I Thought I Knew My Hydration Needs — I Was Wrong
Have you ever had your sweat tested?
Until this past December, I hadn’t. I always suspected I was a salty sweater, but I assumed I could get by with traditional hydration advice and some trial and error.
Ahead of the 2025 Columbus Marathon, I decided to get more precise. I purchased an H Drop device to measure both sweat rate and sodium loss. My goal was to test in different weather conditions so I’d understand what my needs might be on race day.
After several runs — hard workouts, easy days, and long runs — I collected some data. But something didn’t add up.
One easy 10-mile run with strides showed ~1,500 mg of sodium loss. A 22-mile long run with pace work in similar weather showed ~1,600 mg. With comparable conditions and very different efforts, the numbers didn’t seem to correlate.
I also noticed that weather wasn’t being tracked in the app, so I created my own spreadsheet to log temperature, humidity, and intensity. Still, the sodium data felt inconsistent.
As someone who has always struggled in the heat, I suspected I might be losing more sodium than the device was capturing.
So I decided to get lab testing.
The Lab Sweat Test
I connected with Chris Newport of The Endurance Edge, a sports dietitian in Raleigh, NC who specializes in endurance athletes. She offers both in-lab and remote sweat testing.
The protocol was straightforward:
Pre- and post-run body weight to measure sweat rate
Small gauze patches applied to collect sweat
Lab analysis to determine sodium concentration
Recorded temperature (68°F) and humidity (47%)
I ran for 34 minutes:
1 mile warm-up
3 miles at tempo
1 mile easy
That was enough to collect valid samples.
The Results: Higher Than I Expected
My results confirmed my suspicion — but I was still surprised.
I lose 79 mg of sodium per ounce of fluid loss.
If I ran a marathon in those same lab conditions, I would need approximately:
49.75 ounces of fluid
3,926 mg of sodium
…to stay within 1.5% dehydration.
Research suggests performance begins to decline once dehydration exceeds 2%. Beyond that, core temperature rises and risks increase for fatigue, muscle cramps, headaches, and heat illness.
Seeing those numbers in black and white changed everything for me.
Why This Matters
Understanding my sodium losses allows me to:
Fuel more precisely in training
Prevent late-race fade
Improve recovery
Better manage heat stress
Strategically use cooling tools (ice bandanas, cooling towels, ice in sports bras/visors)
High sodium loss athletes also tend to recover more slowly when under-replacing electrolytes. By matching my intake more closely to my losses, I can support both performance and recovery.
This also means that utilizing cooling tools is much more important for someone like me. The better I keep my core temp in check, the better I help reduce skin temperature, cardiovascular strain, and total sweat requirement.
What I Did Before Testing
When I first began addressing hydration, I used SaltStick capsules (~200 mg sodium each).
I started with 2 per hour and didn’t notice much improvement. When I increased to 3 per hour, I felt better — but in hindsight, that still likely wasn’t enough.
I was also running with a single 16 oz bottle and refilling when possible. That plan was likely under-replacing fluid, especially in warm conditions. The challenge with increasing sodium is that we need to make sure our fluid intake matches what we are losing so that we don’t end up with stomach discomfort from too much sodium with too little water.
Now, I use hydration vests for longer summer runs and plan routes where I can access fluids consistently. I bring cold water to workouts so that I can pour it over my head. I also have a cooling towel and ice bandana and will be purchasing an Omius cooling hat and visor for this summer from the feed.
Is Sweat Testing Worth It?
If you:
Struggle in the heat
Fade late in races
Cramp frequently
Feel like fueling has held you back
…sweat testing can be a powerful tool.
It removes the guesswork.
Endurance Edge provides:
A detailed spreadsheet to track different weather conditions
A hydration calculator for runs and races
A nutrition database to build personalized fueling plans
For me, moving from guessing to knowing was a game changer.
Not Ready for Testing? Start Here.
If you’re not sure you need lab testing yet, start with these evidence-based calculators from RD Megan Featherstone:
Hydration Calculator:
https://www.featherstonenutrition.com/hydration/
Sodium Calculator:
https://www.featherstonenutrition.com/sodium/
They’re a great starting point for dialing in hydration.
Final Takeaway
Practice your plan. Track your response. Pay attention to how you feel.
And if you suspect your needs are higher than average — especially in the heat — consider getting tested.
Knowing your numbers can help you train smarter, race stronger, and recover better.