What is the Fat Burn Zone, and Does it Actually Burn all the Fat During Exercise?
I am sure you have heard about the Fat Burning zone at some point in your lifetime. The Fat Burn Zone, or Zone 2, has become a hot topic of discussion in the wellness and fitness industry, and today, we’ll discuss what it means, what are its benefits, discuss arguments against it, and discuss training program options to maximize the body’s ability to burn fat.
Brief synopsis: How does fat burn happen within the body?
Bear with me here!! This gets a bit sciencey, but it will all hash out in the end!
Fat burning, or oxidation, is the process of dietary or stored fat being used as an energy source within our bodies.
At the start of a workout, blood flow increases within the body, specifically to adipose (fat) tissue and muscle tissue. This increase in blood flow allows for fatty acid release from the fat tissue and fatty acid delivery to muscle tissue. Once the fatty acid is delivered to the muscle, it can either be used to produce energy (oxidation,) or it can be stored within the muscle. In order for the fatty acids to be oxidized, they must be transported to the cell’s mitochondria. In order for the muscle to be able to use this energy (oxidation or fat burn,) the mitochondria have to process the fatty acids into a useable energy currency called ATP. ATP is the sole fuel for muscle contraction. In short, we need ATP to be able to exercise.
The body uses three different energy systems to produce ATP- the phosphagen system, glycolytic system, and oxidative system. Which system you use depends on your exercise intensity. For extremely high intensity work, the phosphagen system is used. As the intensity decreases from extremely high, there is a shift to the glycolytic system, and an even further decrease in intensity will switch you to the oxidative system, which is for moderate to low intensity activity.
For the purposes of this article, we will discuss the oxidative system, which the body primarily uses to make ATP when you are doing low intensity exercise, and moderate intensity exercise lasting more than 3 minutes. This is the only energy system that burns fat.
At the very start of a low to moderate intensity workout, the glycolytic system starts burning carbs. As the workout continues past 3 minutes, the body shifts to using fat (oxidation) to produce ATP (its energy currency,) especially when the workout continues past 30 minutes. Although fat oxidation is the slowest method of producing ATP, it is the most efficient. Because this process is extremely slow, fat loss during exercise requires a fairly long workout, however, fat burn (oxidation) continues for a long while after a long or intense workout to help your body recover and replace the depleted ATP (energy currency.)
This low to moderate intensity level of workout, using the oxidative energy system, is where the Fat Burn Zone, or Zone 2 discussion originates from.
What is Zone 2, How do I Figure Out if I’m There, Why do I want to be There?
Zone 2 is one of 5 heart rate zones, based on your maximum heart rate, that you can be in during a bout of exercise. This zone is also known as the Fat Burn zone, and it usually refers to a longer duration of cardiovascular exercise like cycling, running, or swimming. This zone is achieved usually when the exercise participant is between 60 and 70% of their max heart rate. The exertion level is light, and can be sustained for a long period of time. In programming for my clients, I call this the ‘steady state’ training day. It can be whatever cardio modality they prefer, as long as it’s a slower, longer type of workout. As referenced above, it must be more than 30 minutes- ideally 60 minutes or longer, depending on their conditioning. A 40 to 80 minute window is good to shoot for.
Zone 1: 50-60% of maximum heart rate
Zone 2: 60-70% of maximum heart rate
Zone 3: 70-80% of maximum heart rate
Zone 4: 80-90% of maximum heart rate
Zone 5: 90-100% of maximum heart rate
Visit this link to determine your target heart rate zones:
https://brunnerathleticd.com/fitness-calculators
Most of the fitness tracker options out there today will display your heart rate while you are exercising, whether that’s a watch, or a chest strap heart rate monitor, although the chest strap will give you a fairly more accurate reading. If you don’t have a tracker, you can use the Karvonen Formula to determine your target heart rate training zones:
Target Heart Rate Intensity Zone = ((max HR − resting HR) × %Intensity) + resting HR
If you don’t know your resting heart rate, you can use the value of 70bpm. If you don’t know your maximum heart rate, you can calculate it by subtracting your current age from 220. Use the zone chart above to plug in your intensity percentage. For the purposes of this article, you’ll use 65% for Zone 2.
Visit this link to plug in your information: https://www.topendsports.com/fitness/karvonen-formula-calculator.htm
Howard J Luks, author of Longevity Simplified, shared some insightful info in an interview with Live Science, a phenomenal resource reporting on health and human behavior, among other scientific topics.
