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Hi,
With all of the talk about carbs, it’s easy to forget this simple principle: hydration is far more important for endurance performance.
Take away my gels, and I’ll survive (barely). Take away my drink mix, and I might actually die (walk/jog/cramping to the performance grave).
My 2 favorite drink mixes that I take every day:
- Tailwind Nutrition Endurance Fuel (especially Dauwaltermelon, which is like Courtney’s smile in drink form)
- Skratch Labs Hydration Sport Mix (especially Lemon Lime, which makes my old cramps crap their pants from fear)
Let’s talk real-world performance hydration.
Hydration is an area with lots of infuriating advice. I think it was catalyzed by some books and papers in the early 2000s that talked about “drinking to thirst” as a valid way to approach performance. I came into endurance sports right around that time, and I was a trusting little lemming, so I applied that advice.
I kept having major issues in races over 1 hour, and I couldn't figure out why. It all came to a crappy climax when I had to go to the ER after a race. An IV saved me, but in extreme pain and literally fearing death, I said some mean things to a nurse who was just doing her job, and I still wake up at midnight with anxiety about that.
I learned the error in my under-hydration ways, yet the “drink to thirst” advice was so ingrained in me (along with general guidelines of 8-16 oz of fluid per hour) that it would take years until I figured it out fully. Not only do I need to hydrate, I need to hydrate A LOT.
An underfueled athlete can finish a race. They might even win if they are very talented. A dehydrated athlete will be lucky to finish (the same goes for an overhydrated athlete, who could also face serious health issues, which is why this is so tricky). Yet hydration is still rarely talked about as the pillar of performance it is.
I think that offset can be explained by 3 factors:
- First, we're all scared of the vocal minority who don't understand elite performance coming out of the woodwork to say terrible things about us if we put hydration on the pedestal it deserves
- Second, athletes have wild variations. Whereas carb needs during exercise are relatively stable (with some variance for body size and power output), hydration needs might vary from 10 oz per hour all the way up to 48+ oz per hour. The same goes for electrolyte needs. Any blanket recommendation is wrong as it comes out of your mouth, and overhydrating can be extremely dangerous, so it’s easier to just stay out of it.
- Third, here's a weird wrinkle I have seen in practice. In the old days, marathons and ultras selected for athletes with anomalously low sweat rates. Therefore, the best of the best generally didn't need much fluid or electrolytes, so it created a crappy feedback cycle where drinking-to-thirst hydration recommendations seemed to have real-world backing.
Those three points also underscore why this email risks falling into a black hole of disclaimers. Hydration is all about calibrating your own physiology with the understanding that it's impossible to tell where you fall on the genetic spectrum based on other attributes, like speed or body size or even how much salt appears on your workout clothes. At the Javelina 100 Mile in 2024, I took in 56 oz of fluid per hour when it was 100 degrees F. Anecdotally, I have heard of other athletes winning that race with 1/3 the fluid intake.
To summarize the disclaimers: don't follow any of this advice. I am just a running coach with a keyboard and a dream!
A more serious disclaimer: given the genetic variation, it's best to work with an expert in nutrition who can help you dial this in precisely.
But enough of that.
This is where the best nutrition experts are also pushing our understanding forward. As an example, Vic Johnson has helped revolutionize the game in ultras with his nutrition expertise, working with athletes like Tara Dower. We are friends, and his only text to me before my first Leadville 100, knowing nothing specific about me:
“Good luck tomorrow David! Rooting for you. Hydration is the key that unlocks everything. You stay hydrated, you’re able to keep slurping gels.”
We had only texted a few times in the year since we met. He might have changed my life with that text. So consider that the syllabus header: HYDRATION IS THE KEY THAT UNLOCKS EVERYTHING.
Sweat Rate
Sweat rate varies from under 0.5 L per hour (16 oz) to 2.5 L per hour (80+ oz), depending on effort level. But it's not as simple as replacing sweat loss on a 1:1 basis--that will almost certainly result in overhydration. And that's the big fear here: overhydration carries extreme risks, and athletes have died from hyponatremia.
Precision says that the normal range for sweat rates is generally 1-1.5 L per hour, and that overlaps with what I see. So let's use that for our basic recommendations (temperature, humidity, effort, and tons of other factors play a role here, so take it all lightly):
- Starting point: 16-24 oz of fluid per hour during high effort racing with electrolytes--proper electrolyte ratios are absolutely key for performance and health (more on that in a second).
- If you are a light sweater, dip that down to 12-16 oz per hour.
- If you're a heavier sweater, you might want to try 24-32+ oz per hour.
From that starting point, make changes based on how you feel, the temperature, and the effort level. My personal starting point is 24-32 oz per hour. At Leadville, I went from 24 oz per hour in the cold morning up to 32 oz per hour in the heat on Hope Pass, down to 16 oz per hour at the end when my effort was downregulated and my heart rate dropped. (Anecdotally, I see sweat rates drop after a few hours, so make sure you aren’t applying your fresh numbers to your fatigued numbers without letting feel guide you.)
There are athletes out there who would literally die with my hydration approach. Even for myself, I overlearned this lesson at Western States in 2025, overhydrating in the early parts of the race (likely due to extensive heat training that altered my sweat rate in the cold mountain weather), which led to me dialing back gels to deal with stomach issues, which contributed to me having a panic attack at mile 62. Please please be careful and start where you currently are, rather than trying something new without data.
Electrolyte Content
Here's the other tricky part of hydration: athletes vary wildly in the composition of sweat, with Precision's tests showing athletes ranging from 200 mg sodium per liter of fluid to 2000+ mg sodium per liter fluid. They make very simple tests that I recommend, and companies like Nix make biosensors that are progressing rapidly.
- Starting point: Median levels are generally 800-1000 mg per 32 oz fluid.
- Without testing, going a bit lower than that median range might be best to avoid over-salting, adjusting upward toward the median based on feel
- (under-salting with high hydration is the main cause of hyponatremia, which can result in bad health outcomes including death. Over-salting can cause hypernatremia, which isn't fun either)
I love Tailwind (310 mg sodium, 25g carbs per scoop) and Skratch (200 mg sodium, 20g carbs per scoop) because they make it easy to balance the hydration AND fueling equations. Plus, there is some evidence that these types of mixes absorb better than plain water.
Putting It Together
If you're a low sweater, you probably can just drink to thirst, like the superstars who were selected for that prowess in previous generations. Skratch is usually perfect for these athletes.
If you're a medium or heavy sweater, you want to have a focused hydration plan with electrolytes, and you'll want to do focused GI training. Skratch, Tailwind, and other drink mixes are all great, and you might need to add more electrolytes on top of it.
There is no benefit to practicing dehydration. Hydrate in training to improve performance and adaptation, then deploy the strategy you practiced on race day.
A lot of what feels like an endurance limitation is just a hydration and/or fueling limitation. And drink mixes like Tailwind and Skratch are bringing two big middle fingers to our perceived limitations.
- David
Read more on how to measure your sweat rate from Precision Fuel & Hydration here.
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