It's best not to worry much about the so-called "fat burning zone," especially because exactly when your body switches its preferential fuel from carbs to fat is different for everyone. Instead, focus on habits that burn more calories overall and help your body start burning fat faster.
The point at which your body switches from burning carbohydrates to fat varies according to a number of factors. Unless you're under a physician's care for a specific medical condition, most people will find that the fastest fat burner is actually a combination of exercises and lifestyle choices to help you burn more calories overall.
Your Body's Energy Systems
Your body can use all three macronutrients (carbohydrate, fat and protein) as energy sources to fuel your workouts. However, not all of those macronutrients are created equal when it comes time for them to be broken down into adenosine triphosphate, or ATP, which is the fuel actually utilized in your cells.
Both the energy systems and the fat-burning process in the human body are complex, variable operations — and scientists are still unraveling some mysteries of the fat-burning process, in particular. However, a few key principles stand out as useful information for your weight loss journey.
Your body burns carbs first. As noted by Len Kravitz, PhD, an exercise researcher at the University of New Mexico, in general, carbohydrates are the first and preferred fuel your body uses to power a workout. That's because they can readily be broken down into ATP via both anaerobic and aerobic metabolism. Even if you're doing a so-called "aerobic" workout like dancing, walking, jogging or swimming, your initial efforts will use anaerobic ("without oxygen") energy sources, because it takes a couple of minutes for your heart and lungs to be ready to provide the oxygen needed for aerobic ("with oxygen") metabolism.
Fat use increases over time. In general, the percentage of "fuel" your body gets from breaking down fat instead of carbohydrates increases as your workout time also increases. For example, a review published in a 2014 supplemental issue of the New Zealand journal Sports Medicine noted that several studies have shown fat oxidation to increase by about 15 percent during 30 minutes of exercise as compared to the initial rate.
There's no single answer. However, your body's switch from burning carbs to fat as the preferential fuel source can vary quite a bit depending on many factors, not all of which are entirely understood. As noted in the aforementioned sources, your body composition, level of fitness, diet and overall health all play a role.
About That Fat Burning Zone
Another common perception about exercise is that if you just keep your heart rate within the so-called "fat-burning zone," you'll burn more fat than when you do harder workouts in the so-called "cardio zone." Once upon a time, most exercise machines had those two zones labeled on their consoles — but that phrasing has passed out of use, because the idea of a fat-burning zone is only true in a certain, limited context.
As further explained by Kravitz, it's true that exercising at low to moderate intensities uses fat as an energy source in greater percentages than you'd use when exercising at a high intensity. However, even though the percentage of fat used is lower at that high intensity, the amount of energy used (or calories burned) is so much higher that you still end up burning more fat, overall, at the higher intensity.
So, the outdated implication that you only burn fat while you're in that so-called fat-burning zone isn't true, although those former numbers are still useful for measuring workout intensity — in most cases the fat-burning zone corresponds to a low-to-moderate intensity, while the cardio zone depicts a moderate-to-high workout intensity.
Ultimately, for most people, the top thing you can do to help your body burn more fat is to seek out ways to continually keep moving at whatever intensity you can tolerate.
Read more: 10 Ways to Stay Fit & Healthy
Things You Can Control
Happily, there are a number of things you can do to improve your body's ability to burn fat as fuel. As exercise physiologists with the University of New Mexico point out, blending interval training (alternating short intervals of intense effort with lower-intensity "recovery" intervals) and endurance training (so-called "steady state workouts") helps improve your body's ability to metabolize fat as fuel.
Regular, progressive resistance training also improves that ability. Building lean muscle not only boosts your overall metabolic rate, but also helps improve post-workout fat oxidation and your overall EPOC, or the elevated level of energy your body consumes after each workout as it works to return you to a resting state. Mixing low-intensity "recovery" workouts in with your harder workouts — for example, going for a relaxed stroll or taking an easy bike ride to loosen up tired legs — also helps support your body's overall fat metabolism.
Build a Progressive Workout
Not everybody can jump right into high-intensity training — in fact if you've been inactive for some time, the best advice is to start gradually and work your way up to higher intensities and durations as your body adapts. But everybody can find some way to add more physical activity into their life, whether that means parking at the far end of the parking lot and walking to the store, taking a quick 10-minute stroll during your lunch break, swimming an extra lap in the pool or just kicking the speed on any of those up a notch.
Here's some more good news, based on the evolving understanding of how the human body works: Every little shred of physical activity you can squeeze into the day helps. As noted in the 2015-2020 edition of the Department of Health and Human Services physical activity guidelines for Americans, previous editions of those guidelines — which form the core of health-based exercise recommendations for many institutions — stated that physical activity should last at least 10 minutes to count toward your "quota" of aerobic activity for the day.
But now, the HHS acknowledges that any length of activity counts toward that total — even if it's just five minutes. If you're looking for a good first goal to set, meeting the HHS minimum recommendations for physical activity is a good place to start.
- At least 150 minutes of moderate-intensity physical activity per week, or
- At least 75 minutes of vigorous-intensity physical activity, or a combination of the two;
- Plus twice-weekly strength-training workouts for all your major muscle groups.
But don't stop there. As the HHS points out, you can get even more health benefits by doubling that amount to 300 minutes of moderate activity or 150 minutes of vigorous activity — and doing more exercise will just give you more health benefits to enjoy. The trick to getting there in a healthy way is to progressively increase either the duration, frequency or intensity of your workouts as your body adapts to the new challenges you give it. And ultimately, the more you move, the more fat you'll burn.
- Health.gov: "Physical Activity Guidelines for Americans"
- University of New Mexico: "The Physiology of Fat Loss"
- Sports Medicine: "New Insights into the Interaction of Carbohydrate and Fat Metabolism During Exercise"
- American Council on Exercise: "7 Things to Know About Excess Post-Exercise Oxygen Consumption (EPOC)"
- University of New Mexico: "Fat Facts"