(Reuters Health) - Even though many people swear by fitness trackers to help lose weight and stay in shape, a review of existing research confirms that many of these gadgets are not very good at measuring how much energy we burn.

For the analysis, researchers examined data from 60 previously published studies that tested the accuracy of energy expenditure measurements for 40 different devices worn on the arm or wrist. Accuracy varied widely, but it was a bit better when devices factored in other measurements like heart rate to calculate calories burned.

“Our paper shows that estimates are often poor and they vary depending on the activity being performed,” said lead study author Ruairi O’Driscoll of the University of Leeds in the U.K.

“Consumers should be aware of the potential for error in their devices, especially if they are using it to inform their eating behaviors,” O’Driscoll said by email.

When fitness trackers overestimate exercise, people who need more exercise to maintain or lose weight might get too little activity, increasing their risk for obesity and other chronic health problems, the researchers note in the British Journal of Sports Medicine. At the same time, trackers that underestimate activity might lead some people to overdo it and put too much stress on their cardiovascular system.

All of the studies in the analysis looked at energy expenditure in different ways, and with different types of exercise and activities. They often compared fitness trackers to proven methods of monitoring energy expenditure that are typically used only for research, like locking 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.

Taken as a group, the consumer devices tended to slightly underestimate energy expenditure, the researchers found. The devices with the biggest underestimations of energy burned, according to the results, were: Garmin Vivofit, Jawbone UP24 and SenseWear Armband Pro3.

At first glance, other fitness trackers got measurements more similar to those from proven methods of assessing energy expenditures: Apple Watch, Bodymedia CORE armband, Fitbit Charge HR, Fitbit Flex, Jawbone UP, Nike FuelBand, SenseWear Armband, and SenseWear Armband Mini.

But in many cases, there wasn’t enough data to rule out the possibility that results were random or due to chance.

Devices tended to be more accurate at measuring energy expenditure when they also monitored heart rate or body heat, but this was not consistent across all types of activities.

Accuracy also depended on the activity. Fitness trackers often did a poor job of measuring less vigorous movement like walking, climbing stairs, and doing household tasks.

Participants in the studies were 35 years old on average and typically not overweight. Results might be different for older people or obese individuals, the study authors note.

“Most research has shown that wearable devices and activity monitors are not that accurate for measuring energy expenditure,” said Dr. Mitesh Patel, director of the Penn Medicine Nudge Unit at the University of Pennsylvania in Philadelphia.

“For most people, energy expenditure estimates from wearable devices should be used as a gauge (e.g. high or low) as the numbers may not be accurate but trends higher or lower may be more likely to be correct,” Patel, who wasn’t involved in the study, said by email. “Other measures like step counts may be a more accurate and better indicator of how much activity a person has done.”

Fitness trackers with heart rate monitors might be more accurate because the added data helps assess how hard people are working during exercise, said Lisa Cadmus-Bertram, a researcher at the University of Wisconsin-Madison who wasn’t involved in the study.

“There are several trackers under $150 that do include heart rate technology, so you don’t need to get a top-of-the-line tracker,” Cadmus-Bertram said by email.

“Overall, consumers do need to recognize that the technology isn’t perfect and that trackers are providing estimates; it’s not a magic number that reflects the exact number of calories burned.”

SOURCE: bit.ly/2P1F3Xd British Journal of Sports Medicine, online September 7, 2018.