By Sandi Nypaver
Special thanks to Dina Griffin, MS, RDN, CSSD for reviewing scientific accuracy
Most runners understand the importance of carb intake during training and racing. What fewer people realize is that your hydration plan is just as important. In real-world endurance events, both performance and gut tolerance often fall apart not because athletes fail to eat enough, but because they become dehydrated or mismatched their sodium intake to their own sweat profile.
Hydration is not a one-size-fits-all strategy. Your sweat rate and sweat sodium concentration are unique to you, and these values play a major role in how well your body regulates temperature, absorbs fluids, fuels your muscles, and maintains steady blood volume during training and racing.
My goal with this article is to help you understand why hydration matters just as much as carbohydrates, how your sweat is uniquely yours, and how you can build your own personalized hydration strategy for better performance and fewer GI issues.
What Happens When You Get Dehydrated?
Even mild dehydration affects performance. As fluid loss accumulates, the following changes occur:
- Thermoregulation becomes harder which increases core temperature
- Sweat rate decreases which makes cooling less efficient
- Blood volume drops which reduces the oxygen and nutrients delivered to working muscles
- Heart rate rises to compensate for reduced plasma volume
- Perceived effort increases at the same pace or power
- Mental sharpness declines
- Severe dehydration can lead to low blood pressure and dizziness
- Kidney perfusion decreases which increases the risk of rhabdomyolysis
- Carbohydrate metabolism becomes less efficient because glycogen breakdown requires water
These physiological changes do not happen in isolation. They compound. The result is a slower pace, earlier fatigue, and a higher chance of GI issues. Even the best fueling plan cannot overcome the impacts of dehydration.
Why Your Sweat Loss and Sodium Needs Are Unique
Two key variables define your hydration needs.
Sweat rate
This is how much fluid you lose per hour.
Typical ranges vary dramatically.
Some athletes lose 0.5 liters per hour while others lose 2.5 liters per hour in the same conditions.
Sweat rate also changes with:
- Air temperature
- Humidity
- Intensity
- Heat acclimation
- Clothing and gear
- Running surface and solar load
- Your hydration status going into the session
This is why repeated testing in different conditions matters.
Sweat sodium concentration
This is how much sodium you lose per liter of sweat.
It is primarily genetically influenced and stays relatively stable for each athlete.
Ranges can be as low as 200 mg per liter or as high as 2000 mg per liter.
This is a ten-fold difference.
Heat acclimation and consistent training can reduce sweat sodium concentration slightly because the body becomes more efficient at reabsorbing sodium in the sweat glands. However, these changes are moderate. An athlete losing 1400 mg per liter will not suddenly become a 300 mg per liter athlete through training alone.
Understanding your personal sodium concentration is one of the most powerful steps you can take to avoid both under-replacement and over-salting during long events.
Understanding Sodium’s Role
Sodium supports performance in several important ways.
• Helps maintain blood volume
• Supports fluid absorption in the small intestine
• Prevents excessive dilution of blood sodium during long events
• Helps preserve neuromuscular function
• Improves palatability of drinks which encourages adequate intakeSodium does not prevent cramping on its own but it supports the whole system in a way that reduces the conditions where cramps are more likely.
How Much Dehydration Can You Tolerate?
It is often recommended that endurance athletes try to keep body mass losses within 2 to 4 percent during long events. For many athletes, losses beyond this range are associated with greater performance impairment and an increased risk of heat-related issues.
However, well trained or well heat adapted athletes sometimes tolerate losses up to 5 to 6 percent without significant performance decline. These athletes are the exception, not the rule. For most runners, even modest dehydration feels noticeably harder.
What matters most is understanding your own tolerance and planning hydration so fluid loss stays within your safe range.
