If the treadmill says it, it has to be true, right? Not so fast. Calorie trackers may not always be as accurate as you’d think.

Do Calorie Monitors Really Work?

Calorie monitors may be the necessary motivation to keep on truckin’ through a tough cardio workout, but are they really 100 percent on the money? The experts agree: Calorie monitors often overestimate burn, and may not be equally accurate for all exercises (and exercisers) Comparison of activity monitors to estimate energy cost of treadmill exercise. King, G.A., Torres, N., Potter, C., et al. Department of Kinesiology, The University of Texas at El Paso, El Paso, TX. Medicine and Science in Sports and Exercise, 2004 Jul;36(7):1244-51. .

Burn Notice — Why It Matters

First, let’s take it back to Bio 101 (no dissections required!). When we burn more calories than we take in by exercising more or eating less, the body starts chipping away at fat reserves. To help aspiring Biggest Losers keep track, the first calorie counter was invented way back in 1977.

In a lab setting, researchers rely on highly specialized monitors (like electrocardiogram, or EKG, and the metabolic cart), explains Dr. Mark Gorelick, assistant professor of kinesiology at San Francisco State University. Researchers then use heart rate and oxygen exchange to directly calculate calories burned.

But calorie counters in the real world (read: the gym) need to rely on more simple measurements— unless dragging one of these into the gym sounds like fun. So calorie monitors cheat the system a bit. According to Lauara Streeper, a fitness specialist at the Human Performance Center at the University of California, San Francisco, they use a formula (which varies from one monitor to another) to predict how many calories a person burns while exercising rather than actively measuring complex data.

So while a treadmill at the gym may only ask for weight and age, calorie burn is affected by weight, gender, age, body composition, and exercise intensity. Whatever data the monitor doesn’t ask for, it automatically assumes a value— based on piles of research, of course— but still not tailored to the individual.

Down to the Numbers — The Answer/Debate

According to Gorelick’s research, higher-end monitors subjects wear on their wrist or arm can be accurate to within 20 calories per hour. Specifically, FitBit, MotoActv, BodyBugg, and Basis appear to have fairly accurate prediction formulas. But some monitors (including those at the gym) could be off by as much as five to 25 percent. So on the calorie tracking device podium: First place goes to wearable monitors that directly measure heart rate and other data, second place goes to calculators on exercise equipment, and a distant third place goes to calculators on websites and apps, which use no direct feedback at all. In general, “the more parameters you can have, the more accurate your equation could be,” says Gorelick.

But with all the fancy equipment manufacturers have at their fingertips, we had to wonder how some monitors can be so far off. For starters, don’t be fooled by the heart rate monitor on that exercise bike. While it may be relatively accurate, says Gorelick, many machines don’t use heart rate to calculate calorie burn. They instead calculate based on pace. A machine can’t necessarily tell how much we’re actually participating, though (a.k.a. not hanging on to the handle bars for dear life).

Streeper also points out, “Someone who’s more skilled will have a lower caloric expenditure compared to someone who is less skilled.” So fit folk are likely to have an overestimated calorie burn.

But it may not matter, after all. For those aiming to lose weight, calorie calculators may be an important tool. But they’re far from the only tool. Heart rate and exercise intensity (as measured by the “talk test”) can be equally effective for gauging a good workout. Instead of relying on pure numbers, focus on lifestyle changes that will naturally burn more calories (like taking the stairs or riding a bike to work). And for those bigger goals— like running a 10K or losing 10 pounds— consider talking to a trainer or physician to get cracking on a more specific master plan.

The Takeaway

Higher-end, wearable calorie monitors can be especially accurate, but those on gym equipment tend to be less reliable.

Have you used calorie monitors before? What’s your favorite way to monitor exercise intensity?

Photo by Ben Draper

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