Step on the treadmill or check your wearable fitness tracker, and you’ll likely find an array of settings aimed at helping you optimise your workout. Toggling through the menu, you might see a prompt to pick a “fat-burning” or “cardio” mode for your session—and then scratch your head in confusion. What’s the difference? And is one mode more effective than the other at helping people lose weight?
These labels, which reflect workout intensity and are usually tied to heart rate, can be puzzling, acknowledges cardiologist Nieca Goldberg, MD. Read on as Dr. Goldberg and other leading experts decode what each zone really means in terms of weight loss, fitness, and health—and offer evidence-backed reassurance that the best workout is the one that works for your life.
The difference between fat-burning and cardio zones
First things first: Your body needs fuel for physical activity, whether you’re biking, swimming, or rearranging living room furniture. That fuel comes from two main energy sources: Stored fat, or stored carbohydrates in the form of glycogen released from muscle tissue.
The energy source the body taps depends on the intensity of movement, explains exercise physiologist and registered dietitian, Michele Smallidge. Heart rate is considered the best measure of intensity, and that’s what fitness gadgets use to determine “cardio” and “fat-burning” zones.
In marketing-speak, the fat-burning zone is generally billed as the setting for people hoping to lower their percentage of body fat. Meanwhile, the cardio zone—in which the body uses glycogen for energy—is billed as a way to boost cardiovascular endurance.
In science-speak, however, both types of workouts can burn fat and support cardiovascular health, our experts say.
How maximum heart rate factors in
Every person’s “zones” are unique, Smallidge says. They’re calculated according to a number known as maximum heart rate (MHR). This is the uppermost number of beats per minute (BPM) your heart can reach during a super strenuous burst of activity—think, sprinting up a hill. MHR is determined by factors such as age and genetics; it’s not a measure of how fit you are, Smallidge adds.
Whether your workout falls into the so-called fat-burning zone or cardio zone depends on how close you are to hitting your MHR. Here’s how the distinction generally breaks down, according to Dr. Goldberg:
- Fat-burning zone: 50–65% of MHR. Of the two zones, this denotes a lower-intensity workout. In addition to noticing that your heart rate is quickened, you’ll likely find yourself breathing a bit more heavily. People who normally speak are able to do so comfortably. In this zone, the body is more likely to tap stored fat, rather than glycogen, for fuel.
- Cardio zone: 75–85% of MHR. This signifies a higher-intensity workout. Your heart is pumping quickly, and you might find it tough to speak in complete sentences because you’re panting. (You’re probably pretty sweaty, too.) The body fuels this activity mostly by tapping glycogen from muscles, along with a smaller measure of fat.
Most people don't know—and don't need to know—their true MHR, which can only be determined by a medical evaluation that includes a maximal stress test, Smallidge says. Instead, she and many other fitness pros recommend a simple formula for estimating MHR: Subtract your age from 220. Here's how that breaks down for a person who is 40 years old:
- MHR would be 180 BPM (220 minus 40).
- Their fat-burning target heart rate would be 90–117 BPM.
- Their cardio target heart rate would be 135–153 BPM.
Which type of workout is best for weight loss?
The short answer is that any kind of physical activity can help support weight management. When it comes to the number on the scale, the body’s inner fuel source is generally less important than the overall number of calories burned during exertion, says certified strength and conditioning specialist Hayden Steele.
Minute for minute, higher-intensity workouts (those in the cardio zone) burn more calories than less strenuous workouts, Dr. Goldberg says. So getting comparable results from lower-intensity workouts (those in the fat-burning zone) might require exercising for longer periods of time.
A small 12-week study published in the Journal of Diabetes Research in 2017 supports that statement. Researchers assigned groups of women to one of three plans: Short high-intensity workouts, low-intensity workouts roughly twice as long in duration, or no exercise at all. As indicated by body mass, body fat percentage, and levels of subcutaneous belly fat, women in both exercise groups had similar levels of fat loss.
So if dropping kilos is your goal, feel free to choose what works best for you on any given day. Prepare to sweat in a high-intensity session, or block out adequate time for a longer, low-intensity workout. “You can have fat loss through both,” Steele says. “Both methods create a caloric deficit.”
The bottom line: Physical activity of any kind can be beneficial
Of course, the number on the scale isn’t the only reason to include consistent physical movement in your life. “[Activity] is the foundation of not just weight management but improving glucose control, preventing heart disease, and lowering blood pressure, among many other benefits,” Dr. Goldberg says. “By engaging in any activity, you can’t lose.”