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The Ultimate Guide to Sweat and Electrolytes for Endurance Athletes

The Ultimate Guide to Sweat and Electrolytes for Endurance Athletes

Master the Art of Hydration and Electrolyte Replacement with Sweat Testing Insights

Have you ever noticed how some people sweat after just a few minutes of exercise while others can run marathons in scorching heat and barely break a sweat? As an intern, I’ve had the opportunity to shadow countless consultations with athletes, and I've seen firsthand the wide range of people's sweating experiences. Some athletes struggle with excessive sweating, leading to rapid fluid loss and electrolyte imbalances, while others hardly sweat at all, even during intense training workouts. 

Regardless of how much one sweats, every athlete needs to understand their sweat. 

Sweat is a natural fluid produced by the sweat glands in our skin, primarily composed of water and salts (electrolytes). It helps regulate body temperature by evaporating and cooling the skin. Understanding sweat helps us discover the correct electrolyte and fluid replacement strategy needed for optimal athletic performance. 

Sweating runner drinking from handheld INFINIT waterbottle

What are Electrolytes?


Electrolytes are substances that have a natural positive or negative electrical charge when dissolved in water. The five main electrolytes include sodium, potassium, chloride, calcium, and magnesium. 

Electrolytes are crucial for bodily fluid distribution in and out of cells and nerve and muscle stimulation. Fluid distribution helps keep the body’s fluid balanced, which aids in maintaining blood pressure for heart health. Nerve and muscle stimulation rely on action potentials which are like electrical signals that travel through your body, carrying messages from your brain to your muscles and back again. They're essential for communication and movement, helping your body respond to its environment and function properly. This assists in muscle contraction for proper functioning, eliminating muscle weakness and cramps.  

Kayla Bowker riding triathlon bike along a river

Most athletes know that taking in electrolytes is important, but why? 


Replacing electrolytes is important for endurance athletes to improve performance, minimize hydration related injury, avoid thermal stress, reduce fatigue, and maintain blood volume. 

The kidneys regulate most of the body's electrolytes. These vital organs consistently filter blood, helping regulate the levels of sodium, potassium, calcium, and other electrolytes in the bloodstream. When there are high amounts of electrolytes in the body, the kidney increases urine excretion to maintain balance.10  


Sodium — The most well known electrolyte.


Sodium is a vital electrolyte and the one most commonly known to athletes, and for good reason. It is the most abundant electrolyte found in the body,  and it aids in regulating water and fluid balance and acid-base balance, as well as stimulating  muscle and nerve cells. An imbalance of sodium in the body can cause muscle cramping and insufficient blood sodium levels.  

Studies show that sodium consumption increases physical performance, decreases blood sodium concentration, and increases fluid in the blood during endurance activities. But if every athlete sweats differently, how do you know how much sodium you need to take in?   

High sodium intake can cause hypernatremia or high blood sodium. Hypernatremia can cause irritability, muscle weakness, and extreme thirst. Low sodium intake leads to hyponatremia or low blood sodium levels. Hyponatremia causes muscle cramps, nausea, and dizziness. In some cases, it can cause a coma or even death. If an athlete is preparing to complete a long-distance race, like a triathlon or an ultra-marathon, they need to understand how their body will respond to the weather and environmental conditions where the race will happen. They should also create a plan to provide the body with enough fluids and electrolytes to avoid dehydration and a condition called exercise-associated hyponatremia, where the salt levels in their blood get too low.18

Performing sweat tests to determine individual sodium losses can be beneficial for tailoring a personalized electrolyte intake recommendation so that one does not consume too much or too little sodium.

Sweating male athlete on an indoor bike trainer with INFINIT towel and bottle

Fluid and Electrolyte Importance

Athletes need to keep their fluid and electrolyte levels in check during their workouts for peak performance. If they don't replenish these, it can lead to issues like muscle cramping, dehydration, and fatigue, all of which can negatively affect performance. 

When the body's fluid levels drop and you become dehydrated, it can raise the body temperature and heart rate, making physical activity harder. Athletes experiencing cramping, dizziness, or irregular heartbeat need to pay attention to these signs and examine their fluid and electrolyte replacement strategy to see if improvements could be made to prevent or reduce these side effects. 

Trail runner with hydration belt running up a steep hill during a race

Understanding Sweat

Sweat is the body’s way of keeping us cool during activity. Every athlete sweats differently, and each individual experiences different fluid and electrolyte loss levels based on genetics, personal fitness, and heat acclimation. Heat, activity type/intensity, and diet play significant roles in electrolyte loss. Sweat can also change based on environment, exertion, and diet. As outdoor temperature increases, sweat rate increases, which typically leads to higher electrolyte loss as well.


Sweat rate is the amount of fluid an athlete loses through sweat during a workout session. 

Sweat composition is the amount of electrolytes lost in a person's sweat during a workout.  


