Fitness Trackers: Good at Measuring Heart Rate, Not So Good At Measuring Calories
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Patricia Neighmond
PATTI NEIGHMOND
Researchers had participants wear the fitness trackers while walking or
running on a treadmill and while riding an exercise bike to determine
how well the trackers measured heart rate and energy expenditure.
Paul Sakuma/Courtesy of Stanford University School of Medicine
Sleek, high-tech wristbands are extremely popular these days, promising
to measure heart rate, steps taken during the day, sleep, calories
burned and even stress.
And, increasingly, patients are heading to the doctor armed with reams
of data gathered from their devices. "They're essentially asking us to
digest the data and offer advice about how to avoid cardiovascular
disease," says cardiologist Euan Ashley, associate professor of medicine
at the Stanford University Medical Center and Stanford Hospital and
Clinics in northern California. And, being somewhat near Silicon Valley,
he says he gets a lot of tech-savvy patients bringing fitness-tracker
data to appointments.
The problem, he says, is that he just didn't know how reliable that data
was. So, he and colleagues decided to study seven of the most popular
devices and compare their accuracy to the gold-standard tests that
doctor's use.
They looked at two metrics: heart rate and calories burned. For heart
rate, the fitness trackers were compared to findings from an electro-cardiogram, or EKG. It turned out the devices were "surprisingly accurate", says Ashley. "Most devices most of the time were 'off' by
only about 5 percent."
However, when it came to measuring how many calories a person burned,
the findings were way off, says Ashley, showing a degree of inaccuracy
that ranged from 20 percent to 93 percent, meaning 93 percent of the
time the worst-performing device was wrong. Researchers compared the
findings of the wrist devices to a sophisticated system of calculating metabolism which measures oxygen and carbon dioxide in people's breath.
"This is a very well-designed and well-done study," says Dr. Tim Church,
a professor of preventative medicine at Pennington Biomedical Research
Center at Louisiana State University who was not involved in the study.
Church routinely consults with companies about how to introduce wellness strategies into the workplace. Being wrong 93 percent of the time means
the findings from the fitness tracker are more "fiction than fact," he
says, which can actually undermine a healthy diet. "It's just human
nature. People are checking these inaccurate counts and they think
they've earned a muffin or earned some ice cream and they're sabotaging
their weight-loss program."
Church points to a study last year which found participants in a
weight-loss program who also wore fitness trackers actually lost less
weight than participants who didn't wear the trackers. "It's an instance
of no information is probably better than having bad information," he says.
The Stanford study was published in the Journal of Personalized
Medicine. It was relatively small, with 29 men and 31 women. In addition
to the primary results, there were some other interesting findings. In
certain groups of people — for example, those with darker skin, higher
BMIs and men — the error made by devices was actually greater than for Caucasian women with a more healthy weight.
Researcher Euan Ashley and his team in a testing lab at Stanford
University School of Medicine.
Paul Sakuma/Courtesy of Stanford University School of Medicine
"So, for those for whom it might matter the most, who are trying to lose weight, the error was actually greater," says Ashley, who doesn't know
why this may be the case. He speculates that it could be that companies
use a fairly narrow group of people for testing the equations they use
to measure heart rate and calories burned.
The study didn't look at how well devices count steps or monitor sleep
or stress. The take-home message, says Ashley, is to not rely on the
devices to measure total calories burned. Instead focus on eating what
we know is a healthy diet, which is low in sugar and high in fiber, and
to "eat not until you're full but until you're no longer hungry."
And, of course, people should exercise, he says, adding, "we have no
more important intervention than exercise for the prevention of any
number of diseases."
Makers of two devices, Fitbit and PulseOn say they remain confident in
the performance of the trackers both in measuring heart rate and
calories burned. In a statement, PulseOn questioned the study's
methodology, saying that the high errors for calorie measurements
"suggest that the authors may not have properly set all the user
parameters on the device."
Mark Gorelick, the chief science officer at the device-maker Mio Global,
says, "we agree that more accurate calorie estimation is important for
the industry as a whole, since most individuals are monitoring calorie
deficits for weight loss." The other device makers did not immediately
respond to requests for comment.
http://www.npr.org/sections/health-shots/2017/05/24/529839681/fitness-trackers-good-at-measuring-heart-rate-not-so-good-at-measuring-calories
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