In 2007, Wired Magazine editors Gary Wolf and Kevin Kelly coined the term ‘quantified self’. A fancy phrase for ‘self-tracking’.
Two years later, in 2009, Fitbit launched its first product–the eponymously named ‘Fitbit.’
A small matte-black device that users could clip on to their clothing, allowing them to track metrics like daily step count, calories burned, and even sleep.
Fast forward to today, and it’s hard to deny the self-tracking movement has firmly embedded itself into our culture with the proliferation of fitness trackers and apps.
In 2014, five million smartwatch units were sold, and by 2018 this number had ballooned to 141 million. As of 2018, Fitbit is the most popular health and fitness app with 27.4 million users. 1
Invariably, the self-tracking movement is in full swing and the use of these devices is only set to increase as companies continue to innovate and make their devices more intuitive and allow its users to track and collect more data on themselves–from heart rate, to sleep quality and quantity, calorie expenditure, and yes, even their monthly cycle.
While these devices can be great tools, they can also cause problems and backfire. One of these problems has become more pronounced over the last few years as fitness apps and devices have begun allowing cross-integration. The problem I’m referring to is Net Calories.
MyFitnessPal, the popular calorie-tracking app, describes Net Calories as, ‘a daily budget of calories to spend’.
For example, if you need to eat 2000 calories to lose weight, and you burn 700 calories through exercise–MyFitnessPal shows you this:
In the example above, even though you’ve hit your daily goal of 2000 calories from food, MFP is telling you to ‘eat back’ the calories you burned through exercise (based on the number given to it by your activity tracker).
But you shouldn’t be eating back those calories because your activity tracker is most likely overestimating the number of calories you’ve burned.
A 2016 study looked at the energy expenditure on several Fitbit products and the Jawbone UP24
And found the devices and found the devices overestimated energy expenditure by 16-40% during activities like walking, jogging, and climbing stairs. 2
A year later, in 2017, researchers from Stanford University investigated the accuracy of several commercially available wrist-worn devices, including the Fitbit Surge and Apple Watch.
While the devices were pretty accurate for heart rate, they were all off for energy expenditure by 27.5% to 93%. 3
These aren’t trivial numbers.
At a 30% error rate, this number could be overestimated by ~180 calories; and a 40% error rate could mean an overestimation of ~240 calories.
This is why if you’re eating back the calories your activity tracker is reporting, you could very easily wipe out the calorie deficit and best-case, it slows down your progress and worst-case, it completely stops progress.
Another thing that’s worth remembering is that as you lose weight and get leaner, the number of calories you expend through daily movement and exercise decrease. These devices don’t take these changes into account. So as you lose weight, you’re actually burning fewer calories than the device is reporting.
Even if we ignore these inaccuracies, eating back the calories you burn through exercise defeats the purpose of increasing movement.
Doing more steps or even engaging in formal cardio, like running, should be used as a tool to increase the calorie deficit when reducing food intake further isn’t feasible. But if you eat back those calories, it defeats the entire point of increasing activity in the first place. 4
It’s great we have access to these tools but it’s important to use them correctly.
Use activity trackers as a way to quantify movement and to encourage you to be more active. If you do cardio as a way to increase your calorie expenditure, that’s fine, just don’t eat back those calories.
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