Use lean body mass for weight

[Solidizzle]

So i am very active 4 - 5 days a week with fitness, mostly in the gym and I have been measuring and computing body fat, lean body mass and calorie estimates since 10 years. I got a versa recently more out of curiosity for fitness trackers and looking into using the heart tracker for more control. I was sorely disappointed to realize that Fitbit is just using normal weight to estimate the bmr and therefore vastly overestimating.

So I was playing around with the body fat % logging and it didn’t seem to do anything. So my question is here: Does any formula actually use the body fat % as input or is it merely a statistic for self tracking. If it is the latter then i was wondering if I can just use my lean body mass directly as weight to get a more reasonable BMR. I just did that and it did correct significantly downward and is more in line with my expectation.

[JohnnyRow]

Assuming your are really talking about BMR (basal metabolic rate), and not BMI (body mass index), you can search the web for BMI calculators.  Ones I have seen all give results very close to Fitbit's, and I am not aware of any that use fat % as a factor, though I can see that might make sense though I doubt it would make much difference.

However, I think you are thinking that it would go the wrong direction.  If you have  more muscle mass, muscles burn more calories at rest than fat does, therefore a greater BMR.  So if anything, not accounting for that would give you a lower BMR than actual, though I doubt it is much different.

[LZeeW]

@Solidizzle   The Katch-McArdle formula is an equation that uses lean body mass for estimating BMR.  Fitbit is said to use the Mifflin-St Jeor equation to calculate BMR.

The Mifflin-St Jeor is an equation that was developed to fit the averages.  Based on your self description, I imagine that you have a very lean, athletic build.  This puts you at the extreme of the curve and explains why you get numbers that seem off.

So, yes there is a formula that uses %BF.  It's not the formula that Fitbit uses.

I also suppose that if your %BF dropped below essential body fat, the Fitbit number would approach the Katch-McArdle number.  But don't do that, it's just plain unhealthy.

[Solidizzle]

Ok so basically the mifflin st jeor formula must implicitly kind of cover body fat %. As long as your body fat stays within a certain range of the average person you should be good. But that also means that using lean body mass directly as input for weight in the Fitbit app gives nonsense my BMR. So I'll stick with my normal weight for now.

I am quite athletic and big. I have 100kg with a height of 1.92m. my body fat is somewhat high at the moment with 20%. I was just surprised how many calories my versa told me I burned throughout the day when I also exercise. My BMR is at 2k and with exercising it says I burned 5k for the day. It's much more than I expected. But I guess the calorie estimate mud the taken with a grain of salt since my exercise consisted of weight training mostly.