Is your fitbit making you fatter

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Hi All, just did a 85 minute session on the treadmill. Consisting of 4 miles fast walk (4mph) and a 2.25 run walk run

Treadmill Cals burnt 560
HRM Cals burnt 668
Fitbit Cals burnt 839

So if i were to eat back my fitbit cals...Surely i would be gaining weight?