Remember when Health and Human Services Secretary Robert F. Kennedy Jr. caused a national stir in the summer of 2025 when, while testifying before the House Subcommittee on Health, he said "...we think wearables are a key to the MAHA agenda — Making America Healthy Again. My vision is that every American is wearing a wearable within four years."
He explained at the time that wearable trackers are "a way people can take control over their own heath. They can take responsibility. They can see, as you know, what food is doing to their glucose levels, their heart rates and a number of other metrics as they eat it. and they can begin to make good judgments about their diet, about their physical activity, about the way that they live their lives."
Of course, untold tens of millions of Americans already own and use these products.
But the 72-year-old Kennedy's launch of this initiative led critics to pounce, citing serious concerns about data privacy — and how breaches could negatively impact users.
Experts also caution about users having their private health and fitness data — glucose levels, fertility levels, sleep scores, etc. — sold to entities that want to profit from that information.
As the saying goes in today's digital world, "data is currency."
The Washington, D.C.-based Progressive Policy Institute was perhaps the most forceful in elucidating what users of wearables need to be aware of: "While wearable devices and their connected apps are already common in the U.S. and other countries, the scale of Kennedy’s plan raises serious data privacy and security concerns around how health data from these devices is collected, stored, and shared on their connected apps. Many U.S. users are under the mistaken impression that health information collected through health tracking apps is protected by U.S. privacy laws. However, the main federal health privacy law, the Health Insurance Portability and Accountability Act (HIPAA), covers only health care providers, insurers, and their business partners. Commercial health apps fall outside HIPAA’s protection."
FIU research leads to latest concerns about fitness trackers
Well, it turns out that data privacy isn't the only caveat about fitness trackers. Some of the most popular ones are also prone to producing inaccurate calculations and measurements.
At least that's according to a recent study by researchers at Florida International University that was published in the journal PLOS One.
FIU researchers tested four popular fitness trackers — the Apple Watch Series 8, Fitbit Sense 2, Samsung Galaxy Watch 5 and Garmin Forerunner 955 — against clinical-grade laboratory equipment that measured participants’ true energy use and found that smartwatches can significantly overestimate how many calories people burn during exercise.
What's more, the calorie expenditure calculations became progressively less accurate, the higher the subjects' body fat was.
Researchers found typical errors in smartwatch calorie estimates off anywhere from 15% to 25%, with averages for the worst-performing watches climbing much higher (we're looking at you, Fitbit — more on that in a moment).
That means if a smartwatch says someone burned 500 calories during a workout, the real number could easily be closer to 350. The discrepancy could be especially important for people who use fitness trackers and calorie-burn estimates for weight management.
Apple’s device was the most accurate while the ones made by Garmin and Samsung overestimated the most. "You can't treat the calories burned number it gives you as an accurate number, because it's not," said Jason Kostrna, the study's lead author and an associate professor of kinesiology and exercise science in FIU's College of Arts, Sciences & Education. "You could be hundreds of calories off each week and easily end up in a calorie surplus when you think you're in a calorie deficit."
How the FIU study worked
For the study, researchers had 58 adults ride a recumbent bike through a controlled workout alternating between moderate and vigorous intensity. Each participant wore one of the smartwatches while a COSMED K5 metabolic analyzer — the industry’s gold-standard used in clinical labs — measured their true energy use. Fitbit users will find the device’s performance troubling. During testing, it produced repeated glitches — in some sessions reporting implausibly small numbers, such as a single calorie for an entire workout, and in others failing to generate a reading at all. Because many of the measurements were erroneous, the researchers filtered out the data. Kostrna says the team doesn't have a definitive answer for why these trackers struggle with higher body fat percentages. He believes the algorithms used to estimate calorie burn may play a role. "When the companies are developing those algorithms, we don’t know where the test data is coming from," he noted. "That's why I think it's beneficial for the companies to see this type of data, so they can refine their products over time."
Maximizing your wearable tracker's efficacy
If you are a wearable user, take heart — the devices can still be a useful part of your overall health and fitness journey.
Experts say you just have to understand your fitness tracker’s limitations. They recommend treating calorie-burn numbers as a ceiling rather than a precise measurement and rounding down for a more realistic estimate. Because the study found accuracy worsened as body fat increased, people using wearables for weight management may want to build in additional margin. But researchers caution against trying to compensate by cutting food intake too aggressively. The body still needs fuel to recover and train consistently. “Instead of calories burned during a single workout, users can lean more on the measurements that wearables tend to handle better, such as heart rate and time spent in different heart-rate zones,” said Kostrna. “Tracking personal trends over time also can be more useful than focusing on a single day's calorie total.”
Steve Dorfman is a journalist for The Palm Beach Post, part of the USA Today Network of Florida. He writes about all aspects of health, fitness, and wellness. If you have news tips, please send them to sdorfman@pbpost.com. Help support our local journalism, subscribe today.
This article originally appeared on Palm Beach Post: Smartwatches overestimate calories burned by up to 25%, FIU study says
