Is Your Cardio Machine Misleading You? Say Goodbye to Fat-Burning Zones

One of the most frustrating concepts you’ll hear in fitness circles is the idea of the “fat-burning zone.” In essence, it’s the idea that you burn a greater percentage of fat when your heart rate is lower than at higher rates. In other words, you’ll see fat-burning zones on cardio machines at the gym set to about 65% of the max heart rate for age brackets.

On the surface, it might sound like a great idea to be cranking away at the elliptical machine at 65% of your max to burn more fat, but it’s a lot more complicated than this. If you exercise at this intensity for 30 minutes versus doing high-intensity intervals, you’ll burn more total calories at the higher intensity. In other words, you’ll need to be on that elliptical machine a little longer if you want to work towards a calorie deficit. That’s really how to lose weight overall–you must burn more calories than you consume (but not too much at one time).

Now researchers at Mount Sinai Hospital are finding that these fat-burning zones set on machines aren’t very accurate. It makes sense because these aren’t adjusted for sex or level of fitness, only by age. The study, recently published in Nutrition, Metabolism & Cardiovascular Diseases, had people exercise at different intensities while the researchers measured their watts. The researchers also measured their breathing and oxygen/carbon dioxide exchange.

The researchers used a mathematical model to see how the exercisers used fat for energy relative to their peak VO2 and heart rate. They compared this maximum fat-burning rate with various percentages (i.e. 50%, 70%, and 80%) and found that the optimal fat-burning zone was off by an average of 23 beats per minute. 

“We hope that this work will inspire more individuals and trainers to utilize clinical exercise testing to prescribe personalized exercise routines tailored to fat loss. It also emphasizes the role that data-driven approaches can have toward precision exercise,” said senior author Girish Nadkarni, MD, MPH in a news release.

If fat loss is your goal, it might be best to ignore those numbers on the cardio machine and exercise at an intensity that feels right for you. It’s better to exercise at a comfortable (but not TOO relaxed) intensity so you don’t burn out after 10 minutes. As your endurance improves, you’ll eventually want to extend the time and intensity of your cardio workouts. It’s also probably better for you to do a little every day than doing long bouts of exercise less frequently–that way you get yourself into the mindset of getting activity every day.

Rather than rely on the calorie counter on the machine (which overemphasizes how many calories you burn), it’s better to use a personal fitness tracker that can give you better estimates of your fitness and fat-burning progress. These days, the heart rate monitor on personal fitness trackers are much more accurate than they used to be, but even they can be off depending on their position on your wrist or whether you’ve taken decongestants that day. That’s why I pay more attention to how I feel during each moment of my workout to make sure I’m not extending myself too much for that given day.