Calories Burn

[danparker70]

Due to this COVID pandemic and going back into lockdown here in southern California, I haven't been that active and noticed that my weight is creeping upward.  I want to start using my calories burned as a guideline (I have a Fitbit Charge 2) / sounding board for the amount of calories I take in every day.  If I don't lose  the weight, at least I won't gain anymore.  How accurate is the calorie burn on my Charge 2?  How is it determined?

Thank you.

[Heybales]

The biggest part of your daily burn is your Basal Metabolic Rate (BMR), considered your sleeping burn rate - Fitbit uses that for all non-moving time.

Based on gender, age, weight, height - they use formula very close to Mifflin-St Jeor BMR calc.

Of course if awake, or standing with no steps - you burn slightly more, so it's slightly under-estimated.

Your moving around burn is based on distance and mass being moved - very accurate formula based on decades of research studies of people on treadmills.

Getting the distance from steps though could be interesting depending on how average you are - your default stride length is based on gender and height - and you may be close, you may be no where near.

If you have 5K or less steps really doesn't matter - start racking it up though, now it matters for accuracy and calorie burn.

Test walk a known 1 mile distance at about 2.8 mph to confirm what Fitbit reports as distance.

You use a middle of daily used paces from grocery store shuffle to exercise pace walk for that test, the Fitbit will dynamically adjust calculated distance up or down based on each step impact it sees.

Above daily into exercise it'll use HR-based calculated calorie burn.

That calc is only valid for steady-state aerobic - same HR for minutes on end, not intervals or anaerobic like lifting.

So potential for inflated calories if your workouts are intervals or lifting, or if it starts using that mode on when it should be doing daily level stuff by distance calculated calories.

And those calculations even for range it can be best estimate for, are based on some rather lose assumptions of HRmax and fitness level, and your HR can be inflated for stress, dehydrated, cooling, ect that doesn't mean you are burning more calories just because the HR is higher.

So if you are average person it can work well on, in about 2 weeks after it gets some stats from your daily life - you could have decent estimate.

If you get tons of steps daily, do nothing but a lot of interval workouts and lifting, and take meds that elevate your HR all day - it's going to be terrible estimate.

But either way - it is merely getting you honed into an estimated figure.

Very accurate logging of what you eat by weight tells the other half of the story, and your actual weight change for normal reasons.

You'll be adjusting no matter what, the amount of adjustment and reliability as activity level changes is what can make it easier or harder.

So how accurate?

Basically it totally depends.

One month ramping up for a triathlon I logged very accurately for foods, and used the best non-Fitbit calculations for my workouts - my weight change was 5% of expected on paper by the numbers.

Considering food labels are allowed to be upwards of 20% off - that's probably about the best you'll get.

Currently doing the same thing as far as losing some injury and Covid weight - and my estimated burn with good workout estimates, seems to be for the last month, about 700 cal less daily than my real daily burn based on actual results. Food logging this time isn't as tight, but I know it can't all be there.

[emili]

@danparker70 as mentioned in the previous post it is a combination of what your body burns doing nothing and what your body burns doing something. The charge 2 has HR built in so it also measures your calories out by the rate of your HR as you are doing something. I would use an average to determine what you are burning daily provided that your activities are similar. this way if it is overestimated or under you still have a pretty good baseline to help with decisions about food.