Is the "Fat-Burning Zone" a Myth?


When it comes to losing weight, people often want to know the best way to shed excess weight. One theory suggests that exercising at intensities around 60% of your maximum heart rate will bring our bodies into a so-called “fat-burning zone” that is optimal for losing weight.


But does this “fat-burning zone” even exist? And if so, how effective is it?


“Fat-Burning Zone” Theory:

How this theory came about is not completely based off inaccurate information. There are two main sources of energy that fuels our body: carbohydrate and fats.

(Diagram adapted from ultrarunning.com)


During low to moderate levels of exercise intensity (40-70%), the body predominantly uses fats as its main energy fuel source. However, as the exercise intensity increases (>70%), energy requirements increase. As fat metabolism is slow, the body naturally shifts towards burning carbohydrates which is a quicker source of energy. Hence with increasing exercise intensities, the proportion of energy that comes from carbohydrate increases while the proportion from fat decreases.


The fat burning zone theory seeks to help adherents lose weight by tapping on the body’s fat storage rather than carbohydrates. They argue that the body burns a greater percentage of fat with lower-intensity exercises than at higher intensities because the body does not require “fast energy” from carbohydrates. As such, this theory promotes longer and lower-intensity cardio workouts that maintain your heart rate within the “fat-burning zone”.


Reality:

In reality, exercising at higher intensities consistently and pushing yourself is more beneficial and effective for optimising fat metabolism because:


  • Cell Adaptations

After regular moderate to high intensity exercise training, your body adapts to the exercise stimulus. Your body becomes more efficient at burning fat at higher exercise intensities instead of carbohydates. Additionally, during higher intensity exercises, although your body uses your carbohydrate stores first for “fast energy”, it depletes the carbohydrate stores rapidly enough to force your body to tap onto the fat storage. This means that high-intensity workouts are more efficient in burning more total calories – both carbohydrates and fats. Ultimately, the total number of calories you burn leads to the most weight (and fat) loss.



  • After-Burn Effect

Many people forget that this “fat-burning zone” concept does not consider the “after-burn effect”. Performing low to moderate intensity workouts will not induce a high level of metabolism post-workout. However, when performing high intensity exercises, an elevated level of fat and carbohydrate metabolism remains up to about 48 hours post-exercise. This means that even after two days post-exercise, you can still be experiencing an elevated metabolism.


How to Measure Exercise Intensity Using your Heart Rate:

The intensity of your workout can be estimated by your heart rate during the activity. The first step to this is to determine your maximum heart rate, which is the upper limit of what your cardiovascular system can handle during physical activity.


To calculate your maximum heart rate, subtract your age from 220. For example, a 50-year-old will have a maximum theoretical heart rate of 170 beats per minute (BPM).

Next, calculate your desired target heart rate zone. This is the level at which your heart is being exercised and conditioned but not overworked. The following target heart rates are generally recommended:

· Moderate Intensity: 50-70% of maximum heart rate

· Vigorous Intensity: 70-85% of your maximum heart rate


Do remember not to rush into achieving a vigorous exercise intensity. If you are just beginning an exercise routine, always have a slow warm up and slowly build up your heart rate.


Finally, to know whether or not you are in your target heart rate zone, you can either use an activity tracker (Actxa Spark+, Fitbit, Garmin, Polar, Apple Watch) or measure it yourself using the following steps:

1. Briefly stop your exercise

2. Take your pulse for 15 seconds by placing two fingers on your wrist, nearer to the thumb.

3. Multiply this number by 4 to calculate your BPM.


Conclusion:

If fat loss is truly your main objective than you should be trying to perform exercises at a higher intensity. That being said, if you are an untrained individual, performing high intensities exercises can be daunting and unsustainable. “Slow and steady wins the race” has never been more accurate and truer when it comes to developing good and sustainable habits such as regular exercise. Start small and slow, build up your cardiovascular health before embarking on higher intensities.