How many times have you heard that if you want to lose fat, you should do cardio in the “fat-burning heart rate zone” to maximize your results?
This idea has been afloat since, well, basically forever, but it really took hold after the jogging boom of the 1970s. And although other workout trends have gained ground recently (witness high-intensity interval training), many people are still convinced that fat-burning zone workouts are the ticket.
Here’s the deal, though: the fat-burning zone is largely a myth, at least the way most people conceive it.
In other words, doing all of your cardio workouts at a low to moderate intensity isn’t necessarily any better than training at a higher intensity for fat loss.
In this article, you’ll learn what the fat-burning zone is, why it’s overrated, and what’s really the best kind of cardio for fat loss.
The fat-burning zone is a workout intensity at which your body burns predominantly fat, and many people claim that by training at this intensity you can lose fat faster.
It’s also referred to as the fat-burning heart rate zone, because workout intensity is often expressed as a percentage of your maximum heart rate.
In order to understand how the idea of the fat-burning zone came about, you have to understand how your metabolism works during exercise.
Your body uses several different sources of energy to stay alive and power physical activity, with the main ones being carbohydrate and fat. It also uses different physiological processes, known as energy systems, to transform said carbs and fats into usable energy for your cells.
At rest (when your heart rate is low), your body gets almost all of its energy from fat, which is a very efficient and plentiful fuel source. Of course, you also aren’t burning many calories, and thus the absolute amount of fat you burn is miniscule.
You can burn more calories (and thus fat) by increasing your workout intensity, but this has a side effect: your energy systems aren’t able to process fat quickly enough to keep up with the demand from your muscles, so you begin metabolizing more and more carbohydrate.
Once you’re exercising at about 60 to 80% of your maximum heart rate, your body gets about half of its energy from carbohydrate stores and half from fat stores (a point scientists refer to as your maximal fat oxidation rate, or MFO). Push the envelope even further, and at near-maximal intensities, such as sprinting, your muscles get the majority of their energy from carbs and very little from fat.
Once you connect the dots, you can see how the idea of the fat-burning zone emerged—it’s the intensity at which your body burns as many calories as possible while still getting the majority of those calories from fat.
Sounds nice in theory, but there are three reasons it turns out to be all hat and no cattle.
The first and biggest problem with the idea of the fat-burning zone, is that the amount of fat you burn during a workout isn’t all that important when it comes to long-term fat loss.
When you burn more fat and less carbs during a workout, you just burn less fat and more carbs later in the day, and vice versa. This is because your body calibrates how much body fat and carbohydrate you burn over a 24-hour period so that you burn the same amount of each over time regardless of what you burn during your workouts.
For example, whether you burn 400 calories during an intense 30-minute run, with most of those calories coming from carbs, or 400 calories from a leisurely 90-minute walk, with most of those calories coming from fat, you’ll burn about the same amount of body fat at the end of the day.
In other words, how much fat you lose in response to a cardio workout depends on how much it contributes to a calorie deficit, not how much fat you burn during the workout.
The second problem with the idea of the fat-burning zone is the way most people implement this concept.
Specifically, they often hold themselves back in their training, thinking that relatively easy workouts are superior to moderate or high-intensity workouts because it keeps them in the fat-burning zone. As you learned a moment ago, though, you’re still burning mostly fat up to about 60 to 80% of your maximum heart rate, which most people would consider a moderate- to fairly high-intensity workout.
For instance, if you walk off, say, 100 calories, 85 of which come from fat stores, that isn’t as effective as spending that time running at a moderate pace and burning off 400 calories with 250 coming from fat. Or, better yet, burning 600 calories on a slightly more intense bike ride with 300 coming from fat.
Finally, the last problem with this idea is that it implies the fat-burning zone is a static intensity that occurs at a very specific heart rate. Cardio machines reinforce this idea by showing pretty graphs indicating exactly where your heart rate should be for “fat burning” versus “cardiovascular training.”
In reality, though, the fat-burning zone (or more accurately, your maximal rate of fat oxidation) can vary widely depending on your fitness level and sex. Specifically, people who are in better shape burn a higher percentage of fat at a higher intensity than people who are less fit, because exercise improves your ability to metabolize fat for energy at higher intensities.
Specifically, some obese people max out their fat burning abilities when working at just 24% of their VO2max (the maximum amount of oxygen you can metabolize during exercise—another measure of exercise intensity), whereas some endurance athletes don’t reach this point until they’re close to 80% of their VO2max. Women also burn a higher percentage of calories from fat than men at most intensities.
The bottom line is that the fat-burning zone is a fluid, moving target that you’re never going to pinpoint.
Luckily, you don’t have to, because it’s not necessary or useful to do so.
So, if the fat-burning zone is mostly bunk, what kind of cardio is best for burning fat?
Whatever burns the most calories over time . . . which is going to be whatever you can stick to . . . which is going to be whatever you enjoy most.
If you enjoy slow, steady runs in the fat-burning zone (whatever that is for you), go for it. If you enjoy leisurely walks, that’s fine, too. And if you enjoy high-intensity interval sprints on an exercise bike, that’s also fine.
Just make sure you follow a cardio program you can maintain and, ideally, that compliments your strength training routine.
+ Scientific References
- Knechtle, B., Müller, C., Willmann, F., Kotteck, K., Eser, P., & Knecht, H. (2004). Fat Oxidation in Men and Women Endurance Athletes in Running and Cycling. International Journal of Sports Medicine, 25(1), 38–44. https://doi.org/10.1055/S-2003-45232
- J, A., MC, V., & AE, J. (2003). Fat oxidation rates are higher during running compared with cycling over a wide range of intensities. Metabolism: Clinical and Experimental, 52(6), 747–752. https://doi.org/10.1016/S0026-0495(03)00068-4
- Cao, L., Jiang, Y., Li, Q., Wang, J., & Tan, S. (2019). Exercise Training at Maximal Fat Oxidation Intensity for Overweight or Obese Older Women: A Randomized Study. Journal of Sports Science & Medicine, 18(3), 413. /pmc/articles/PMC6683615/
- Purdom, T., Kravitz, L., Dokladny, K., & Mermier, C. (2018). Understanding the factors that effect maximal fat oxidation. Journal of the International Society of Sports Nutrition 2018 15:1, 15(1), 1–10. https://doi.org/10.1186/S12970-018-0207-1
- K, H., T, S., & D, S. (2005). The effects of exercise on the storage and oxidation of dietary fat. Sports Medicine (Auckland, N.Z.), 35(5), 363–373. https://doi.org/10.2165/00007256-200535050-00001
- J, A., M, G., & AE, J. (2002). Determination of the exercise intensity that elicits maximal fat oxidation. Medicine and Science in Sports and Exercise, 34(1), 92–97. https://doi.org/10.1097/00005768-200201000-00015
- Loon, L. J. C. van, Greenhaff, P. L., Constantin-Teodosiu, D., Saris, W. H. M., & Wagenmakers, A. J. M. (2001). The effects of increasing exercise intensity on muscle fuel utilisation in humans. The Journal of Physiology, 536(Pt 1), 295. https://doi.org/10.1111/J.1469-7793.2001.00295.X
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