Can fitness trackers accurately measure calories burned?

Can fitness trackersactually measure your calories?
Are fitness trackers reliable at measuring your energy expenditure during exercise?
 
This was actually a very small study and only looked at seven fitness trackers (Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2).  This said, the results were interesting  and on a positive note the results showed that heart rate monitoring was accurate in six devices with an error rate of <5% – thats pretty good.  However most of the trackers were incorrect in displaying actual calories burned when it came to ‘calories burned’ compared to energy expenditure tested on each individual.  All trackers varied significantly, and their error of estimating calories burned were between 27%-93%.  Not only does this show the variability of these devices, but more importantly how vastly inaccurate they can be.
 
I feel like this is no surprise, however this is worrying information for those relying heavily on these results to alter their dietary intake.  For example, if a fitness tracker indicates Howard burned 800 kcal and as a result Howard then ate an extra 800 kcal, if we use a tracker with a 25% accuracy, Howard would have actually only burned 200 kcal!  This would result in a positive energy balance (weight gain) because Howard would have eaten an extra 600 kcal, which was not burnt off (sorry Howard).  
 
Advice
Counting calories is not really the best way to monitor your health and dietary needs. Individual energy needs can vary greatly between individuals and just because inputting your height and weight into a fitness app, will not result in these being super accurate.  Despite this being a small study, the results question how strictly those using these trackers should rely on their data.
I would certainly avoid monitoring the kcal function on these trackers.  Having said that, I think the fitness trackers can have a great influence on individuals monitoring their fitness and using these as markers to help improve themselves.  
In terms of diet forget counting calories as this can be tiring, not always accurate and can lead to a negative relationship with food.  Instead focus on a healthy balanced diet full of variety while eating energy dense foods in moderation.  Choose a diet abundant in fruits and vegetables, try and fill half you plate with these.  Swap to whole grain carbohydrates and avoid having too large a portion here.  Include a variety of protein sources (rather than just meat look at fish, eggs, beans, pulses, tofu & low fat diary products).  Minimise the consumption of foods high in fat, sugar & alcohol and instead of removing these completely find healthier alternatives.  If you look at your plate; 1/2 is vegetables, 1/4 wholegrain carbohydrates and 1/4 protein.  Many may find this more reliable for weight loss and therefore reduce the obsession with calories (which mentally will make you feel better too).
This story relates to news articles from The BBC, The Guardian & The Daily Mail.
Note: Howard is actually my dog! 
Shcherbina, A.; Mattsson, C.M.; Waggott, D.; Salisbury, H.; Christle, J.W.; Hastie, T.; Wheeler, M.T.; Ashley, E.A. Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. J. Pers. Med. 2017, 7, 3.
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