Don’t trust your fitness tracker’s calorie estimations

A study by Stanford University of Medicine has revealed the inaccuracies of fitness trackers, and the results will be pretty alarming for anyone who depends on their fitness tracker to decide how much they can eat after their workout.

The study tested seven different fitness trackers - the Apple Watch, Basis Peak , Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2. The test subjects were a variety of ages, races, genders, and fitness levels.

The subjects were measured engaging in a range of activities, including sitting, walking, running and cycling while wearing fitness trackers. The results from the trackers was then compared to the industry standard tests for measuring heart rate and energy expenditure.

Across the board the bands were more efficient in gauging heart rate than energy expenditure, with the Apple Watch being the most efficient at heart rate measurement. Where the results get interesting is the inaccuracies in measuring energy expenditure: “No device achieved an error in [energy expenditure] below 20 percent.”

As a minimum level of fault that’s very high. The Fitbit Surge, the leader of the pack, was out by a whopping 27.4%. What’s even more alarming is that the level of error ranged all the way up to a frankly unbelievable 92.6% for the PulseOn.

Calorie counting

The principle of ‘calories in versus calories out’ is still a contentious one in the fitness industry but is the basis for how most people measure the amount of food that they can eat and the frequency and intensity of their workouts.

The basic idea is that food = energy (measured in calories). Using your body (we intentionally haven’t written exercise because exercise makes up a very small amount of daily calorie expenditure) burns calories. When you consume more calories than you burn, you put on weight.

There are now apps to measure calorie consumption, that helpfully synch with fitness tracker apps to measure how you’re doing with your calorie balance.

The difficulty with these apps is that they rely on honest reporting by the user, which often doesn’t happen. Sometimes because people unintentionally under-report, but more often because they require you to weigh your food, and frankly who has time for that?

Add into the equation incorrect tracking for calorie expenditure and you are very quickly in a position where you are consuming far more calories than you’re burning, and gaining weight where you should be losing it.

One size doesn't fit all

One of the reasons that these trackers are so much more efficient at heart rate than energy expenditure is to do with how they are measured. Heart rate is measured directly, by sensors detecting the pulse.

Energy expenditure is measured using algorithms, and according to one of the report’s authors Anna Scherbina: “My take on this is that it’s very hard to train an algorithm that would be accurate across a wide variety of people because energy expenditure is variable based on someone’s fitness level, height and weight, etc.”

And that's fundamentally the problem, when it comes to fitness, one size really doesn't fit all and the algorithms needed to accurately measure calorie expenditure aren't there yet.

The positive that can be taken from this is that algorithms can and will improve, so the technology will only get better over time. So don’t give up on fitness trackers just yet. Unless you’re using the PulseOn.