements of athletes across a variety of exercise scenarios – Identifying when to test and target, or season to taste', European Journal of Sports Science, DOI: 10.1080/17461391.2022.2083526

McCubbin & Costa 2023, 'Effect of Personalised Sodium Replacement on Fluid and Sodium Balance and Thermophysiological Strain During and After Ultraendurance Running in the Heat', International Journal of Sports Physiology and Performance, ahead of print.

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