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You already track your heart rate. Heck, you can probably reel off your numbers for zones three, four, and five and are pretty good at ballparking all those ranges during a workout without any monitoring device.

But, dude, that’s so 2006. 

Because heart rate is, at best, a trailing indicator. That means it’s difficult to analyze not only what zone you’re in during a workout but also how your body is reacting to stress and whether that zone is an accurate measure of your fitness. Maybe you’re nowhere near your lactate threshold at that heart rate on a given day. 

Two companies are racing to change this model by adding sweat-sensing capabilities to our wearables. Turns out the salty liquid is good for more than just cooling you down: It’s packed with vital information about your health. Unlike heart rate monitors, sweat sensors give you actual diagnostics, meaning they can tell you what workouts you should be doing and, over time, even create a fueling and training regimen.  

To read perspiration, both San Diego–based  (owned in part by billionaire Mark Cuban) and Swiss startup  have created paper-thin sweat sensors, designed for 24/7 wear, that they claim can diagnose hydration levels, electrolyte balance, lactate threshold, glucose level, and calories burned—all by analyzing sweat in real time.  

Kenzen plans to make a nickel-size device called the that sticks to your rib cage or abdomen. It will communicate in two ways: via vibrations (one buzz to, say, tell you to drink, two buzzes to indicate you’re going too fast) and an app on your phone. The hardware will house a rechargeable battery, heart rate monitor, and Bluetooth unit that will communicate with third-party fitness apps (like ). The sweat sensor will be a disposable adhesive that’s meant to be replaced once per week. 

Electrozyme will also make a disposable product, but unlike Kenzen, it plans to license its sensors to product designers, which can then stick the patches on the backs of their wearables. 

Each device relies on the same basic technology: They monitor chemical signatures on the wearer’s skin to determine everything from hydration levels to lactate threshold. Take dehydration—one of the most important metrics the sensors will relay. In past lab studies, scientists had to measure body weight to know hydration levels. A slight weight loss meant an athlete had become dehydrated. 

But you can’t rely on scales to provide live feedback in a race, says Sonia Sousa, co-founder of Kenzen. Instead, her company created technology to analyze sweat rate, skin surface temperature, and ion concentration. Sousa says Kenzen also measures pH “since the body has a tendency to become very acidic as toxins build up.” The Echo H2 uses all this data—and takes into account environmental factors like air temperature and humidity—to create a detailed outline of hydration needs and tell you the pace at which you need to drink as temperatures rise and fall. 

Kenzen also looks at what you sweat away, telling you precisely what nutrients to add back to your system. For electrolyte loss, the sensor measures sodium, chloride, calcium, and potassium loss in perspiration. This way it can tell you not just when to hydrate but also how many nutrients you need to supplement the water.

The device uses all that data, plus lactic acid amounts and glucose levels in sweat, to predict fatigue and measure caloric output. The ultimate goal is to create a measurement for your live metabolic rate so you can see how exercise and nutrition affect your weight, says Sousa. 

Kenzen’s wearable will also track heart rate, a capability that, when paired with the sensors that can test for chemical signatures, could let the device prescribe rest days and intervals to avoid over- or undertraining.  

These wearables might one day even predict whether you’re predisposed to an injury or heart attack. Certain blood biomarkers tell us if we’re at risk for a disease. Both Kenzen and Electrozyme are working on ways to identify these indicators in sweat chemical signatures via a wearable. In theory, you’d be able to see predispositions toward injury, stroke, and heart attack, as well as other diseases on your wrist. Many diseases, from kidney problems to cystic fibrosis, can already be diagnosed via sweat, but either the needed tool is too bulky to move from a lab or the markers exist and accurate modeling is still a challenge. 

The key for all of this to work is 24/7 wear. The sweat sensors know you ate chicken wings after going to the gym because they’re designed to be worn continuously and can thus monitor all your activities, movements, and dining habits.   

Kenzen will be available later this summer. While the final cost has yet to be announced, . Electrozyme will be licensed as an add-on to smartwatches that will likely debut in early 2016. Both companies say the long-term goal is to integrate bioanalysis directly into clothing, taking wearables to an entirely new level.