Because all Fitbit existing tracking feature (steps, active minute, distance, calories) are very easy to cheat: just manually log your exercise from Fitbit app and it will sync to beeminder, I suggest to implement new “active zone minute” feature (only available on Charge 4 I think). This new feature can not be cheated since it needs your heart rate to be able to achieve.
Read more on this link: https://help.fitbit.com/articles/en_US/Help_article/1379
Great idea! Thanks for suggesting this! (Psst, others should click 
on this to help spur us to prioritize it…)
I would also be very interested in this feature!
I would like this too, not because of cheating but because “active minutes” aren’t a very useful thing to maximize. AZM is much more useful.
You can fetch active zone minutes by getting time in each heart rate zone using this API:
Then you can calculate: AZM = minutes of “fat burn” + 2*(minutes of “cardio” + minutes of “peak”)
EDIT: I find (by viewing the source code) that actually all this is in there already. We can record Active Zone Minutes: by setting the target to “zoneminutes”, the code will check the “ActiveZoneMinutes” field and record that (writing an error out if it’s not there). Further down the code I also see that asking for “fairly_active_time” actually records “fairlyActiveMinutes” plus “veryActiveMinutes”, if the latter is there at all.
I think this is getting more important as more people are moving to newer Fitbits: What are Active Zone Minutes or active minutes on my Fitbit device?
Right now I think we are using the Get Daily Activity Summary (fitbit.com) API call. This returns (amongst other things) “lightlyActiveMinutes”, “fairlyActiveMinutes”, and “veryActiveMinutes”. As @dmalik says, we can simply calculate Active Zone Minutes from:
AZM = lightlyActiveMinutes + 2*(fairlyActiveMinutes + veryActiveMinutes)
Since users of these newer fitbits are looking at displays in their apps showing Active Zone Minutes, we really need to reflect this in Beeminder, or people will (rightly!) be confused why we aren’t showing the values they are seeing!
Ah, yes, thanks @clivemeister!
See also:
http://beeminder.com/changelog#4043
http://beeminder.com/changelog#4044