Why Tracking Sweat Loss Might Be the Most Important Metric You’re Missing

Hydration has always been part of endurance sport. Everyone knows it’s important. Drink early, drink often. Grab the electrolyte tabs. Maybe weigh yourself before and after a session.

But here’s the reality: most athletes still don’t know what their sweat is costing them.

We’re talking about real losses - fluid and sodium - that vary from person to person, and change depending on conditions, intensity, and duration. And if you’re not measuring it, you’re relying on instinct or generic advice. That’s where things start to fall apart.

The signs are there - you’ve just learned to normalise them

Cramps that hit halfway through your session. The fade-out in the last third of your race. Feeling fine one day, and totally depleted the next.

These aren’t always the result of poor fuelling, undertraining, or bad luck. More often than not, they’re a hydration issue - specifically, a sodium and fluid imbalance that wasn’t identified, and wasn’t addressed.

And despite all the tracking we do - pace, power, heart rate, sweat remains a blind spot.

Generic plans don’t cut it

You’ve seen the advice: drink X litres per hour, or take Y milligrams of sodium every 30 minutes. But that assumes we all sweat the same. We don’t.

Some athletes lose 400mg of sodium per litre. Others lose over 2,000mg. Some sweat heavily in cool conditions. Others barely sweat at all in the heat.

So following the same plan as someone else - even someone elite - doesn’t guarantee anything. In fact, it could be what’s holding you back.

The value of knowing your sweat profile

This is where FLOWBIO comes in. The FLOWBIO Sensor tracks your sweat rate, sodium loss, and skin temperature in real-time - during your actual training sessions. No patches, no lab work, no assumptions.

It builds your personal Sweat Profile, so you can:

• Understand exactly how much fluid you’re losing

• Know how much sodium you need to replace

• Adapt your intake based on heat, humidity and intensity

• Plan your hydration before, during and after sessions

It’s not about drinking more. It’s about drinking what your body actually needs.

From lab testing to on-the-road insight

This level of data used to be limited to lab testing. And even then, it was a one-off snapshot that didn’t account for changes over time.

FLOWBIO makes that kind of insight available every session, in real-world conditions, using a sensor you wear on your arm or chest strap. It integrates with your existing setup - Garmin, Zwift, Wahoo, Coros - and feeds into the FLOWBIO App for clear, actionable guidance.

Who’s using it?

Athletes training for Ironman. Runners preparing for their first 100K. Riders putting in long days across changing terrain. Coaches looking for more precise inputs.

Because whether you’re chasing podiums or just trying to stay consistent, hydration is a strategy - not a side note.

Final thoughts

We measure nearly everything in endurance sport. But until now, sweat’s been ignored. That’s changing.

Knowing your numbers—your actual losses, not estimated averages—gives you a better chance at holding pace, avoiding cramps, and recovering faster.

And once you have that insight, you don’t go back.

Explore the FLOWBIO Sensor → https://flowbio.com/products/flowbio-sensor