Fitness trackers may be a trendy way to monitor every step we take, but a new study suggests there’s not very good at keeping tabs on how many calories we burn.
Scientists from Japan pitted 12 devices such as the Fitbit Flex and Jawbone Up24 against two proven methods of monitoring energy expenditure: monitoring people in a room to assess every calorie consumed and burned or asking people at home to drink specially treated water that makes it possible to detect energy output with a urine test.
In the first experiment, measurements from the fitness trackers deviated from the lab results in a typical day by underestimating energy expenditure by as much as 278 calories or overestimating by up to 204 calories.
With the second experiment, the devices ranged from 69 to 590 calories lower than the urine tests.
When fitness trackers overestimate exercise, people who need more exercise to maintain or lose weight may get too little activity, increasing their risk for obesity and other health problems, said the lead researcher.
Underestimating exercise might be just as dangerous for some people if inaccurate recordings of their activity and exercise were used to make medical decisions.
Even for healthy users, it may be difficult to promote health and wellness if these devices are proving inaccurate or variable feedback.
To test the accuracy of fitness trackers for monitoring energy expenditure, the study team asked nine men and 10 women, ages 21 to 50, to wear 12 different devices while participating in the two experiments.
Eight devices, popular with consumers in Japan, used in the experiments included: Fitbit Flex, Jawbone UP24, Misfit Shine, Epson Pulsense PS-100, Garmin Vivofit, Tanita AM-160, Omron CalorieScan HJA-403C and Withings Pulse O2.
The other four gadgets have been validated in previous research – Panasonic Actimarker EW 4800, Suzuken Lifecorder EX, Omron Active style Pro HJA-350IT, and ActiGraph GT3X.
Fitness trackers overestimate and underestimate energy expenditure
For the first experiment, participants went into what’s known as a metabolic chamber, a room specially designed to monitor calories consumed and burned, for 24 hours. They got three meals, and they could work at a desk, exercise on a treadmill, watch television, do housework and sleep while they were in the room.
In this airtight chamber, scientists use a technique known as indirect calorimetry to assess energy expenditure by measuring carbon dioxide production and oxygen consumption.
Compared with these measurements, half of the fitness trackers underestimated energy expenditure and the rest overestimated it.
For the second experiment, each participant wore the devices for 15 days and collected urine samples on eight days. Every fitness tracker underestimated energy expenditure, the study found.
It’s possible some of the underestimation might be due to people removing the devices to bathe or to charge batteries, the authors note.
In addition to its small size, other limitations of the study include its reliance on participants who weren’t obese and who didn’t have health problems that would limit their ability to exercise.
Still, the findings suggest that consumers may not have an easy time finding a reliable fitness tracker to monitor exercise. With little research to show their accuracy, it’s impossible to know if their feedback is accurate.
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