Does Fitbit detect a lowered BMR due to "starvation response"?

[Daves_Not_Here]

After an initial large water weight loss, I'm losing about 2 pounds per week over the last month.  So far, my weight changes as expected - 1 pound lost per 3,500 calorie deficit.  I'm curious if Fitbit can detect the lowering of my BMR due to a "starvation response" if it occurs.

At the same time, my resting heart rate has gone from 65 to 52 -- does anyone know if that will be reflected in a lowered BMR calculation by Fitbit?

Last question: as I lose weight, will Fitbit adjust my BMR downward?  My activity level is so variable, I can't figure out what is exercise versus BMR.

Just trying to head off the plateau.  Thanks for any help!

[JenniferinFL]

It definitely adjusts for your heart rate. It won't notice other metabolic changes though. You'd have to go to a hospital and have your expired lung gas measured to tell how many calories you were actually burning. Fitbit gives you an estimate based off your heart rate and activity. 

[Baltoscott]

It will adjust exercise burn based on hr rate and activity detected by the device's motion sensors (unless you manually add an activity to overwrite what the tracker detects). Age weight and sex are also used in the exercise burn calculation..  BMR is calculated by age weight and  sex only -- using one of.the standard formulas.  To keep your burn calculations as consistent as possible update your weight as it changes.  (For me the caloric burn, or perhaps my food logging, and the calculated caloric deficit between those two measures, predicts a greater weight loss than what I have actually experienced over the past six months, so I just adjusted the deficit to keep my loss at a little over a pound a week).

[Daves_Not_Here]

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@JenniferinFL wrote:

It definitely adjusts for your heart rate. It won't notice other metabolic changes though. You'd have to go to a hospital and have your expired lung gas measured to tell how many calories you were actually burning. Fitbit gives you an estimate based off your heart rate and activity. 

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Thanks Jennifer!  

My understanding is that Fitbit uses heart rate to calculate calories burned during exercise.  My question is whether Fitbit uses resting heart rate to calculate changes in Base Metabolic Rate.

For example, in the last 3 weeks my resting heart rate went from 65 to 52.  Will Fitbit lower my BMR assumption based on that drop? 

[Daves_Not_Here]

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@Baltoscott wrote:

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...  BMR is calculated by age weight and  sex only -- using one of.the standard formulas.  To keep your burn calculations as consistent as possible update your weight as it changes.  (For me the caloric burn, or perhaps my food logging, and the calculated caloric deficit between those two measures, predicts a greater weight loss than what I have actually experienced over the past six months, so I just adjusted the deficit to keep my loss at a little over a pound a week).

What I'm hearing from you is that the lowering of my resting heart rate will not affect Fitbit's BMR calculation, correct?

As far as your having to adjust the deficit, do you think this was due to a lowering of your BMR not accounted for by Fitbit?  And did it seem like the prediction was initially accurate but over-predicted later in the six months?

My weight loss is almost exactly as predicted, but I'm only 1 month in. 

[Baltoscott · Accepted solution]

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@Daves_Not_Here wrote:

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@Baltoscott wrote:

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...  BMR is calculated by age weight and  sex only -- using one of.the standard formulas.  To keep your burn calculations as consistent as possible update your weight as it changes.  (For me the caloric burn, or perhaps my food logging, and the calculated caloric deficit between those two measures, predicts a greater weight loss than what I have actually experienced over the past six months, so I just adjusted the deficit to keep my loss at a little over a pound a week).

What I'm hearing from you is that the lowering of my resting heart rate will not affect Fitbit's BMR calculation, correct?

As far as your having to adjust the deficit, do you think this was due to a lowering of your BMR not accounted for by Fitbit?  And did it seem like the prediction was initially accurate but over-predicted later in the six months?

My weight loss is almost exactly as predicted, but I'm only 1 month in. 

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Correct.  Your lower heart rate will not be taken into account when fitbit calculates your BMR.  I'm not even sure lower resting heart rate correlates very well with BMR such that fitbit could use it as an adjustment factor if they wanted to.

I think my true BMR is just lower than fitbit's calculation of that number.  For virtually every week I have lost weight, fitbit's predicted weight loss based on the calorie deficit it calculated was higher than my actual weight loss.  I just kind of worked things backward.  So long as I was losing about a pound a week according to TrendWeight (which uses a moving average of my daily weigh-ins), I figure was operating at about a 500 daily calorie deficit even if fitbit thought it was more like a 1000+ calorie deficit.  

The Wikipedia article on BMR is interesting reading.  In particular, the short paragraph on "Causes of individual differences in BMR,"  might be a surprise to some folks who assume (as fitbit and most internet calculators do) that everyone of the same sex, age and weight has the same BMR. One factor fitbit doesn't  take into account is lean body mass, even though that factor accounted for about 62% of the difference across study group.  That is not really fitbit's fault though as getting accurate estimates of body fat and lean body mass is itself very hard, and adding one more unreliable factor to the equation won't make it any more accurate. It is also notable that 26% of the variation in BMR was unexplained.  

And the differences can be dramatic. The article cites a study of individuals with the same lean body mass where that top 5% of BMRs were 28-32% higher than the lowest 5% of BMRs.  Two individuals with the same lean body mass in that study had a difference of 715 calories/day attributable to their BMRs.  Think of that!

[Daves_Not_Here]

Hey Baltoscott -- really appreciate your indulging me with such an in-depth response.  It was exactly the information I was looking for.  I'm smarter after having read your post.

Subject for another thread:  when researching this question, I've come to be open to the idea that having a slower metabolism may not be such a bad thing after all, assuming you don't overeat and gain weight.  There is some thought that a slower metabolism is associated with lower stress and increased longevity, and that that is one reason why women outlive men.  Requiring less calories to live seems advantageous.