We wear sensors that track steps, heart rate, and calories burned. Soon, we may measure our sweat, too! What will those measurements tell us?
Yesterday morning, I was riding my bike up a pretty significant hill. To get up that hill, I was pedaling as hard as I could. I had nearly reached the top when I felt that familiar trickle of moisture run down my forehead. Even though it was only 6ºC (42ºF), I was sweating.
Our human meat-sack bodies work optimally when their internal temperature hovers around 98.6ºF (37ºC). When the body gets warmer than that, the brain doesn't like it, so the hypothalamus (the part that controls temperature) sends a message to your body saying "Let's cool down!" That's when your sweat glands spring into action, and you start perspiring.
Perspiration is made almost completely of water, with smaller amounts of other chemicals in it as well. (We'll get to that in a moment.) Perspiration leaves our body through tiny holes in our skin called pores, and when the sweat meets the air, it begins evaporating (turning from a liquid to vapor) and cools us down.
Sweat is a great cooling system, but it is also a great way for our body to remove certain elements from our bloodstream. Salt, ammonia, potassium, glucose, lactate, and urea, to name a few. Our sweat also contains biochemical markers such as metabolites, electrolytes, and even heavy metals, which can give doctors and researchers a window into a person's health and even aid in diagnosing some diseases.
In recent years, scientists have developed sweat sensors in the form of patches, bandages, and tattoos that can make these types of measurements on the fly. A team of scientists at the University of California, Berkeley, is developing wearable skin sensors that can detect what's in your sweat. But before we get into that, let's look at how we measured sweat in the past, and how we're looking at it today.
Measuring sweat in the past
Back in 2014, I reviewed a product called a Fuelstrip, which the manufacturers called a Sweat Testing Sports Nutrition Technology. Not surprisingly, they don't seem to exist anymore. And yet, they had seemed promising.
Fuelstrips were small pieces of paper you were supposed to dab in your sweat periodically during a workout. The results would tell you whether or not it was time to replenish your glycogen stores. The darker the color on the strip, the more you needed to refuel. Luckily—and I say with my tongue firmly planted in my cheek—they also sold a "scientifically formulated" sports drink powder along with the strips.
You can watch the video of my (mostly) scientific experiment. The upshot was that the strips did indeed change color as my workout progressed. But the strips were not convenient, easy to use, or all that accurate. According to the strips, even after fasting for 12 hours, I still had to ride for 60+ minutes, at a near-threshold effort, before I was allowed to sip some of their sugary nectar.
Even though the strips were not all that impressive, they did seemingly measure my sweat ammonia levels (which correlate with plasma ammonia levels.) Since plasma ammonia levels will increase when muscle protein is metabolized for fuel, this is an indication that your glycogen is depleted. So, they did work ... in a manner of speaking.
Measuring fluid loss
Another way sweat, and sweat rates, have been tracked in the past is by using the formula (sweating rate = pre-exercise body weight - post-exercise body weight + fluid intake - urine volume/exercise time in hours). This rather cumbersome formula gives a pretty reliable representative range of sweat rate. And if repeated enough times, in many locations and conditions, it can be used for various environmental conditions, practices, and competitions.
The idea behind measuring fluid loss through sweat is to get athletes to focus on their hydration needs by teaching them to compare pre-exercise and post-exercise body weights. For us recreational exercisers, if you lost weight, then you need to drink more at the next practice or competition. If you gained weight, you would drink less. But for the true professionals, the formula was created and refined.
In an article called How to Calculate Your Sweat Loss, QDT's Nutrition Diva wrote this:
Half a liter of fluid weighs approximately one pound. If you weigh the same amount before and after, then your fluid losses were roughly equal to the amount of fluid you took in. Good work! If you are one pound lighter, then your sweat losses exceeded your fluid intake by approximately half a liter. For example, let's say, you weigh 150 pounds before you exercise. You drink a liter of water, which weighs about two pounds. After exercising, you weigh 149 pounds. That suggests that you lost about a liter and a half of sweat, and only replaced a liter. Go have a glass of water!
Measuring sodium loss
Athletes lose water during exercise, but they also lose electrolytes as a consequence of what is called thermoregulatory sweating. It is well known that the rate and composition of sweat can vary considerably between individuals based on physiology and also on how much salt you consume (more salt eaten, more salt to sweat out). So, many scientists and practitioners conduct sweat tests to determine the water and electrolyte losses of athletes during practice and competition.
To do this, sports labs have generally used regional patch sweat collection (literally placing patches of material in various locations on the athletes body to collect sweat), syringe sweat extraction (which is exactly what it sounds like), and LAQUAtwin analysis (a pocket sampling kit that measures electrolyte concentration). The coach or nutritionist then uses this information to develop electrolyte replacement recommendations or to formulate a replacement beverage that tailored to fit the athlete's needs specifically.
All of these manual, mathematic, and cumbersome testing protocols definitely work and can add to an already robust training regimen. But a team at UC Berkeley is on the verge of something so much easier and more meaningful that it makes me feel like I have been banging stones together for most of my athletic and coaching career.
The future or sweat analysis
At UC Berkley, a team of researchers has designed a flexible sensor system that can measure metabolites and electrolytes in sweat, calibrate the data based on skin temperature, and sync the results in real-time to a smartphone. No more slips of paper, pads of material in your armpits, or weighing yourself repeatedly.
In a recent paper, you can read how the researchers fitted the device onto smart wristbands and headbands and then put the device on dozens of volunteers. The volunteers then went through various indoor and outdoor exercises—on stationary bikes or tracks and trails—for a few minutes to more than an hour. The researchers also put the device on volunteers who were experiencing what is frighteningly referred to as "chemically-induced perspiration."
Ali Javey, a professor of electrical engineering and computer science at UC Berkeley and senior author on the paper told Science Daily "The goal of the project is not just to make the sensors but start to do many subject studies and see what sweat tells us—I always say 'decoding' sweat composition."
To better decode what sweat can tell us about the health of the human body in real-time, the researchers tried placing the sweat sensors on various spots on the athletes' bodies. They put the athletes on stationary bikes and tested the forehead, forearm, underarm and upper back, all the while measuring their sweat rates, sodium and potassium levels and more.
Researchers used the sensors to compare sweat glucose levels and blood glucose levels in healthy and in diabetic patients. In this experiment, they hoped that non-invasive sweat tests could replace blood-based measurements for diagnosing and monitoring diabetes.
They concluded that local sweat rate could indicate how much the athlete's body lost liquid during exercise, which could be a way to give athletes a real-time heads up when they may be pushing themselves too hard. Which on it's own isn't that impressive. But the researchers also used the sensors to compare sweat glucose levels and blood glucose levels in healthy and diabetic patients. In this experiment, they hoped that non-invasive sweat tests could replace blood-based measurements for diagnosing and monitoring diabetes.
In a Berkeley News article, principal investigator Ali Javey is quoted saying “Human sweat contains physiologically rich information, thus making it an attractive body fluid for non-invasive wearable sensors.” That's as exciting as it is weird to think about.
Although I was not on-board with the Fuelstrips I tested back in 2014, and I don't think anyone outside of the highest performing athletes in the world needs to do all the sweat tests that I outlined in this article, I'm optimistic that this new sweat tracking technology will not only save us time in the sports lab but will also lead to breakthroughs in the medical lab.