Why Your Core Runs Hotter Indoors (and What That Means for Sweat)

Indoor training has a way of turning steady sessions into sweat-fests. Without airflow, your core body temperature rises faster, pushing up sweat rate and sodium loss. That means the hydration demands of an indoor ride or treadmill run often look very different to the same effort outdoors. Some athletes use core body temperature sensors to track the heat, but temperature alone doesn’t tell you what you’re losing in sweat. That’s where the FLOWBIO Sensor comes in — the world’s most accurate sweat sensor, measuring fluid and sodium loss in real time. By combining your sweat profile with session data, FLOWBIO helps you spot dehydration risks before they cost you performance, recovery, or sleep.

Why sweat feels different indoors

Outdoors, airflow does half the cooling job for you. Whether it’s a breeze on a run or the slipstream on the bike, moving air helps your body regulate temperature. Indoors, the heat hangs around. With less natural cooling, your core temperature rises faster, and sweat has to pick up the slack.

That’s why a tempo session on the turbo or treadmill can feel tougher than the same power outside — sweat rate climbs, sodium loss increases, and effort feels higher even when the numbers say otherwise.

Sodium loss and dehydration in hotter conditions

It’s not just about fluid. Sweat carries sodium, and the concentration varies massively from one athlete to another. Some riders finish with kit streaked white with salt; others drip litres but lose relatively little sodium. Either way, miss what you’re losing, and you edge closer to dehydration — legs fading mid-session, cramps, longer recovery, or disrupted sleep after hard efforts.

Core Body Temperature Sensors and Sweat.

There’s growing interest in the core body temperature sensor, especially among athletes training for hot races. They show how your internal temperature responds to effort and heat stress, and that can be useful for planning sessions or heat adaptation blocks.

The missing piece: fluid and sodium loss

But temperature is only half the story. As your core temperature rises, your sweat rate and sodium loss rise too. That’s the part that directly affects hydration needs — and ultimately, how you perform and recover.

This is where the FLOWBIO Sensor comes in. It measures sweat rate and sodium concentration in real time, giving you the data temperature sensors can’t. Used together, you get a clearer picture: not just how hot you’re running, but what that heat is costing you, and how to balance it with the right hydration strategy.

Where FLOWBIO Fits.

The FLOWBIO Sensor tracks your sweat as it happens. Every litre of fluid loss, every milligram of sodium — measured, not guessed.

Building your indoor vs outdoor sweat profile

Over time, the FLOWBIO App builds your personal sweat profile, learning how your body responds in different conditions. That means you can see how your sweat shifts indoors versus outdoors, and get hydration strategies that adapt to your intensity, environment, and physiology.

No more guesswork. No more one-size-fits-all. Just actionable hydration guidance for the sessions that matter.

The Big Question for Athletes - What are you actually losing indoors?

Indoor training is the backbone of most athletes’ consistency. But it’s also where dehydration risks creep up quickest. The heat, the lack of airflow, the rising core temperature — it all adds up.

So the question is: next time you roll out the turbo, what are you really losing in sweat — and what’s it doing to your training?