We’ve been talking about the idea of tracking hydration for a while now – and, no, not the kind that means manually inputting how many cups of water you drink in a day.
Instead, we mean hydration-tracking wearables that tap into your physiological data, interpret your hydration requirements, and inform you how different activities can influence those needs.
In the past, we’ve spoken about wearables that have promised to do that, but, for whatever reason, they haven’t made it to athletes – until now.
Monitors from startups like Nix, hDrop, and FlowBio – the tracker we’ve used in this test – are now all ready to keep an eye on your hydration levels.
Like Nix, FlowBio’s S1 wearable sensor is built for athletes (primarily runners and cyclists) and costs a pretty hefty $405/£329. It’s also designed to be used alongside a sports watch or indoor trainer.
We managed to get our hands on one of those sensors for the 2024 London Marathon – let’s see if FlowBio’s wearable can tell us anything useful about getting sweaty over 26.2 miles.
How Flowbio tracks your hydration
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The FlowBio S1 sensor is essentially a small-ish, 3D-printed, plasticky device with an IP65 waterproof rating. It features a USB-C port to charge it up, with a small button up top to turn on the tracking and sync data to the companion phone app, (iOS-only, currently).
There’s also a small LED light to indicate when it’s tracking, syncing, and is still packing some battery.
When you flip the sensor over, you’ll find the key component to how the FlowBio works.
At its core, it uses microfluidic technology, which is something similarly used on the Gatorade’s Gx smart patch we tested back in 2021. This technology captures liquid through your sweat and uses electrochemical sensors to measure the total loss of fluid and sodium.
The latter is an electrolyte, and, if you’ve been running or riding for a while, you’ll know about the importance of topping up your electrolytes during more intense exercise. If not, your body is going to have a hard time operating properly when it’s under greater strain.
Flowbio takes all this information – along with data from an onboard skin temperature sensor – and combines it with its algorithms to measure sweat rate and loss of sodium.
Unlike the Gatorade patch, this isn’t a single-use device. And, once you’ve given it a quick rinse after a sweaty session, you can throw it on to tap into your sweat again.
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The sensor doesn’t work alone, however.
It doesn’t track the traditional metrics of your workouts, like pace or duration, and therefore needs something that does. That’s where the wearable integration comes into play, with Flowbio’s sensor currently able to link up with a Garmin, Coros, or Wahoo watch.
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This means you’ll need to track your workout as you typically would, and, when your session is completed and you’ve synced your workout, you’ll need to stop the tracking on the device and the two will be brought together in the FlowBio app to provide your hydration insights.
FlowBio offers the ability to use the S1 with bike computers, and it works with the indoor bike training platform Zwift, too.
Getting set up
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The first thing you’ll need to do is download the S1 companion app.
It’s a pretty simplistic affair – you can view workout history, there’s a dashboard to allow for workout syncing, and a settings menu where you can connect to compatible apps.
The device uses Bluetooth to connect the companion app to sync data and also has ANT+ connectivity to pair it up to other supported devices.
The sensor can be worn threaded through a bundled strap worn on your upper arm. Alternatively, you can thread it through a heart rate monitor chest strap and wear it on your upper back.
When you’re ready to track, you hit the button on the sensor until it flashes green multiple times, and then start tracking your workout on your watch.
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When you’re done, you’ll save your workout on your wearable as you normally would and then sync the S1 data to the Flowbio app. From there, you’re able to adjust the average temperature and the position you wore the sensor and see details of your sweat composition.
Flowbio eventually plans to let you view your fluid, sweat, and sodium loss in real-time during workouts and offer advice on what to do before and after you go for a run or big ride.
Right now, though, those things are very much in the works.
The London Marathon test
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For our marathon test, we decided to wear the FlowBio clipped to a heart rate monitor, which was pretty straightforward to set up, ensuring the sensor was sitting flat against our back to make sure it could collect that sweat. It never felt uncomfortable to wear, and didn’t move around or threaten to fall off.
In terms of battery life, using the sensor for three hours and 26 minutes of running saw the battery drop around 20%. FlowBio actually says you should enjoy over 100 hours of battery life before it needs charging, though our experience would see it only manage around 20-ish hours when extrapolated out.
There’s not a lot of context for battery life expectations within hydration monitors at this time, but – even if you use that shorter battery estimation – we think it’s a respectable amount of drain for a first-gen product.