An Example of How Hydration Can Go Wrong
Imagine a runner with the following sweat profile:
Sweat sodium concentration. 350 mg per liter
Sweat rate. 1 liter per hour in 75 degree dry heat
Their race-day plan looks like this:
Fluid intake: 0.5 liters per hour
Sodium intake: 750 mg per hour
Where the problems begin:
This athlete is losing 1 liter of sweat per hour but drinking only half that amount. This creates a fluid deficit of 0.5 liters every hour. Over ten hours, this would be an estimated eleven pounds of body weight lost. Even accounting for glycogen use and metabolic weight changes, this is well beyond the range most athletes can tolerate without a significant decline in performance.
At the same time, the athlete is consuming more sodium per hour than they are losing through sweat. Combined with dehydration, this increases blood sodium concentration and raises the risk of hypernatremia, which occurs when sodium levels in the blood become too high.
This example shows why pairing high-sodium products with insufficient fluid intake can create problems even for low sodium sweaters.
A Second Practical Example. Why Two Runners Can Need Completely Different Sodium Amounts
Two runners head out for the same three hour long run.
They run at similar intensities and sweat the same amount.
Their sweat rate is 0.7 liters per hour which equals a total of 2.1 liters of sweat lost.
The only major difference between them is their sweat sodium concentration.
Runner A
Sweat sodium concentration. 450 mg per liter
Total sodium lost. 945 mg
Runner B
Sweat sodium concentration. 1300 mg per liter
Total sodium lost. 2730 mg
Even though they ran side by side and sweated the same volume, Runner B lost almost three times as much sodium.
Sports dietitians generally recommend replacing about 50 to 80 percent of sodium lost during longer training sessions and races.
Which means:
Runner A typically needs 470 to 750 mg total
Runner B typically needs 1360 to 2180 mg total
Their hydration plans should look completely different.
Runner A may do well with a moderate sodium drink mix or occasional electrolytes.
Runner B may need a higher sodium concentration or more frequent electrolyte intake to maintain blood sodium levels, support fluid absorption, and keep performance steady.
This example shows why knowing your sweat sodium concentration helps you avoid both under-salting and over-salting. Two athletes can run the same route, in the same weather, at the same pace, yet require very different hydration strategies.
A Note on Lab Tests vs Real-World Wearables
In fall 2024, I completed a Precision Hydration sweat test and received a baseline result of 573 mg of sodium per liter. This value represents my physiology under controlled conditions. Since then, I have used the hDrop hydration sensor during more than ten training runs, where it consistently reports 850 to 1050 mg/L.
Precision Hydration’s internal testing shows that hDrop tends to overestimate sodium by about 170 mg/L and underestimate sweat rate by about 0.5 L/h. Sweat sodium concentration also naturally increases during harder or hotter sessions because sweat glands have less time to reabsorb sodium.
When adjusting for device tendencies and real-world intensity, my “working” sweat sodium concentration during training is likely closer to 650 to 900 mg/L.
For my own strategy, I use:
• Lab value (573 mg/L) in cooler or moderate sessions
• Adjusted range (650 to 900 mg/L) in heat or higher-intensity conditions
• Upper range approaches ~1000 mg/L only in demanding long runs in hot weather
Both the lab test and the wearable data are helpful. Trends across conditions matter more than any single number.
Hyponatremia
Hyponatremia is a condition where blood sodium levels drop too low. While athletes often worry about not getting enough sodium, hyponatremia is far more commonly caused by drinking more fluid than you are losing, especially when that fluid is mostly plain water.
When you overconsume fluid relative to your sweat rate, blood sodium becomes diluted. Symptoms can include nausea, headache, swelling in the hands or face, confusion, and in severe cases seizures or loss of consciousness.
Hyponatremia does not develop from failing to consume high amounts of sodium. It develops when fluid intake exceeds your body’s ability to clear that fluid. This is why knowing your sweat rate is just as important as knowing your sweat sodium concentration. Matching both appropriately keeps you in a safe and optimal range.
How to Figure Out Your Individual Hydration Needs
Step 1. Determine your sweat sodium concentration
A professional sweat test provides individualized data that rarely shifts outside of moderate heat adaptation. Wearables such as the hDrop can provide useful trend data during real-world training.
