My Fitbit is making me fat!..??

FitBit One.  photo: Donna P Feldman

FitBit One

The flurry of emails from colleagues this week screamed

My Fitbit is making me fat!

Whaa?  How is that even possible?  Does the popular step counter emit anti-metabolism rays?  Does it mysteriously rev up appetite?

So I ‘Searched’ it and found the evidence.  And I was not surprised.

Here’s the basic problem: Fitbit and other handy digital step counting gadgets were designed by engineers — techno geeks with zero understanding of nutrition.  They think the human body is some kind of predictable machine, running like a car.  You put energy (gas) in, you burn it off based on a predictable calorie count (mileage), you put more energy in; repeat.  Unfortunately the human body and human metabolism are vastly more complicated than that.  But when you design a gadget like this with no input from nutrition experts, you end up with a gadget that can make you fat.

Here’s how it happens:

  • Users of step counters and activity trackers believe they are getting extremely accurate calorie burning estimates, because it comes from a computer.
  • The tracker gave a calorie estimate, based on steps taken, activities logged and weight loss goals, but the estimate is too high.  Why?  Because these are, at best, estimates, based on averages; because the users picked overly optimistic exertion levels for their activities.
  • The user believes the calculation, ignores hunger, and eats all those calculated calories.  After all, it’s coming from a computer, it must be accurate, right?  Why pay attention to actual hunger?
  • The user feels entitled to backload (eat back) calories they theoretically burned with all those steps.  Wow, great!  I can reward myself with food.


The human body is not a machine.  Calories in food, calories burned with activity, portion sizes and day-to-day calorie needs are very inaccurate and vary wildly.   Your 2 mile walk on Monday may burn far fewer calories than your 2 mile walk on Wednesday, even if you go the same pace along the same route.  Yet a tracker will simply calculate 2 miles = 6000 steps = X calories.

Take Jane Doe.  She wears a step counter, and added a 3 mile run to her day.  She went at a very modest 10 mile/hour pace, but entered her pace at 9 mph.  She also over-estimated the time she spent actually running.  Her calculated calorie burn is now 200 calories too high.

She decides to reward herself with a 200 calorie bowl of ice cream that evening.  Except her bowl is a cereal bowl, while the calorie tracker’s bowl is the size of a coffee cup.  Plus her ice cream is premium.  The total is not 200 calories; it’s 300.   She’s now at a +500 calorie difference, although she thinks she’s still on track to lose weight.  Repeat this miscalculation every day for a week and she might gain a pound.  And she probably never once felt hungry.

There’s another problem with this type of system: backloading calories — rewarding yourself for exercising by eating back those calories, typically as some high calorie late night treat.  There’s actual evidence that backloading calories results in poor weight control, more body fat and less fitness, compared to eating those calories before exercise, to be burned as fuel.

Does this mean activity/step trackers are useless?  Yes and no.  I would never pay attention to those calculated calorie levels.  Eating by numbers is a bad idea.  If you want to lose weight, you need to eat fewer calories than you burn, and the only good way to know that’s happening is if you’re hungry.  Hunger is Mother Nature’s way of telling you you haven’t eaten enough.  And hunger can take hours or even days to catch up to you after a strenuous activity.  Rewarding yourself with treats when you’re not even hungry is a sure way to defeat your weight control efforts.  Use a step counter to do just that: track your steps.  It’s a useful way to nudge yourself to stay active; just don’t mistake random steps taken back and forth in your kitchen or around the office as having the same calorie burning potential as steps you take walking at 4 miles/hour up a steep hill.  And don’t reward yourself with high calorie treats.

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