Smartwatches are continually adding more and more “life-saving” technology to keep users safe, such as fall detection, SOS calling, atrial fibrillation warnings, and the Pixel Watch’s latest lost pulse feature. But there’s a less obvious, more common concern that most watches are relegated to the back burner: tracking hydration and reducing sweating.
I’ve long wanted a better hydration tool on my watch, and last weekend 35-year-old runner Bobby Graves reported: Man dies of cardiac arrest after completing Disneyland half marathon That was front and center in my mind last weekend, the day after I self-diagnosed myself with heat stroke in 100-degree heat.
This man was my age, had run multiple half and full marathons in the past, and was not at an age where he was worried about his heart health. Perhaps a medical report will shed light on mitigating circumstances, but there’s a reason why the experts quoted by SFGate specifically warned about “staying hydrated” during hot weather.
Smartwatches have the ability to estimate sweat loss from exercise, and Samsung claims it’s clinically accurate, but most other brands (outside of Garmin) ignore it, and those that do track it don’t take full advantage of it.
I think it’s time for hydration to become a higher priority for fitness brands. Excessive heat is only going to get worse over time, and your watch needs to be able to keep up.
Hydration tracking is as basic as it gets
There are several apps for Wear OS and WatchOS that track hydration. Waterminder And Waterllama: Samsung’s One UI Watch has a first-party hydration tile, and Garmin watches let you download hydration tracking glances from the Connect IQ store.
They vary in appearance and niche features, but they all work pretty much the same way: Open the app or tile, tap the button, and tell it you’ve had a glass of water. The screen will tell you how many fluid ounces you have left to drink that day, and it can even give you periodic reminder popups to check if you’ve had a drink.
It’s convenient enough for daily life. However, the daily hydration reminder doesn’t take into account circumstances such as temperature and whether you’ve exercised, so you have to change the goal yourself. Also, the combination of hourly hydration and exercise reminders is very annoying, so most people ignore it or disable it.
Fitness watch brands like Coros and Polar allow you to set nutrition and hydration reminders during exercise, but the intervals are preset and, again, there’s no information about how much fluid or electrolytes your body really needs.
Ideally, the watch would know if the user has sweated (or is sweating) more or less than usual and dynamically suggest drinking more water to avoid issues with dehydration and heat stroke.
Smartwatches can accurately measure sweating
Earlier this month, Samsung boasted that a clinical study from the University of Michigan had proven how its Galaxy Watch compared to medical-grade sensors in terms of accuracy for heart rate (90%), VO2 Max (82%), and sweat rate (95%). That’s pretty much what you’d expect from a wrist-worn optical heart rate sensor, but the accuracy of the sweat rate surprised me.
Samsung doesn’t track sweat loss directly, instead “estimating it based on your body size, age, gender, intensity of exercise, heart rate, ambient temperature, and other factors.” I’d previously been guessing at how much sweat loss the Galaxy Watch Ultra would cause after a run, but it looks like that was an educated guess.
Only Direct The only consumer sweat reduction tool I know of is the Nix Biosensor, which sticks to your bicep and pumps sweat into an “inlet” with electrodes on each end, then calculates the “velocity” of sweat to determine how much sweat is being lost, and then, because different parts of the body sweat differently, estimates how much sweat is being lost by the rest of the body.
Nix’s sensors could be useful to serious athletes, and we’ll test them in a future column, but they’re niche and only worn in athletic endeavors. To be an authority on sweat, we need mainstream smartwatches that are worn daily.
The Pixel Watch 3’s cEDA sensor can detect “small changes in skin sweat rate” for stress data, but I don’t think it’s designed to track overall sweat rate. Apple has patented a sweat sensor that would “measure the amount of moisture lost over a period of time” and show real-time sweat rate, but a patent is no guarantee that a company can make the concept work in the real world.
For now, it would be nice to see more brands follow Samsung and Google’s lead and use standard heart rate and body data to estimate how much sweat you’ll lose after a workout, but ultimately they need to go further than Apple’s patent.
Imagine this: you’re running a half marathon in warm weather, wearing your Galaxy Watch 7. As you run, the data screen shows you a real-time estimate of your sweat loss, perhaps calculating a new total every few minutes, or every mile. At a certain threshold — say, every 500ml of sweat loss, or a customizable number — your wrist vibrates to suggest you hydrate immediately.
The amount of sweat produced by the body Slow If your heart rate drops below 100 while you’re running, that’s a serious sign of dehydration. Just as your Galaxy Watch does if it detects irregular heartbeat or low heart rate, it will alert you to stop running immediately and rehydrate or seek medical attention.
Once you cross the finish line, the watch will estimate your total sweat loss, just like it does now, but it will also automatically add the total amount of sweat lost to the Hydration tile. Samsung recommends “replenish 150% of lost volume within 1-2 hours,” so I think it’s easy to multiply the total milliliters of sweat lost during your workout by 1.5 and add fluid ounces to that.
And it could send you notifications one and two hours after exercise to remind you to record the total amount of water you’ve drunk in your hydration tile to make sure you’re staying properly hydrated.
Here’s my vision for how this would work: And ideally, the sweat sensor would not just kick in during exercise, but would continuously measure sweat (like the Fitbit cEDA sensor does) and then start tracking more frequently when you hit a certain sweat rate or heart rate threshold, or if your local weather and humidity is especially high.
This would be useful not only for athletes, but also for farm workers, delivery drivers in hot vans, construction workers and many other outdoor workers.
Garmin comes the closest to my ideal when it comes to sweat expenditure, however, by shamelessly embedding a Hydration Tracker tool in Connect IQ. Once downloaded, you can add sweat expenditure from workouts to your standard daily goal by opening the app’s settings and turning on “auto-increase goal.”
I tested this with a Garmin Forerunner 965, and if you sweat that much during a 10km run on a hot day, that would add about six cups of water to your total daily fluid intake, but it seems like the Garmin underestimated my sweat compared to the Samsung. and I wouldn’t follow the “150% change” advice, as you will always need more water than Garmin thinks, so it’s more theoretically useful than practical.
For runners determined to finish a race even when their body is telling them to stop, a fitness watch needs to be ready to step in at any moment, not just warn them when they’re drenched in sweat at the finish line.
Instead of relying on algorithms, you might want to start measuring your body’s sweat directly to get more customized data, because relatively accurate brands like Garmin and Samsung give different results on their heart rate estimates. With sweat data, the warnings to drink water and electrolytes would feel more appropriate.
