Best Workout for Fat Loss: The Truth About the Fat-Burning Zone
I love data, so I use wearable devices to track pretty much everything. While the numbers aren’t as precise as gold-standard testing, they do a decent job of showing patterns and trends. One of the metrics I look at is heart-rate zones during exercise. Zone 2 is often nicknamed the “fat burning zone” on these devices, but I don’t understand the logic. Zone 2 is low to moderate intensity, and I burn far more calories in 30 minutes of higher-intensity exercise than I do in Zone 2. So, if I’m burning more calories at higher intensities, how is Zone 2 the “fat burning zone”? The math isn’t mathing, so I decided to look into it.
What is Zone 2 and Why Is It Called the Fat-Burning Zone?
Zone 2 exercise is considered low to moderate intensity activity. It is defined in several ways: estimated 65-75% maximal heart rate, the maximum rate of fat oxidation, blood lactate threshold, and/or ventilation threshold.1 Fat oxidation is the process of using fat for energy. Historically, Zone 2 has been referred to as the “fat-burning zone” because fat is the preferred substrate utilized for energy production at this level of exertion. However, some researchers are now calling this into question.
Much of the data about Zone 2 training comes from studies on elite endurance athletes. These athletes participate in both low and high intensity exercise, so it is unclear if their physiologic efficiency is due to Zone 2 training or higher intensity training. Additionally, many of these athletes exercise greater than 20 hours per week, which is well beyond public-health recommendations.2 The argument is that data extrapolated from these studies cannot be applied to the general population, and questions remain regarding whether it is the best form of cardiovascular exercise to increase mitochondrial capacity.3
Benefits of Zone 2 Training for Health and Fat Loss
Zone 2 provides several benefits. First, it is a well-tolerated intensity of exercise and is associated with lower risk of injury and is the preferred intensity level for individuals who may have chronic illness or low fitness.4,5 Beyond this very practical application, it also provides multiple physiologic adaptations.
Glucose metabolism: Zone 2 training is superior to high-intensity interval training in improving long-term glycemic control through enhanced insulin sensitivity and more efficient non-insulin dependent glucose metabolism.6,7
Vascular adaptations: While higher intensity exercise improves blood flow to muscles during exercise, Zone 2 training is thought to increase artery diameter at rest, reflecting improved baseline vascular tone.8
Improved endurance: Increased skeletal muscle capillary density, improved cardiac output, and enhanced ability to tolerate prolonged physical activity all result from Zone 2 training.9
Lipid metabolism: Zone 2 training preferentially uses fat oxidation as its primary energy source, even in the non-athlete.3,7
So, if it is true that Zone 2 offers all of these health benefits, including preferentially burning fat, why even exercise at higher levels of intensity?
Where Does Zone 2 Fall Short for Fat Loss?
One area where zone 2 falls short is improving VO2 max. VO2 max is the highest rate at which your body can consume oxygen to produce ATP (adenosine triphosphate). A higher VO2 max not only results in more efficient ATP production, but it is also associated with a lower all-cause mortality rate.10
Higher intensity exercises also result in a larger caloric expenditure. This offsets caloric intake and can lead to increased weight loss. A very simplified equation is caloric intake – caloric output = net effect. If the net effect is negative, the body taps into energy reserves (typically stored fat) for more energy. If the net effect is positive, our bodies store the excess energy, usually in the form of fat.
One other advantage of higher intensity training is increased post-exercise fat oxidation (EPOC). This is the increased rate of fat oxidation that occurs in the hours following exercise.11 The magnitude of EPOC is influenced by exercise intensity and duration. Higher intensities and longer durations result in greater EPOC compared to lower intensities or shorter bouts.12
And this is where the math kicks in. While zone 2 utilizes fat oxidation for energy during exercise, higher intensity exercise leads to EPOC along with a potentially larger net negative caloric deficit. Together, these can produce comparable or greater reductions in fat mass and do so with a shorter duration of effort.
How to Choose the Best Cardio for Fat Loss?
This is tough because I believe everyone needs to have a personalized exercise plan based on their long-term goals, physiologic capabilities, and realistic life limitations, but I can offer a generalization.
For beginners, Zone 2 intensity is a great place to start. A 2015 JAMA article demonstrated a very clear dose-response relationship between physical activity and reduced all-cause mortality with the biggest benefit in those who transitioned from a relatively sedentary lifestyle to minimal activity.13 This, plus the added benefit of preferential fat oxidation, intensity tolerance, and lower incidence of injury makes Zone 2 an ideal starting intensity. The key is to build a healthy habit, so taking it slow and being consistent is the approach I take with my patients. Once people have incorporated exercise as a regular part of their lives, we can increase the intensity and start working on other physiologic parameters such as VO2 max.