Once I remembered in the haze of finishing the race to end the session, it was a pretty straightforward process to sync the data to the app.
The Flowbio was connected to the Garmin Connect app (with the Garmin Forerunner 965 linked to it) and, once synced over to the S1 app, provided the crucial workout duration and temperature details.
Below that data lies your sweat composition where you can see your total fluid loss, sweat rate, sodium loss, and total sodium concentration.
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So, as shown above, I’d lost 1.73 liters of fluid over the race at a total sweat rate of 0.5 L/h (liters per hour), and a sodium loss of 3,417mg.
It also tells us that our total sodium concentration is 1,818 mg/l (milligrams per liter).
Now, that data presented doesn’t feel hugely insightful in isolation – and that’s where the hydration recommendations presented by FlowBio come into play. They tell you how much water and sodium you should consume to rehydrate.
The bad news is that those recommendations didn’t appear for us after the marathon.
So, how useful or accurate was this data overall?
We’ll start with what FlowBio says – that the S1 has a 90% accuracy. When you look at the individual elements, it’s tracking sodium loss with an accuracy of 83% and a difference of 23mg compared to lab conditions testing. When monitoring fluid loss, Flowbio claims a difference of ~250ml compared to gold standard testing.
We decided to speak to someone well versed in all things sweat and hydration – and has even spent some testing time with the FlowBio sensor to delve a bit deeper into the data.
Chris Harris is a Sports Scientist at Precision Fuel & Hydration, a company that helps elite athletes and mere mortals like ourselves manage their hydration needs with its energy and electrolyte products.
The first question we wanted answered was whether or not it’s useful to see data from just one run – even if that run was a marathon.
“The short answer is no,” says Harris. “You could do a run the following weekend and see totally different numbers. One run in isolation is not enough.
“I believe the minimum is ten tests to build your hydration profile,” says Harris. “What these wearables are aiming to do is to develop a hydration profile over time, so several tests can build a picture to show the sort of the limits to your hydration profile. So, essentially saying: ‘You sweat roughly this amount in these conditions’.”
Harris hones in on the data recorded about my sodium concentration, something Precision Fuel & Hydration captures with its own at-rest sweat tests.
“Your sodium concentration is very high, 1000 milligrams per liter is salty – and that could lead to quite an aggressive rehydration strategy. Potentially, if you were to do another marathon, you might need to introduce a higher concentration of electrolytes.”
That concentration of sweat should influence how you build your electrolyte strategy. If that concentration data is accurate, you would have drinks containing similar concentrations.
However, as Harris tells us, there can be variations in sodium concentration because of individual variations in the genetic makeup of the sweat gland.
While making scalable assumptions about your sodium concentration is something Harris says isn’t too difficult, he has a different take on capturing fluid loss, which is also something the FlowBio sensor promises to do.
“The volume of fluid loss in your sweat rate is incredibly difficult to measure from a two-centimeter patch located somewhere on your body.
“What we’ve seen with most of the sensors we’ve tested is that measuring or estimating total body sweat rate is challenging.
“We feel that it’s still probably more accurate to weigh yourself before and after exercise while the technology and the algorithms develop to give you that good data,” he says.
Is FlowBio the future of hydration tracking?
The major caveat from our testing is that we’ve only used the sensor twice, so this isn’t a full verdict on whether you should buy it. This is more about whether FlowBio has something here that can give you useful, reliable insights.
We like the idea of promised support to view fluid and sodium loss in real-time on a compatible device, so you can know in real-time to address your hydration needs. That’s where we think it’s going to have the greatest value for runners and riders. That isn’t here yet, though.
Then there’s the question of accuracy, because if it’s not accurate, how useful will that data truly be?
Getting the most meaningful data from FlowBio requires regular testing, which is fine, but you also need to factor in that there are also going to be variations for different exercises, temperatures, and humidities. Are your hydration needs for a marathon going to be the same for a bike ride in hotter conditions? Likely not.
It feels like FlowBio is on the cusp of something great here, especially with the form factor – and it’s good to see we’ve moved on from single-use sensors like Gatorade’s.
There are still questions about how the Flowbio sensor fares over long-term use and whether it can truly match the gold standard – weighing before and after workouts, as Harris mentions – but we expect the startup to only get better at telling us what’s going on inside our bodies.
