Fat burn zone changed

[SunsetRunner]

since the Fitbit update a couple of days ago my fat burn zone has shifted from 91 up to 105. Any idea why this has happened and can it be changed back?

Moderator edit: subject for clarity 

[SteveH]

This will be down to the change from Active Minutes to Active Zone Minutes that the latest update has introduced.

With the old way Fitbit estimates your maximum heart rate with the formula of 220 - your age, and calculates your heart-rate zones based on that number

With the new Active Zone Minutes Fitbit personalizes your heart-rate zones using your heart rate reserve, which is the difference between your maximum heart rate and your resting heart rate

It seems that once we apply the update and switch to Active Zone Minutes, it's not possible to switch back.

There's much more on how this works at https://help.fitbit.com/articles/en_US/Help_article/1565.htm and there's a help file for the new Active Zone Minutes at https://help.fitbit.com/articles/en_US/Help_article/1379.htm

[SunsetRunner]

So can your zones change and reduce the start of fat burn back to a lower number?

Sent from my iPhone

[SteveH]

Yes. The breakpoints of all your heart rate zones can change due to the move to Active Zone minutes.

The start of the fat burn zone used to be 50% of your max heart rate (which was calculated as 220 minus your age).

The new calculation is 40% of your heart rate reserve (which is calculated from max heart rate - resting heart rate) plus your resting heart rate. 

There's a good worked example of the calculations of the zones in those help files.

[VioletaSR]

So, does it mean the new way of calculating is supposed to be more accurate?

My zones have shifted a lot. I used to be in fat burn and cardio every time I work out but now, half of the time of my workout is "below zones". Also, now, when I go for a walk, I am usually 100% "below zones".

[SteveH]

I think their intention was to make the zones tailored to us individually rather than having everyone's zones based just on our age. The new zone targets are based on external guidance:

"your goal is to earn 150 Active Zone Minutes each week. The American Heart Association and World Health Organization recommend at least 150 minutes of moderate-intensity activity or 75 minutes of vigorous activity, or a combination of both, each week. If you earn an average of 22 Active Zone Minutes daily, you meet these recommendations"

It's not clear whether Fitbit's calculation of the zones to meet that guidance is their own or whether they are using published algoriths. 

Whether the new zones or the old zones are more accurate is difficult to say. They are different and judging by the posts I've seen, the new zones are harder to hit but that not make them less accurate - perhaps the old zones were wrong!

[MrMarv]

@SunsetRunner -

Actually @SteveH answered your question regarding the Heart Rate Zones being recalculated by explaining Active Zone Minutes which was rolled out at the same time.

The Heart Rate Zone calculations have moved from the Traditional formula which are straight fixed percentages (50, 70 and 85%) of your Max Heart Rate which is defined as subtracting your age from 220. For me, as a 68 year old male, my Max Heart Rate is 152. Traditional zone values for me are shown in the first data row below.

The new formula being used by many FitBit devices use a value called Heart Rate Reserve. Subtracting your resting heart rate from your max heart rate results in that reserve.

40% of the reserve is added to your resting heart rate to establish the beginning of the Fat Burn Zone.

60% of the reserve s added to your resting heart rate to establish the beginning of the Cardio Zone.

85% of the reserve s added to your resting heart rate to establish the beginning of the Peak Zone.

The new Heart Rate Reserve formula zones for me are shown in the second data row below.

Actually, my Max Heart Rate is 120 (80% of traditional max) because I take Beta Blockers that throttle down my heart rate even during exercise. This was confirmed with my Cardiologist yesterday during my biannual office visit and is discussed in the Harvard Medical School Harvard Health Letter from November of last year. My heart rate zones utilizing the Heart Rate Reserve formula and using my redefined Max Heart Rate is represented by the third data line below.

BTW, even with the zone changes, my actual calorie burns for the same amount of exercise remain the same. Maybe that is why many references on the web regarding Heart Rate Reserve talk about "Training Zones" and are targeted towards athletes, including Olympic hopefuls. This isn't the first time FitBit has tried to be cute without coming completely clean about it.

I hope this helps - and welcome to the boards.

[Msheathersmith]

Thank you!!!! I was wondering! I went from an average of two hours plus fat burning in an average day, right down to currently about 15 mins! My resting heart rate is usually in the 50’s so obviously with the latest update I have to work harder to go into fatburning zone. 

[Wexford_Lass]

Hi, so glad I searched the Community forum for the answer to my question rather than start a new thread. Thank you for your detailed response. I have just upgraded from a Charge 2 to a Charge 4 and was more than a little perturbed to discover that my Fat Burn zone had shifted considerably from starting at 85 to 104 now and cardio from a starting point of 116 up to 125 now. 

Now I understand why and yes the zones are harder to maintain but as you rightly point out maybe this system is more accurate & we simply need to move faster for longer 😅  

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@SteveH wrote:

Whether the new zones or the old zones are more accurate is difficult to say. They are different and judging by the posts I've seen, the new zones are harder to hit but that not make them less accurate - perhaps the old zones were wrong!

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[Meknessy]

It is very annoying and hard to customize anything with fitbit. I will no longer buy their products!

[SunsetRunner]

Is there a way to let the app know you are on beta blockers?