Calories burned during exercise lower after weight loss

[EL90_Lnd]

Hello,

Just a quick question about how fitbit calculates calories burned during a workout.

Over the last 8 weeks I participated in a fitness challenge, have lost 12lbs and 7% fat, and gained 4% muscle. Last week, at the end of the 8 weeks I updated my fitbit profile to reflect this. Since then I have noticed that after doing the same workouts my 'calories burned' during the workouts are lower (between 75-100 cals less).

I feel like I am working as hard during workouts as I was a week ago, so has my changing my weight etc on my profile resulted in the 'calories burned' being lower?

[SteveH]

A lower weight will certainly cause fitbit to lower its estimate of calorie burn.

It calculates 2 types of calorie burn. First there's the BMR calories. These are the calories you burn just keeping your body alive (even when asleep). You burn these at the same rate whether you are active or resting and the calculations depend on your profile settings for age, height, weight etc so reducing your weight will reduce your BMR calories.

The second type of calorie burn is your activity calories. I would expect these to also reduce if you reduce the weight in your profile. Think of it as the effort required to move around your body - the more you weigh the more effort it takes to move that weight around.

Fitbit's guide to how they calculate calories is below (but it's not very detailed):

https://help.fitbit.com/articles/en_US/Help_article/1381

[Dominique]

The impact on your basal metabolic rate (BMR) – as estimated by Fitbit – of being 12 lbs lighter should be minimal. You can quantify it using an online calculator such as this one. Let’s say you are 30-yo, 5’ 5 tall and weighed 155 pounds: your calculated BMR would be 1424 calories. Now you’re 143 lbs, your BMR would be 1370. Energy expended during activity depends on BMR, so it would make sense that for the same intensity level and the same duration, you would burn less calories if your BMR is lower.

Note that the equation used by Fitbit doesn’t take into account lean mass vs. fat mass, it works purely with age, gender, height and weight.

Can you explain what you mean by "lost 7% fat" and "gained 4% muscle"? Do you mean that your total fat mass reduced by 7% (e.g. from 47 to 43.7 lbs), or that your body fat % reduced by 7 percentage points (e.g. from 30% to 23%)? 

[bikerhiker]

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@EL90_Lnd wrote:

Hello,

Just a quick question about how fitbit calculates calories burned during a workout.

Over the last 8 weeks I participated in a fitness challenge, have lost 12lbs and 7% fat, and gained 4% muscle. Last week, at the end of the 8 weeks I updated my fitbit profile to reflect this. Since then I have noticed that after doing the same workouts my 'calories burned' during the workouts are lower (between 75-100 cals less).

I feel like I am working as hard during workouts as I was a week ago, so has my changing my weight etc on my profile resulted in the 'calories burned' being lower?

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As @Dominique has said, the Fitbit app accounts your lower body weight accounts for a lesser caloric burn for your exercise routine.  This is normal as less mass burns less calories.  However, I noticed the Fitbit app will not take account of your metabolism oxidation rate as well as your mitochondria content in your muscle growth.  What is mitochondria?  The easiest explanation is that it's like a little cell factory that burns fat through caloric oxidation.  Now typically, if you don't do intense exercises that promote the process known as mitochondria biogenesis, you will only have very few little cells of this fat burning cell furnace and contributes very little in terms of caloric burn as your muscle mass grew.  This is what I'm suspecting is Fitbit doing to be on the conservative side.  So with a few mitochondria content in your muscle mass, you'll probably be burning no more than 6 calories extra/day despite the fact that you work really hard AND is losing weight at the same time.  I noticed this as well with the Fitbit app.  Trendweight, however, is a better predictor of my weight loss.

Anyhow, the only explanation is that your exercise routine is promoting the process of mitochondria biogenesis, which means that they are a lot of mitochondria content in your muscle mass growth than say a typical person who works out, but perhaps not at the intensity and duration as you do.

So what promotes mitochondria biogenesis?

1, Endurance exercise

2, Caloric deficit

3, Acute energy crisis (100% maximum muscle failure)

It just so happens that recent research had found that acute energy crisis (the most intense 100% maxed out workout you can do as hard as you can till you almost die) promotes rapid mitochondria biogenesis through Type 2X muscle failure, which allows cell duplication of more mitochondria in your muscle growth.  You can ONLY know your mitochondria content through a muscle biopsy and look through an electron microscope.  You can't calculate this with equations.  

What this does is that, now with a high content of mitochondria, you are literally burning more calories than you would with the same muscle mass growth.  And the reason is that, for every 1lb of muscle growth you gain from Type 2X muscle failure inducing exercises, after the recovery you can burn up to 300 calories/day (figure provided by Dr. Ted Naiman M.D/ABFM from the Virginia Mason hospital in Seattle Washington) in your sleep as opposed to just a measly 6 calories/day through a normal exercise routine.  This is why the Fitbit app is underestimating your caloric burn.  It is basing its caloric burn of your exercise NOT on your mitochondrial content, rather on your muscle mass percentage you had input through known equations.  Fitbit is using what can be applied to everyone.  The Fitbit app can't do a muscle biopsy, so therefore it has to provide an average general caloric consumption on your muscle gain %.

And for this reason, I had stopped using the Fitbit app tracking my calorie deficit.  It's no where accurate for what I do anymore.

Hope this helps.