Can Your Fitness Tracker Detect Illness?

Fitness trackers can obviously log your exercise metrics and give you info on calorie burn and cycling routes, but because they can do so much more in terms of tracking heart rate, blood pressure, body composition, and sleep, it’s possible that you could be getting more health info than you realize.

In some cases, that might even translate to an early warning about an issue that may worsen if ignored. Here’s a look at how the data can be used as a prevention tool as well as a workout resource.

Heart Health

As you exercise regularly, your cardiovascular system becomes more efficient, according to Robert Greenfield, M.D., co-founder of California Heart Associates.

“With conditioning, your resting heart rate (RHR) will start to lower, because your heart won’t have to work as hard at rest, thanks to a more efficient system,” he tells Bicycling. “Being able to track this particular measure can be useful for heart health, especially if it’s going in the other direction.”

You’ll also be able to see how quickly your heart rate goes back to normal after intense exercise, another measure of heart health that’s helpful to watch. If you see that your rate is still elevated for 10 to 15 minutes after you’ve cooled down, Greenfield says that’s a concern and should get checked out. If that comes along with extreme fatigue, shortness of breath, or chest tightness or pain, get seen ASAP.

If you don’t have those type of intense symptoms, an elevated heart rate could mean you’re feeling stressed, you’re dehydrated, you’re more fatigued than you thought, or you have some metabolic concerns like prediabetes. Also, keep in mind that beta blockers, which are used to treat high blood pressure, tend to affect exercise heart rate. If this is happening for you, check in with your doctor to make sure you’re staying within a normal range.

Sleep Quality

When it comes to both athletic performance and overall health, sleep is another top variable, and a 2019 analysis of fitness trackers in Current Sleep Medicine Reports found they were helpful in detecting issues related to health outcomes like stress, asthma, and high blood pressure.

For example, researchers noted that deep sleep is associated with better regulated systolic (the top number) and diastolic blood pressure (the bottom number), which means that poor quality sleep could have negative effects.

There’s still debate about whether trackers really gauge sleep properly, and the researchers in the analysis note that a sleep study with polysomnography (PSG)—a comprehensive test used to diagnose sleep disorders—is still considered the gold standard for measurement.

However, having such a widely accessible tool that can pick up on sleep disruption early provides more opportunity to make meaningful tweaks to your sleep habits, says W. Christopher Winter, M.D., president of Charlottesville Neurology and Sleep Medicine.

“Sleep is related to your health in such a breadth of ways, including immune system response, brain health, gut health, pretty much everything,” he tells Bicycling. “If you’re seeing changes in how much you’re getting, especially with deep sleep, you have the ability to adjust and perhaps prevent some issues that might come with chronic sleep issues.”

A few possibilities that your tracker might pick up on are chronic insomnia, disrupted sleep that includes lack of deep sleep, and even sleep apnea. However, you’d need a sleep study for a diagnosis of apnea—a 2018 study looking at trackers and apnea suggested the devices could be useful for detecting problems at an early stage. For example, since you stop breathing temporarily with sleep apnea, that causes a heart rate drop, which a tracker would be able to detect.

Cold, Flu, and COVID-19

When RHR and sleep duration are combined, changes in what’s normal for you could be a sign that you’re under the weather—even if you don’t have any symptoms yet.

That’s according to a study published in The Lancet Digital Health, which reviewed two months of data from about 200,000 Fitbit users in five states. When that data was compared to flu trends, there was significant overlap, says study author Jennifer Radin, Ph.D., researcher at Scripps Research Translational Institute.

Everyone has a unique resting heart rate, Radin tells Bicycling, and that rate begins to climb upward when there’s an infection, most likely as a response to inflammation in the body, especially when there’s a fever.

That inflammation and fever can also sabotage sleep quality, so when the two are paired up, there’s a higher risk there may be infection, says Radin. If this data is seen on a state-wide, or even a country-wide level, that has the potential to make flu predictions and reporting faster and more accurate.

On an individual level, seeing changes in these data points could prompt actions to support your immune health more effectively, like getting more rest, doing more gentle exercise, eating nutrient-dense foods, hydrating, and even just washing your hands more often, Radin says.

This kind of data may be helpful for detecting COVID-19 as well, and not in the distant future. A November 2021 study in Nature Medicine details the use of a specially built device that detects anomalies in heart rate and step counts. In a trial, researchers found it could generate alerts for presymptomatic and asymptomatic COVID-19 infection in 80 percent of those who subsequently tested positive.

That doesn’t mean you get to skip using COVID tests, but it is a nod toward using health data in a way that goes well beyond your fitness goals. Keep in mind that trackers aren’t a substitute for checkups and seeing a doctor when you have particular concerns, but they can be one more way to stay on top of your health goals.

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