Bottom Line
Zone 2 is great for endurance, metabolic health, recovery, and sustainability. But the idea that it is the “fat burning zone” is confusing, outdated, and not fully supported by the evidence. All intensities of exercise can burn fat, and the type of exercise needs to align with long-term health goals and realistic constraints, like time, that life throws our way. No matter what, the best workout for fat loss are the ones done consistently. It’s not as catchy as “stay in the fat-burning zone” but it’s true.
References
- BISHOP DJ, BECK B, BIDDLE SJH, et al. Physical Activity and Exercise Intensity Terminology: A Joint American College of Sports Medicine (ACSM) Expert Statement and Exercise and Sport Science Australia (ESSA) Consensus Statement. Medicine & Science in Sports & Exercise. 2025;57(11):2599-2613. doi:10.1249/mss.0000000000003795
- Piercy KL, Troiano RP. Physical Activity Guidelines for Americans From the US Department of Health and Human Services. Circulation: Cardiovascular Quality and Outcomes. 2018;11(11):e005263. doi:doi:10.1161/CIRCOUTCOMES.118.005263
- Storoschuk KL, Moran-MacDonald A, Gibala MJ, Gurd BJ. Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population. Sports Med. Jul 2025;55(7):1611-1624. doi:10.1007/s40279-025-02261-y
- Brun JF, Myzia J, Varlet-Marie E, Raynaud de Mauverger E, Mercier J. Beyond the Calorie Paradigm: Taking into Account in Practice the Balance of Fat and Carbohydrate Oxidation during Exercise? Nutrients. Apr 12 2022;14(8)doi:10.3390/nu14081605
- Ross R, Blair SN, Arena R, et al. Importance of Assessing Cardiorespiratory Fitness in Clinical Practice: A Case for Fitness as a Clinical Vital Sign: A Scientific Statement From the American Heart Association. Circulation. 2016;134(24):e653-e699. doi:doi:10.1161/CIR.0000000000000461
- Mattioni Maturana F, Martus P, Zipfel S, Nieß AM. Effectiveness of HIIE Versus MICT in Improving Cardiometabolic Risk Factors in Health and Disease: A Meta-Analysis. Medicine and Science in Sports and Exercise. 2021;53(3):559-573. doi:10.1249/MSS.0000000000002506
- Chen X, Wu L, Zheng Y, et al. Effective of High-Intensity Interval Training and Moderate-Intensity Continuous Training on Body Composition, Glycolipid Metabolism, and Cardiopulmonary Function in Patients With Pre-Diabetes: A Randomized Controlled Trial. Frontiers in Endocrinology. 2025;16:1614149. doi:10.3389/fendo.2025.1614149
- Sawyer BJ, Tucker WJ, Bhammar DM, et al. Effects of High-Intensity Interval Training and Moderate-Intensity Continuous Training on Endothelial Function and Cardiometabolic Risk Markers in Obese Adults. Journal of Applied Physiology (Bethesda, Md : 1985). 2016;121(1):279-288. doi:10.1152/japplphysiol.00024.2016
- MacInnis MJ, Gibala MJ. Physiological adaptations to interval training and the role of exercise intensity. J Physiol. May 1 2017;595(9):2915-2930. doi:10.1113/jp273196
- Laukkanen JA, Zaccardi F, Khan H, Kurl S, Jae SY, Rauramaa R. Long-term Change in Cardiorespiratory Fitness and All-Cause Mortality: A Population-Based Follow-up Study. Mayo Clin Proc. Sep 2016;91(9):1183-8. doi:10.1016/j.mayocp.2016.05.014
- Lundsgaard AM, Fritzen AM, Kiens B. The Importance of Fatty Acids as Nutrients during Post-Exercise Recovery. Nutrients. Jan 21 2020;12(2)doi:10.3390/nu12020280
- Warren A, Howden EJ, Williams AD, Fell JW, Johnson NA. Postexercise fat oxidation: effect of exercise duration, intensity, and modality. Int J Sport Nutr Exerc Metab. Dec 2009;19(6):607-23. doi:10.1123/ijsnem.19.6.607
- Arem H, Moore SC, Patel A, et al. Leisure Time Physical Activity and Mortality: A Detailed Pooled Analysis of the Dose-Response Relationship. JAMA Internal Medicine. 2015;175(6):959-967. doi:10.1001/jamainternmed.2015.0533