It is essential to replenish fluid and electrolytes in high temperatures. Losing more than 2% of your body weight from not drinking enough can make cardio exercise and thinking clearly more difficult, especially in warm or hot weather. The more body weight you lose via sweat, the more stress you put on your heart and body overall.4

Anyone who’s done a HIIT workout knows your sweat rate increases with activity intensity. But did you know what you eat can also change your sweat composition? A higher-sodium diet results in more sodium in the sweat. The type of exercise you’re doing also plays a role in sweat rate. For example, research shows fluid shifts are more significant during running workouts than cycling.19  

Sweating female runner wearing Nix Hydration Biosensor on her bicep and holding a handheld water bottle

Sweat Testing

With the rise of wearable technology, there has been an increased focus on athlete sweat testing — But what exactly is sweat testing? Sweat testing is a scientific procedure measuring sweat losses, electrolyte composition, or a combination of the two. It can be a valuable tool to help athletes discover their electrolyte and fluid needs.

Sweat testing can help athletes dial in their nutrition. After all, how do you know what to replace if you don’t know what you’re losing? You can test sweat either by looking at sweat from the whole body or just from one specific area.2 For example, scale testing or using a Nix Hydration Biosensor patch. 


How to Calculate Your Sweat Rate at Home

Sweat testing using a scale is a simple, low-tech method you can do at home to determine your sweat rate. In a scale sweat test, you start by weighing yourself, then exercise for a period while monitoring hydration. Afterward, you weigh yourself again and use these weights to calculate how much weight was lost from sweating. Learn more and download guided worksheets to perform a scale sweat test here.  

High Tech Sweat Testing

There are multiple sweat testing options available on the market. These include Nix Hydration Biosensor, Precision Hydration, Levelen, Gatorade, HDrop, and Endurance Edge to name a few. All sweat tests typically recommend conducting a forty to sixty minute workout for the best results. 

The Nix Hydration Biosensor is a small wearable device that attaches to your bicep and connects to an app to evaluate your sweat. It assesses the biomarkers in your sweat and ​​analyzes your sweat from moment to moment. The Nix Biosensor notifies you when to replenish fluids and how much to replenish.12 

The Precision Hydration sweat test is worn on the wrist. This method does not require the tester to actually workout. Instead, the technology in the device pulls out a sweat sample from the skin to analyze the sweat.13 

Levelen's Electrolyte Replenishment System sweat test uses a patch with gauze under it to absorb sweat during a workout. Once the workout is complete, athletes mail in their sample to get their results. In addition to the patch, Levelen also recommends weighing yourself before and after the workout.9

The Gatorade Gx Sweat Patch is a wearable device that is worn on the forearm. After a workout the patch is scanned and loaded onto your cell phone.6

T-shirt hanging on a clothesline with sweat rings

How much fluid and electrolytes should be replaced?

It is crucial for athletes to stay hydrated during activity, but how much do athletes need to replace? First, individuals should start a workout sufficiently hydrated. A simple rule of thumb is to drink about 13 to 20 ounces of fluid within two hours of exercising. 

During exercise, the goal is to drink enough to prevent your body weight from dropping more than 2% to prevent dehydration.15 Once a workout has started, an athlete needs to focus on mitigating sweat losses by consistently sipping on fluids. Light sweaters may be able to get away with simply drinking small amounts of fluid every fifteen minutes of exercise. However, heavy sweaters or those training or racing for multiple hours need to pay attention to their body’s unique fluid replacement needs and plan accordingly. Eventually, you want to aim to replace all the fluid you lose during exercise, but if you can't do it right away, you can always catch up after your workout.

Personal Nutrition Coaching Kit


Taking personalization to the next level

Work with a certified sports dietitian to sweat test, analyze your results, and come away with a foolproof nutrition plan and custom-tailored nutrition formulas based on your data.

Plain water is typically suitable for fluid replacement when exercise lasts less than ninety minutes. During exercise lasting over ninety minutes, carbohydrate and electrolyte drinks provide a crucial source of nutrition to sustain endurance performance. Electrolyte replenishment should reach at least 50% of what is lost in sweat.11 The most accurate way to measure this is through a sweat test. You can also come up with a rough estimate by the amount of salt you see an feel on your body and clothes after a workout — If you feel a grit on your skin or see salt rings on your clothing, odds are you are a salty sweater.

Although many athletes struggle with dehydration, some also can run into overhydration. Drinking too much water (and not enough electrolytes) during exercise can make your blood less concentrated, causing water to move into your cells. This can make the cells swell up, leading to a serious condition called exertional hyponatremia, which can be fatal for otherwise healthy athletes during exercise.1 Symptoms of overhydration include lightheadedness, nausea, and vomiting. The best way to avoid overhydration during exercise is to determine your sweat rate and replace only what is needed. Athletes can also monitor urine color to keep track of their hydration levels. Light yellow is generally a sign of optimal hydration. Darker yellow and browns indicate underhydration, while completely clear is a sign that you may be overhydrated.


The big, sweaty takeaway.

Every athlete is unique. To really optimize a hydration and nutrition plan, athletes need to understand their sweat. Knowing your sweat can provide the pieces of the puzzle and eliminate the guesswork behind electrolyte and fluid replacement strategies. Understanding how to properly replenish what you’re losing in your sweat is the key to astounding performance. 

About the Author

Payton Williamson was a 2024 Dietetic Intern who spent time with INFINIT during her final undergraduate year at the University of Cincinnati and in preparation to become a Registered Dietitian Nutritionist. INFINIT Nutrition is proud to partner with the University of Cincinnati’s Coordinated Program in Dietetics (CPD) to provide our next generation of nutrition professionals an opportunity to spend time with the INFINIT Team and gain real-life experience working at a sports nutrition company.

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