Typical ranges
- Low. 200 to 700 mg/L
- Moderate. 700 to 1100 mg/L
- High. 1100 to 1500 mg/L
- Very high. 1500 to 2000+ mg/L
This number helps guide sodium intake rather than relying on generic recommendations.
See how mine and Sage’s sweat test with Dina Griffin went here.
Step 2. Measure your sweat rate in different conditions
The simplest and most accurate way to measure sweat rate is by weighing yourself before and after training.
- Weigh yourself naked before your run
- Track all fluid intake
- Weigh yourself naked again after your run
- Subtract post-run weight from pre-run weight
- Add the amount of fluid consumed
- Subtract urine output if applicable
- Divide by hours exercised
Sweat rate = fluid lost per hour.
It is helpful to repeat this in several environments since sweat rate changes significantly with temperature, humidity, and intensity.
You can also use wearable devices such as the hDrop hydration sensor to gather sweat rate data. These devices can provide real-time estimates, though it is still valuable to compare wearable data with the pre- and post-run weigh-in method for accuracy.
Here is the Nutrition Mechanic sweat rate spreadsheet to make tracking easier:
https://nutritionmechanic.mykajabi.com/sweatrate
Step 3. Estimate your hourly losses
Once you know your sweat sodium concentration and your sweat rate, you can estimate what you typically lose during training.
Fluid loss per hour
This is simply your sweat rate.
If your sweat rate is 1 liter per hour, then you lose about 1 liter of fluid per hour.
Sodium loss per hour
Multiply your sweat sodium concentration (mg/L) by your sweat rate (L/hour).
For example, if you lose 700 mg per liter and you sweat 1 liter per hour, you lose about 700 mg of sodium per hour.
These hourly estimates do not need to be perfect. They simply give you a clearer sense of your typical losses so you can match your fluid and sodium intake more effectively.
Step 4. Match intake to your needs
General guidelines used by many sports dietitians:
For long events
Replace about 50 to 80 percent of sodium lost
Fluid replacement
Usually 60 to 90 percent of sweat rate depending on conditions and GI tolerance
Most sports drinks
Contain 400 to 1000 mg of sodium per liter
More on why you should not aim for 100 percent like-for-like fluid replacement can be found here.
Step 5. Practice in training
Practice combinations of:
- Fluid volume
- Sodium concentration
- Drinking frequency
- Fuel timing
Your best race-day plan is built through consistent training, not guesswork.
Common Hydration Mistakes
Assuming more sodium is always better
Drinking too little because carrying bottles feels inconvenient
Using the same plan year round
Overdrinking plain water which increases hyponatremia risk
Using drink mixes that exceed your sodium needs
Not practicing high fluid intake in training
Trying new products on race day
Understanding the Hydration and Carbohydrate Connection
Carbohydrate metabolism requires water.
If you are dehydrated, your muscles have less access to stored carbohydrate and your gut becomes less efficient at absorbing fuel. This is why dehydration often feels like “my stomach shut down” even when carb intake is appropriate.
Your hydration and fueling plans work together.
Final Thoughts: Why This Matters
Hydration is not secondary to carbohydrates. They are equal partners in performance. Understanding your sweat rate and sweat sodium concentration gives you the tools to create a personalized hydration strategy that supports your training, keeps you safe, and helps you perform at your best.
Taking time to get hydration right pays off on every long run and every race.
If this article helped you, sharing it supports our ability to keep offering free education through Higher Running.
Resources:
Dina Griffin’s website: Nutrition Mechanic
Find a sweat test near you: Sweat Testing Locations (Or just do a quick internet search!) Dina is a great choice if you’re in or near Boulder, CO. Sage and I drove 3 hours just to see her.
https://pubmed.ncbi.nlm.nih.gov/12834575/
American College of Sports Medicine position stand. Exercise and fluid replacement
Exercise-induced hypohydration impairs 3 km treadmill-running performance in temperate conditions
Dehydration and endurance performance in competitive athletes
Drinking behaviors of elite male runners during marathon competition