What is the Fat Burning Zone?


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You may have already heard of heart rate zones - whether from your doctor, social media or on your Apple Watch -- they’re a common buzzword in the health and fitness industry. When talking about heart rate zones, the most popular point of discussion is the existence of the "fat burning zone" -- a particular heart rate zone that supposedly maximizes fat burning.

Woman Checks Heart Rate On Smart Watch

As such, the fat burning zone has garnered a reputation for being an effective way to lose weight, more so than other activities in other heart rate zones. But is it real? In this article we’re going to give you the scoop on heart rate zones, specifically the fat burning zone.

What is your heart rate?

It may seem like a question with an obvious answer, but to prevent any confusion, let’s clarify what we mean by your heart rate. The definition of heart rate is the number of heart beats per minute, characterized by the contractions of the ventricles.

Why is your heart rate important?

In terms of your general health and wellbeing, heart rate is a way to monitor your heart health. The heart is responsible for transporting blood and oxygen around the body. If your heart rate is too slow (bradycardia) you’re at risk of heart failure, low blood pressure or hypotension. If your heart rate is too fast (tachycardia), you’re at risk of stroke, heart failure, and cardiac arrest.

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Ultimately when it comes to heart rate, there is a "goldilocks" value for beats per minute. A healthy, normal resting heart rate ranges from 60 to 100 beats per minute. The lower end of the spectrum is an indicator of efficient heart function and cardiovascular fitness.

Resting Heart Rate Chart

Source: https://agelessinvesting.com/what-is-a-good-resting-heart-rate/

So how does this all relate to fitness?

Your heart rate is a helpful and accurate indicator of how intensely you are working out, based on your own fitness levels. For example, an athlete might only elevate their heart rate substantially during very high intensity exercise, whereas a sedentary, obese individual may elevate their heart rate walking up the stairs.

Trying to aim for certain heart rate zones can be a useful way to monitor your exercise activity and progress; rather than focusing on metrics like duration or frequency. Plus, for each heart rate "zone," there is a different level of exertion and relying on different fuel sources; which we’ll come onto now.

Aerobic vs. anaerobic heart rate zones

There are two main heart rate "zones": aerobic and anaerobic zones. They are expressed as percentages of your heart rate max.

Low intensity exercises, like walking, do not significantly increase your heart rate, but they are still beneficial for your cardiovascular health. These exercises involve working in the aerobic zone.

Exercises that cause a high heart rate include sprinting, circuit training and explosive movements. These exercises work in the anaerobic zone, and can only be done for a minute or two at a time due to the energy required to perform the movement.

Man Running Through A Park In The Morning

The main difference between these two zones is the source burned for fuel: glycogen (carbohydrates) or triglycerides (fat).

What is your heart rate max?

The recommended heart zones during exercise are expressed as percentages of your ‘heart rate max’ (HRM) - the maximum healthy heart rate you can go up to during exercise.

To calculate your heart rate max, follow this equation:

220 - your age = HRM

What is the fat burning zone?

60-79 percent of HRM - The Fat Burning Zone - "Aerobic"

This is the low intensity zone. This includes walking or cycling and can be done for long periods of time as it doesn’t create an oxygen deficit, meaning you don’t usually have to stop to catch your breath. This type of exercise is beneficial for your general cardiovascular health and endurance and doesn’t require long periods of recovery.

80-100 percent of HRM - High Intensity Exercise - "Anaerobic"

This is considered high intensity, anaerobic exercise and cannot be sustained for long periods. Anaerobic exercise creates an oxygen deficit which causes you to have to ‘catch your breath’ during recovery periods.

Woman Takes A Break During Run In Autumn

Studies show that working in the anaerobic zone helps to build endurance, by improving your VO2 max, which is the maximum amount of oxygen your body can convert to energy. During this zone you are burning glycogen (stored carbohydrates) for fuel.

Is the fat burning zone real?

Research shows that training in the fat burning zone helps you to:

  • Metabolize fat as fuel more efficiently
  • Boosts mitochondrial function
  • Improves markers of heart health.

In fact, it got its name the ‘fat burning zone’ because at this exercise intensity, the body relies on fat stores as the preferred fuel source. This was examined in a study published in 2009 in the Journal of Strength and Conditioning Research that tested the maximal fat oxidation during exercise, concluding that 58.9 to 76.2 percent of HRM was the heart rate zone preferential for fat burning.

So, you should only train in the fat burning zone if you want to lose weight?

We wish it could be that simple! But unfortunately, that’s not the full picture.

It must be understood that just because fat oxidation occurs during the fat burning heart rate zone, it doesn’t mean that the maximum amount of fat burnt occurs during these workouts. That might be quite confusing to understand but think of it this way: just because you’re burning fat at lower intensities, doesn’t mean you’re burning a lot of fat. This is the relative vs. absolute principle1.

Higher intensity anaerobic training creates an oxygen deficit known as excess post-exercise excess oxygen consumption (EPOC). This means that the metabolic rate is elevated post-workout, whereas with low intensity exercise, calories are only burnt during the workout. Thus, more calories are typically burnt from anaerobic exercise.

To exercise for fat loss, it has been shown as best-practice in research to combine aerobic exercise in the fat burning zone with anaerobic exercise in short bursts. For example: going for a two-mile walk with two-minute sprints added in every quarter mile.

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Final thoughts

The fat burning zone is real, but it can be misleading. While it seems to make sense that doing fat-burning exercise results in fat loss, the reality is, maximum fat is burned during anaerobic exercise, because the total caloric expenditure is much higher.

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