The "Fat Burning Zone." Should You Find It to Burn Calories?

“If you want to lose fat, you should prioritize the forms of exercise that burn mostly fat.”

Sound familiar? It should.

This statement summarizes the argument behind exercising in the “fat-burning zone.” The “fat-burning zone” is a sustained period where your heart rate is moderately elevated. At this intensity, you can easily carry on a conversation.

Your body has three primary sources of fuel during exercise: carbs, fat and protein. Anytime you exercise at a conversational, low-intensity pace (approximately 60% of your maximum heart rate), about 60% of the calories you burn come from fat. Once you increase the intensity (i.e., from an easy jog to a moderate-intensity run), your body shifts to burning carbs in the form of glucose or sugar. Meanwhile, during high-intensity (HIIT) sessions, your body mostly relies on a quick-acting form of carbs known as glycogen.

Fat takes longer to break down and requires more oxygen, so a low-level increase in activity will drive the use of fat over carbohydrate stores. 

To determine if you’re exercising in the fat-burning zone, first calculate your maximum heart rate by subtracting your age from 220. (This is just a general measurement and individual max heart rates can vary based on conditioning.) Then, multiply that number by 0.6. For example, an average 30-year-old will have a maximum heart rate of 190 beats per minute (bpm). To achieve an intensity that’s equivalent to 60%, aim to hover around 114 bpm.

In a word: No.

On the one hand, it’s true you’ll burn a greater proportion of calories from fat when you exercise in the “fat-burning zone.” However, you’ll likely burn more calories overall if you exercise at a higher intensity and ultimately — depending on the length of your workout — more calories from fat.

For example, let’s say you burned 200 calories during a 30-minute session in the “fat-burning zone.” This means 120 of those calories (60%) came from fat. By contrast, if you perform a 30-minute HIIT cardio session, you could burn up to 400 calories, with roughly 140 (about 35%) of those calories coming from fat, according to the National Academy of Sports Medicine. In this example, you burned 200 more calories overall and 20 more calories from fat — in less time.

These are ballpark numbers to give a rough example of how the ‘slow-and-steady fat-burning cardio’ is less efficient than higher-intensity, short-duration cardio.

Comparing sprinters and marathon runners: Sprinters do nothing but explosive sprint training cardio for an hour a day or less and have minimal excess fat. A marathon runner does nothing but long-duration steady-state cardio for 1–2 hours per day and also has little fat. Who would you rather train like, a sprinter or a marathon runner?

While low-intensity, steady-state cardio may not be the most efficient form of exercise for fat-loss, there are still plenty of good reasons to exercise at a lower intensity.

For starters, some people simply prefer to keep their exercise regimen slow and relatively easy. If this is you, don’t feel like you have to force yourself to embrace HIIT. The best form of exercise for weight loss is going to be the kind you enjoy and will do consistently. If you hate HIIT, you’re less likely to do it — so you may as well do the kind of exercise you enjoy.

Even if you love HIIT, you can’t do it every day. If you overdo it with the HIIT, you risk burnout, injury and stalled weight loss. A low-intensity cardio session can be a great way to stay active — and keep your body burning calories — in between your scheduled HIIT workouts.