by Sandi Nypaver (Special thanks to Dina Griffin, MS, RDN, CSSD for reviewing this post!)
While many marathoners and ultra runners focus heavily on carb intake (and rightly so), the elites at the top of the sport are putting just as much—if not more—focus on dialing in their hydration strategies.
I’m not saying carbs aren’t important, because they absolutely are, but overlooking hydration is a huge mistake.
As a coach and someone who closely follows trends in the sport, I’ve seen how tough it can be to get athletes to recognize just how important and individualized hydration really is. Unlike increasing carbs, hydration isn’t always an easy sell. My hope is that this article helps shift that perspective.
Your hydration plan can make or break your performance, and your sweat is unique. This includes how much sodium you lose per liter of sweat and the amount of fluids you lose per hour—your sweat rate—in different conditions.
What Happens When You Get Dehydrated?
To understand why hydration is so crucial, here are some things that happen when dehydration sets in:
- Your body struggles to regulate temperature, making it harder to stay cool.
- Sweat rate decreases, reducing your ability to dissipate heat.
- Blood volume drops, making it harder for muscles to function efficiently—leading to fatigue and weakness.
- The heart works harder to circulate blood, causing an elevated heart rate.
- Mental sharpness declines.
- Severe dehydration can lead to low blood pressure.
- Reduced blood flow affects kidney function, increasing the risk of rhabdomyolysis (muscle breakdown).
- Dehydration significantly impacts carbohydrate utilization. Since glycogen metabolism requires water, reduced fluid availability can limit how well your muscles access and use carbs for energy.
In short, dehydration slows you down and makes you feel terrible. Now, let’s talk about how individualized hydration needs can be.
Why Your Sweat Loss and Sodium Needs Are Unique
Some people may lose only 200 mg of sodium per liter of sweat, while others may lose over 2,000 mg per liter. (In case it’s not obvious, this is a HUGE difference.) Sweat loss varies just as much: on a humid 70-degree day, some runners may lose only 0.7 liters per hour, while others may lose 2.5 liters per hour. Additionally, your sweat rate on a 20-degree Fahrenheit day will be significantly lower than on an 80-degree day. Humidity, effort, and heat acclimation also affect fluid loss.
According to current research, you want to avoid losing more than ~2% of your body weight from fluid loss.
An Example of How Hydration Can Go Wrong
Imagine a runner who loses 350 mg of sodium per liter and has a sweat rate of 1 liter per hour on a dry, 75-degree day. They run an ultra, consuming 750 mg of sodium per hour but are only drinking 0.5 liters per hour. For a few hours, they might feel okay. But they’re not replacing nearly enough fluids, and a hydration deficit is building.
Since they’re losing 0.5 liters more per hour than they’re drinking, that’s about 1.1 lbs of body weight lost per hour (1 liter of water weighs ~2.2 lbs). Over a 10-hour race, this could lead to an estimated 11 lbs of fluid loss.
Now, the body does slow its sweat rate over time to conserve fluid, so the actual loss may be somewhat lower. You could also factor in things like food intake and the fact that 5–15% of weight loss might come from glycogen and fat metabolism, along with some respiratory water loss during prolonged activity.
Still, even a 6% drop in body weight—about 9 lbs for a 150 lb runner—can cause a major decline in performance, not to mention increased risk of overheating and cardiovascular strain.
Additionally, the runner is taking in more sodium (750 mg) than they are losing (350 mg per hour), which, combined with dehydration, could increase blood sodium levels over time. While hypernatremia (too much sodium) is typically associated with excessive sodium intake and low fluid intake, in this case, both fluid loss and excess sodium intake relative to the runner’s sweat profile contribute to the risk. Their drink mix is far too salty for their needs. So not only are they not drinking enough fluid per hour, but they are consuming a sodium solution that is too concentrated for their body.
Lastly, it’s worth noting that hyponatremia (low sodium) is a much more common danger for runners than hypernatremia. Many athletes think they need to drink excessive amounts of plain water to stay hydrated, but that can dilute their blood sodium levels too much, leading to serious issues like nausea, confusion, and even seizures.
How to Figure Out Your Individual Hydration Needs
1. Get a Sweat Test
A sweat test determines how much sodium you lose per liter of sweat. While it can cost ~$200, it’s a one-time investment. During the test, a small electrode stimulates sweat production on your arm. Your sweat is collected, and a machine provides results on the spot. These results can guide your hydration strategy for years. I’ve also heard that the hDrop hydration wearable sensor is a promising alternative, though I haven’t tried it myself.
2. Measure Your Sweat Rate
Figuring out your sweat rate is fairly easy; it’s just more time-consuming. Basically, you need to weigh yourself naked before a run and then weigh yourself again post-run (wipe off any sweat), also factoring in how much food and fluids you’re consuming and how much you’re peeing. You also need to do it in a variety of temperatures and conditions at different efforts (easy, moderate, hard) so you can see how much you need to drink depending on the situation. I know that sounds like work and it is a little complicated, but there are resources to make it easier for you like the Nutrition Mechanic’s sweat rate spreadsheet which can be found here: https://nutritionmechanic.mykajabi.com/sweatrate
When I had my sweat test done with sports dietitian Dina Griffin, she provided this spreadsheet so that I could input my weight before and after runs, along with fluid intake, to calculate my fluid loss per hour. Since I lose a lot of sweat even on a 55-degree day, she suggested I practice chugging fluids in training to reduce how much I need to carry during races.
Final Thoughts: Why This Matters
Taking the time to understand your hydration needs is worth it. Increasing your carb intake won’t help you if you’re dehydrated. Dehydration can also make it harder for your body to digest the carbs you’re consuming, leading to GI distress. You don’t want to train for months only to have a race ruined by poor hydration planning. Even on training runs, I’ve noticed a huge difference—I feel stronger and a lot more energized during the run and after when I hydrate properly.
- Hydration can impact performance as much as carb intake, especially in ultra marathons.
- Losing more than 2% of body weight from fluid loss impairs performance.
- Sweat loss and sodium needs vary widely—personalized testing is best.
- Tracking your sweat rate and sodium loss can help you dial in your hydration plan.
Take the time to get this right—your future race-day self will thank you!
If this post helped you, please share it with others. That simple act helps us to be able to give out more free information.
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/
