If you’ve hopped on any cardio machine ever, you’ve probably seen the graph or different colored hearts (or whatever) identifying the different exercise ‘zones’ that use your heart rate to categorize the intensity of your workout.
Which zone we should be cardio-ing away in, though, isn’t so clear—and the enticing ‘fat-burning’ zone, in particular, is actually pretty misleading.
You’re in the ‘fat-burning zone’ when you exercise at a pace that gets your heartrate up to between 60 and 75 percent of your maximum (220 minus your age). “If you measured your exertion or effort on a scale of one to ten, the fat-burning zone would be a five or six,” explains exercise physiologist Pete McCall, M.S., C.S.C.S., C.P.T., host of the All About Fitness Podcast. This is a pretty low-intensity pace, and you’ll probably be able to carry on a conversation as you move.
Given its name, you’d think the fat-burning zone is where you want to be if you’re trying to lose weight, right? Well, not quite.
Workouts that focus on the fat-burning zone are a rooted in the outdated (but persistent) belief that long, slow workouts are more effective for weight loss than shorter, more intense workouts. Here, exercise experts break down why the fat-burning zone isn’t really your fat-loss friend.
The Fat-Burning Basics
To fuel literally everything we do, our body produces and uses a form of chemical energy called ATP (adenosine triphosphate). What we create that ATP from, though, depends on what ingredients we have in our system (like carbs or fat from food, or stored body fat) and how much energy we need how quickly (depending on whether we’re just hanging out or sprinting, for example).
Technically, the fat-burning zone is legit: At lower intensities, our body’s primary ATP fuel source is fatty acids from food or body fat, whereas at higher intensities—usually an effort level of seven or higher—we primarily use the carbohydrates circulating in our bloodstream as sugar or stored in our muscles as glycogen.
“Our body needs oxygen to turn fat into ATP, and when we’re resting or working out at a low intensity, that oxygen is readily available,” explains Tiffany Chag, M.S., R.D, C.S.C.S., sports dietitian at Hospital for Special Surgery. When we work out at higher intensities, oxygen becomes scarce and our body turns to carbs, instead. It’s never entirely one or the other—just a different proportion: Fat can still account for between 10 and 45 percent of our total energy expenditure during high-intensity exercise.
Exercise Intensity And Weight Loss
While it’s true we burn a higher percentage of calories from fat in the fat-burning zone, that doesn’t translate to quicker fat loss. Burning more calories total—regardless of whether the energy used comes from fat or carbs—is what matters for fat loss, explains running coach and exercise physiologist, Janet Hamilton, MA, RCEP, CSCS, founder of Running Strong.
Unsurprisingly, we burn more calories overall when we work out at higher intensities—like 75 to 85 percent of our max heartrate, or an effort level of seven or eight—because our organs and muscles have to work harder to meet that high energy demand. For example, a 155-pound person burns about 260 calories cycling at a moderate pace for 30 minutes, but churns through about 315 at a more vigorous pace.
That’s where HIIT (high-intensity interval training), which involves alternating between short intervals of max-effort and intervals of low-intensity recovery, comes in. By upping the intensity so much (even just for short bursts of time), we can burn just as much fat, if not more, in less time—even if carbs account for a larger percentage of our calories burned, explains Christi Marraccini C.P.T., Head Coach at Tone House in New York City.
Related: 7 HIIT Workouts That Incinerate Fat
By pushing so hard during HIIT’s work intervals, we increase our body’s demand for oxygen during the rest intervals, and throughout the rest of the day after the workout, explains McCall. (This is called ‘EPOC,’ or excess post-exercise oxygen consumption.) Our body continues working harder-than-usual to deliver that oxygen, and we continue burning calories.
The Time And Place For The Fat-Burning Zone
HIIT is great, but too much high-intensity work can lead to injury or burnout over time—so our experts recommend your weekly workout routine strikes a balance between higher- and lower-intensity exercise. Every other—or every third—cardio workout can be HIIT, but the rest should actually land in that fat-burning zone. “This kind of breakdown will allow your body to recover and your muscle to repair after tough workouts, while still giving you the opportunity to move,” says Chag.