The Science of Heart Rate Zones: Improving Fitness and Fat Burn

Your heart rate zone can offer key insights into your overall fitness and cardiovascular health. But what if your goal is to burn fat along with strengthening your heart?

Mass General Brigham sports cardiologist and director of the Cardiovascular Performance Program at Massachusetts General Hospital, J. Sawalla Guseh, MD, shares how your heart rate correlates with fat loss. Dr. Guseh also serves as the team cardiologist for the New England Patriots and the New England Revolution. 

“People often come to me and are interested in running a marathon,” says Dr. Guseh. “Running a marathon can be a fun goal. But you don’t have to run races to improve your overall health, and you certainly don’t have to do them to achieve fat loss.”

What are the 5 heart rate zones?

The five-zone model makes it easy to understand your heart rate. Each zone correlates with how hard you’re exercising:

• Zone 1: Very light effort
• Zone 2: Light effort
• Zone 3: Moderate effort
• Zone 4: Heavy effort
• Zone 5: Maximum effort

Your level of effort maps directly to how hard you’re breathing and how fast your heart is beating. In zone 1, for example, you can converse and even sing with ease. In zone 2, you will have a harder time keeping a conversation going, and you won’t be able to sing. As your effort increases in zones 3 to 5, just talking becomes even more difficult and you can often only say a few words.

Which heart rate zone is best for fat burning?

In zones 1 and 2, your body relies more on aerobic metabolism, where energy is produced primarily through the citric acid cycle. This series of chemical reactions is how the body slowly burns fat to get energy. This is why you may have heard people say that lower-intensity exercise burns fat more effectively than high-intensity exercise. But that may not always be true: Lower-intensity exercise relies more on fat for fuel, but higher-intensity exercise can burn more calories and improve long-term fat loss.

“High-intensity workouts can also promote fat loss, particularly when combined with adequate recovery and overall energy balance,” explains Dr. Guseh. At higher intensities (zones 3–5), your body increasingly shifts toward carbohydrate metabolism. As you burn more sugars, you burn a little less fat. “As a result, people don’t think of these higher zones as ‘fat-burning zones.’ But you’re still burning a little bit of everything.”

Understanding fat loss

When it comes to fat loss, heart rate is only one piece of the puzzle. “Fat loss and weight loss aren’t the same. Exercise plays a key role in reshaping body composition, but meaningful weight loss typically also requires nutritional changes,” Dr. Guseh explains. “Abs are best earned in the kitchen and not the gym,” he adds.

He notes that there’s a strong correlation between exercise and changing your body composition through fat loss and muscle gain. The correlation is weaker between exercise and weight loss. To see the scale go down, changing how you eat is more effective.

Overall fat loss requires that you focus on:

• Diet: Eating a nutritious diet of fruits, vegetables, whole grains, healthy fats, and lean proteins leads to the best results. You need to eat in a caloric deficit to lose weight. However, you may lose fat along with muscle when you decrease your calories. Eating enough protein, especially when combined with resistance training, helps preserve lean muscle while you lose fat.
• Resistance training: Strength training is key to losing fat while keeping muscle. Lean mass is one of the most metabolically active parts of the body. That means the more muscle mass you build, the more calories you burn even at rest.

Exercising for overall fitness

While lower heart rate zones correlate with slightly increased fat burning, exercising in higher zones is still important. For example, if you aim for 150 minutes of aerobic activity weekly, as per the American Heart Association’s recommendations, try to hit zone 3 or higher for 30 of those minutes.

“People have a tendency to emphasize the benefits of one type of exercise over another,” Dr. Guseh says. “But the reality is that exercise at lighter to moderate intensities allows you to exercise more, which is better for your health overall.”