can I use Fitbit Charge 2 to estimate my calories surplus?

[SunsetRunner]

Hi, I've recently successfully lost 25 kg very quickly using a Fitbit Charge HR to calculate my calorie expenditure. That rapid weight loss came at the expense of losing a lot of lean muscle.

I would like to gain that lean muscle back whilst minimising fat gain by lean bulking. My aim is to be in a calorie surplus of between 100 -200 calories per day (approx 1000 a week). 

Can I wack these additional calories onto what my Fitbit app says I'm burning or is the Fitbit too unreliable to track my daily calories accurately? 

[emili]

@SunsetRunner hey and welcome to the forums. Congrats on your weight loss and continued success in your fitness journey. There isn't a tracker out there that will calc your calories out completely accurate. They all come close. Just like you can't completely accurately track your intake, you come close. you can either ignore the number and do your own calculation or you can trick your fitbit by changing your weight so it thinks you are already there and will increase your daily intake...

[Dominique]

@Daves_Not_Here has a neat Excel sheet with his predicted weight loss calibrated for the inaccuracy of his Fitbit. Maybe you could use it with your own data?

Whatever deviation there was between calories burned reported by your Fitbit and your real energy expenditure during your weight loss should be of the same magnitude during your lean bulking phase.

What you could do also is spend 2-3 weeks finding out your current maintenance level, and then add your 100-200 calories on top of that.

[Daves_Not_Here]

@SunsetRunner - Outside of a lab, I believe it is difficult to measure caloric intake and output so accurately that you could setup a reliable 100 to 200 calorie surplus.  The reason is that I think that such a small surplus would fall within the margin of error.   And there are a number of sources of error, both in your measurement of food weights, and Fitbit's estimates of calories burned.

That said, I came up with a method for estimating and subtracting out the gross measurement error, which then causes the actual and measured results to line up uncannily well (I should maybe sell it to the climate change lobby?).  I'll detail it out in a separate post later this week, but here's a high-level description:  

1. Get a few months of data (calories eaten and burned and put your weight into TrendWeight to get a smoothed curve.
2. Compare your actual end weight with that which would be predicted based on your accumulated calorie deficit (or surplus), and determine the difference - this is your gross accumulated error.
3. Multiply by 3,500 calories per pound and divide by the number of days of data to get the daily error in calories.  If you then subtract this error out of your daily calorie deficits, your predicted weight will match your actual weight.

In my case, the error as of today is -178 calories, meaning Fitbit has historically underestimated my calories burned by 178 calories per day.  You can see how this would defeat setting up a 100 to 200 calorie surplus.