Luks says when you are in zone 2, you should be able to hold a conversation without pausing. “Paying attention to our breathing is also important. There's a moment when you start to take a deeper breath or can no longer breathe for distance through your nose. For many that represents the moment they are starting to transition out of zone 2.” Luks also says “...Most runners run too fast on their slow days and too slow on their fast days. They are stressing their energy systems and not deriving the benefit of a strong aerobic base. Zone 1 and 2 activities will improve all zones above it, even VO2max. So, it is important to remember that although you may be able to work harder than your effort level in Zone 2, to achieve the benefits of working in this zone, you need to be within that 60 to 70% of your max heart rate.
The benefits of training in this zone are myriad and well documented. This type of training can help create a more sustainable exercise program for the individual by avoiding overtraining and burnout, and Luks says it can also improve endurance, resilience, lower your resting heart rate, and improve overall health. “Zone 2 or low heart rate training is also one of the best tools we have to achieve metabolic health and longevity.” “In terms of the overall health of our nation, most people have poor mitochondrial flexibility, which is the ability to burn fat versus glucose for low demand activities. Our mitochondria need to be trained to function properly. That mostly occurs with low heart rate (zones one and two) activities.” Luks says training to increase mitochondrial efficiency will not only benefit athletic performance, but also everyday health and longevity.
Practical application, Arguments For/ Against Zone 2, Other Zones, and Polarized Training Programming
When I program for my clients, I include a mix of endurance exercise prescription, higher intensity movements, eccentric and isometric focused resistance training, and different types of interval training. This varied approach is aimed at causing muscle adaptations to improve the body’s ability to maximize fatty acid oxidation and mitochondrial efficiency. A good rule of thumb if you are creating your own training program is to aim for at least one day of zone 2 training, one day of sport-specific heavier resistance or strength training, one day of a shorter HIIT workout, and one day of intervals. I aim to have my clients move NO LESS than 4 days per week. If you add one or two more days, they will be at a lower level intensity.
This format falls under the umbrella of polarized training, or 80/20 training. This type of training has been used for decades by some of the world’s most elite athletes, however it can absolutely be implemented by everyday athletes, as well as ‘weekend warriors.’ In essence, this type of training involves doing most of your program at a low intensity- 80% of your training at low intensity, specifically. This is usually in that 70%’ish area of your max heart rate. (Refer to the charts above for heart rate percentages.)
We have a tiny bit more science to discuss:
I understand that training in zone 2 can get… tedious. It’s definitely slow, it’s definitely hard to stay within that lower heart rate threshold, it can get boring, and it takes a long time to really be able to see the benefits. HOWEVER: we need to return to looking at the physiological responses that are happening within this zone. We are literally retraining our mitochondria to rely more on burning fat vs. carbs. Training in zone 2 helps mitochondrial flexibility, and increases the number of mitochondria we have in our slow twitch muscle fibers. These are healthy mitochondrial functions, and we are helping to build a phenomenal aerobic base, which helps build resilience, endurance, and speed.
With all of this being said, are you a zone 2 believer yet? There are some that disagree with this training style. They want to look more at the full picture of exercise output over an extended period of time, rather than the 40 to 80 minute fat burn window.
Pete McCall, ACE certified personal trainer and the host of the All About Fitness Podcast, says “Rather than worrying about your heart rate, it’s more important to measure your breath during exercise. If you are breathing quickly and can only say a couple of words during exercise and not a full sentence, then you are expending more net energy, which will ultimately result in more fat burning than a less-intense workout [would].” This higher intensity energy output is actually burning more carbs and thus, calories, than what happens in zone 2.
The argument is that while stored fat within the body is used as fuel while training in zone 2, it does not account for the caloric and fat burn aspect of higher intensity exercise in the aerobic heart rate zone, specifically AFTER the workout is over. This ‘after burn,’ is also known as EPOC, or exercise post oxygen consumption. During this EPOC time frame, the metabolism stays elevated and fat burn continues long after the workout is over. McCall says, “While it’s hard to quantify exactly, the body may burn approximately an extra 200 calories over the course of 12 hours during EPOC. It’s not significant, but it’s something.” That additional caloric burn can lead to additional fat loss over time. In contrast, the fat burn zone has a minimal afterburn effect.
The crux of the argument boils down to caloric expenditure vs. burning fat for weight loss. Which one is better?
I think that all depends on the person, how much time they are able to commit to a CONSISTENT exercise program, and what their goals are. I tend to lean more towards the 80/20 approach, as I feel it is the most sustainable for the general population to keep our bodies healthy, able to move without pain and injury, and enjoyment.
The bottom line is you need to choose a modality and format that is realistic and enjoyable for you. If you hate running and sprinting, choose something else. Your exercise program needs to be sustainable and consistent, and regardless of your heart rate, if you are moving, you are winning!
Resources:
https://pubmed.ncbi.nlm.nih.gov/15896087/
http://www.unm.edu/~lkravitz/Article%20folder/physiologgfatloss.html
https://www.signos.com/blog/fat-burning-heart-rate-zone