225 million of these…

Fitness trackers

…were sold in 2019. I want a motherfucking fitness tracker too!

Well, I wanted it because since they got so freakn popular, I started getting somewhere between “a lot” and “a shitload” of messages asking me hey my smartwatch says I burned 800 calories on a treadmill but how come I haven’t lost any weight, tell me what to do, for free preferably (you can chat with me on Instagram too).

So I took loads of caffeine and went deep into the murky waters of science.

And what I found was pretty much the same that I have been telling all those people messaging and bothering me — stop using fitness trackers for estimating how many calories you burned during the workout

You see, based on research findings the calorie expenditure using these fitness trackers is highly inaccurate (it also varies significantly between different activities). When I say highly inaccurate I mean a man who designed these trackers must have taken 225 million grams of drugs before digging into the whole inventing process.

The 2017 study looked at the Apple Watch, Fitbit Surge, Samsung Gear S2, and other wrist-worn devices during different physical activities and found that no device achieved an error in calorie expenditure below 20%. 

It means that all devices sucked — they were off by 27.4% (the Fitbit Surge) to 92.6% (for the PulseOn). If you already said “Take my money, Apple!” and spend half of your wage for the Apple watch, please know that it was off by 40%:

Percent error of energy expenditure in various fitness trackers

But let’s give some credit to that inventor who created fitness trackers — heart rate measurements were within an acceptable error range. Well done, mate. Well done. 

In 2016, Nelson et al. also examined the accuracy of the various Fitbit devices and Jawbone UP24 for estimating calorie expenditure for specific activities. Once again, they sucked. A lot. They overestimated calorie expenditure by 16% to 40% during walking, jogging, and stair climbing.

The researchers concluded:

With the possible exception of sedentary behaviors, consumer-based physical activity monitors do not provide accurate estimates of energy expenditure and should not be used for estimating energy expenditure.

I honestly think that inventors simply shrink monkeys to five inches tall and put them into these fitness trackers to spew out random numbers once you are done with your sweaty run. But don’t take my word for it, the 2020 systematic review pretty much proved it.

The researchers examined 158 publications investigating Apple, Fitbit, Garmin, Mio, Misfit, Polar, Samsung, Withings, and Xiaomi wearable devices and found that for calorie expenditure, no brand was accurate

So it must be the damn monkeys.

Jokes aside, if you have the Apple watch and it says you burned 700 calories, it could be off by ~280 calories. If you own the Fitbit Surge, an overestimation could be ~190 calories. 

This can slow down your weight loss progress because many people have the reward mentality — they see the fitness tracker (meaning — monkey) reporting “500 calories burned on the treadmill” and they think they have earned a 300-calorie piece of cheesecake. And since those 500 calories are more like 300 calories, they end up eating back the calories they have burned.

It’s great that fitness trackers can encourage people to be more active. They are also pretty accurate in monitoring heart rate. I’m sure they have various other cool features and superpowers. Like warning you when your significant other is calling you and that he/she is pissed at you and you better come home with tickets to Jamaica. 

But please, don’t use them to estimate how many calories you burned. In fact, forget the calorie expenditure part completely. Instead, if weight loss is the goal, track how many calories you consume and adjust it based on your weekly weight loss rate

Don’t make weight loss harder than it already is by tracking something that serves no purpose. Apart from keeping the five-inch monkeys busy.


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Originally published by me on Medium on January 25, 2022

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