from what I see this is a very common problem and one unfortunately I hadn't realised it was occurring in the 1 year I've had a Fitbit and steadily put on 5 pounds.
I'm a very active person regularly running (6 marathons), I bought I fitbit last year as I started training more hours for middle distance triathlons and that was new and I wasn't sure how many calories I was going to be burning (especially in swimming) - I was quite bang on with the running but was unsure with cycling and swimming and wanted to have an idea of how many calories needed to put back in. For this, I was following fitbit. I log calories on my fitness pal and was always "in zone", whilst the weight just kept creeping up; I went through all the other potential reasons (hormonal stuff, liquids, etc and just hoped for it to change). Two months ago I bought a triathlon heart rate monitor I could use during all my training with garmin, and have realised that the actual calories are nowhere near what Fitbit is giving me, for a swim it is giving me something like 50% more, for running and cycling something between 10-20% more. But it's not only during exercise is from every day activities, yesterday I did a 4 hour hike and it gave me 600 calories more than Garmin.
I'm now trying to shift understanding that Fitbit is basically to be completely ignored. All my metrics are correct and I'm quite data-driven so this is not a rant of a post; this is genuinely observation of 1 year of usage (and 5 pounds gained!!); it's an embarrassment of a product; I don't need a step counter, I would certainly not buy one this expensive, I thought Fitbit was good at what it says it does but it's actually appalling. It's a shame seeing so many posts where this is a common issue, and it's a shame it will cost me 5 pounds to revert their mistake.
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