How Fitness Trackers Sabotage Weight Loss

Lots of people use diet and activity trackers to log their food intake and exercise. After all, there’s an old saying that “you can’t manage what you don’t measure.” And yet it seems to be backfiring.

I get email after email from people using these trackers who can’t understand why they’re not losing weight. They’re entering in every morsel of food and logging every activity. According to their trackers, they should be shedding two or three pounds a week. And yet the scale hasn’t budged—or they’ve actually gained weight!

The problem with net calories

Here’s how many of these trackers work: You start every day with a certain number of calories to spend. That number is based on your height, weight, age, sex, activity level, and your goals—that is, whether you’re trying to lose, gain, or maintain your current weight.

How Many Calories Do I Need?

Calories are subtracted from your balance as you log your meals into the diet tracker over the course of the day. Ideally, you don’t get to zero too early in the day. But if you do, there’s a solution. Let’s say it’s 5 pm and I’m down to my last 400 calories. But wait! I can take an evening run, log it into the app and now I’ve got 840 calories to spend on dinner! How awesome is that?

The general principle here is sound: The more you move, the more you can eat. In practice, however, these “net calorie” calculations are inaccurate and misleading—and they are suckering people into eating too many calories. Let me explain.

How logging exercise leads you astray

Although diet tracking apps can help you get an accurate picture of your calorie intake, they are much less reliable in determining how many calories you burn. Here are at three ways they tend to get it wrong.

Mistake #1: Your baseline may be too high.  In order to calculate your baseline calorie requirements, you indicate your activity level: sedentary, lightly active, moderately active, or very active. This does not refer to how much you exercise (we’ll get to that in a moment). This is just about your daily activity level. And guess what? Most people select an activity level that's one or two categories higher than their lifestyle actually warrants. Unless you rope cattle eight hours a day, your lifestyle probably does not qualify as “very active.”

If you use a wearable fitness tracker like a Fitbit or Apple Watch or even a low-tech pedometer or step counter, you can use that to help you select the proper category for your lifestyle. Here's an easy cheat sheet:

• Fewer than 1,000 steps a day is Sedentary.
• Fewer than 10,000 steps or about four miles a day is Lightly Active.
• Ten to 23,000 steps or four...

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