If your goal is to lose weight or optimize body composition, you’ve probably heard the term “fat-burning zone.” Your fat-burning zone simply refers to a specific workout intensity that utilizes your body fat for fuel. We're going to dive more in-depth into what the fat-burning-zone is and how to calculate your fat-burning heart rate.
What Is The Fat-Burning Zone
Your body requires a continuous supply of energy to function. Energy is derived from the foods you eat, which come in the form of carbohydrates, protein, and fats. As you digest those macronutrients they are broken down into their simplest forms, (glucose, amino acids, and fatty acids). These molecules are transported through your bloodstream and converted in your body as energy in the form of Adenosine Triphosphate (ATP).
At rest your body utilizes both glucose (carbohydrates) and fatty acids (body fat) for energy production. Your fat-burning zone simply refers to a target heart rate (THR) or exercise intensity at which you burn recruits more body fat for fuel as opposed to glucose. So what zone, is the fat-burning zone?
Target Heart Rate Zones
Heart rate zones and heart rate intensity are effective and convenient tools to monitor training intensity, to determine which energy systems your body is using to supply itself with fuel during exercise.
There are 4 different heart rate training zones, each is a percentage of your Maximal Heart Rate (MHR) which is the maximum number of times your heart can beat within a minute. Heart rate zones are good benchmarks for developing heart rate training and metabolic energy system training programs.
Zone 1 [Low-Intensity] – This is what’s known as your “fat-burning zone”. This zone is comprised of low-intensity, and steady-state exercise between 50-70% of your MHR. Think low-intensity cardio.
Zone 2 [Moderate] – This zone is between 70-85% maximal heart rate and is used to increase anaerobic and aerobic capacity.
Zone 3 [High-Intensity] – Between 85-95% maximal heart rate intensity, zone 3 is primarily used in interval training. It can increase speed, power, and anaerobic capacity through high-intensity functional training modalities and exercise. This is the best zone to increase endurance capacity.
Zone 4 [Max Effort] – Zone 4 is an all-out sprint. At this zone, most athletes will only be able to maintain it for a relatively short period of time.
Calculate Your Fat-Burning Heart Rate
So we know that zone 1 low intensity is the fat-burning zone. But how do you calculate maximum heart rate, so you can calculate your ability to burn fat?
The regression formula 208 – (0.7 x AGE). This equation is the simplest and most accurate way to give yourself a general starting point to measure your fat-burning zone and your metabolic energy system training intensity.
After you find your maximal heart rate, you would take that number and multiply it by the zone intensity (70% or 0.7) to find your target heart rate or beats per minute (bpm).
For example, a 25 year old trying to burn body fat would look like this
208 – (0.7 x 25) = 190.5 HRmax
To find Heart Rate Training Zone 1 (Fat-burning zone) you would multiply 190.5 x 0.7 = 133.35 bpm
The Truth About Your Fat-Burning Zone
As you can see, the fat-burning zone is also the lowest intensity zone. Seems counterintuitive right? The reason why it’s called the fat-burning zone is because your energy metabolism is primarily focused on utilizing stored body fat, instead of carbohydrates as its primary fuel source when you work out at lower intensities.
The problem with this, however, is that this can easily be misinterpreted. It’s a common misconception that working out at a lower intensity will burn more body fat. Working out at a lower intensity will utilize more body fat for fuel, and that is true, however, it’s really about increasing your intensity and burning more calories, which will ultimately burn more body fat.
Respiratory Exchange Ratio
The reason for the misinterpretation is due to what’s called the respiratory exchange ratio (RER). I know another acronym you have to remember. The RER is a marker of the proportion of fat or carbohydrate that is being used for fuel at different exercise intensities.
An RER of 0.7 burning 100% pure fat. When pure carbohydrates are being used for fuel, the RER is 1. As exercise intensity increases, the RER also linearly increases, reflecting a shift from fat metabolism to carbohydrate metabolism.
At rest, RER is 0.75, which indicates your body is burning approximately 85% body fat and 15% carbohydrates. Therefore, a workout intensity of 50% would likely be burning closer to 70% body fat and 30% carbohydrates with an RER of 0.8. RER is only indicative of energy metabolism during steady-state workout conditions such as low-intensity running, walking, stairs, elliptical, etc.
A study conducted at the University of Wisconsin examined the effects of low-intensity training and high-intensity training on energy metabolism and fat loss. The results showed that for low-intensity exercise, study participants burned a total of 240 calories performing two 30-minute training sessions, with 96 of those calories (41%) coming from fat. During the high-intensity training session, a total of 450 calories were burned with 108 of those calories (24%) from fat. Therefore, during low-intensity training, there was a higher percentage of calories from fat, but the total number of fat calories was less than during the high-intensity trial.
The total number of calories burned is what determines weight loss. but that does not mean, low-intensity exercise can’t help your burn body fat. Low-intensity exercise is excellent for beginners, especially if you can’t sustain a moderate pace, or haven’t built muscular endurance. Exercising for a longer period of time at a lower intensity will still burn more body fat and calories.
What Is The Fat-Burning Zone: Takeaway
If you’re exercising at a lower intensity, you’re burning fewer calories per minute. The average person who walks for an hour will only burn a couple hundred calories. By increasing your intensity by jogging, cycling, or increasing to moderate intensity, you could burn twice the amount of calories in the same given amount of time. If you only have a limited amount of time set aside to work out, then pick up the pace and get moving. Otherwise, you’ll need to spend more time in the fat-burning zone, to burn the same amount of calories. Either way, being more active is the key to weight loss. Just be realistic with your expectations and know what it takes to reach your goals.
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