Last updated on 3 September 2020
If you’re looking to lose weight and keep fit, the general rule of the game is to increase the intensity of your workouts. But what about the ‘fat burning zone’ theory that says you should exercise at lower intensities instead? What is the idea behind this concept, and is it true?
The link between heart rate and fat loss
Your body requires glucose as fuel for your muscles. The 2 main sources of fuel are glycogen (a substance that stores carbohydrate) and fat, which breaks down to form glucose and ultimately carbon dioxide and water. Oxygen is required to oxidise (break down) either the glycogen or fat stores into glucose to fuel the muscles.
During a workout, your body requires more energy. Thus, your heart pumps faster and harder to send oxygen to your muscle cells to break down more glycogen and fat to fuel your muscles.
While 1 gram of carbohydrate contains 4 calories of energy, 1 gram of fat contains 9 calories. This makes glycogen (carbohydrate) a less dense form of energy storage that is readily broken down into glucose, as compared to fats. As such, glycogen is your body's first source of energy during exercise. Since high-intensity workouts require more energy quickly, you tap on glycogen rather than fat in your body for fuel. Your body only taps onto the next fuel, fat, when you start to run out of glycogen.
The truth of the fat burning zone theory
The fat burning zone theory seeks to help adherents lose weight by tapping on the body’s fat storage rather than glycogen. 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 glycogen. As such, this theory promotes longer and lower-intensity cardio workouts that maintain your heart rate within the ‘fat burning zone’.
However, that is a bit of a misconception. While it is true that the body burns fat during low-intensity workouts, the fat burning rate remains low and you have to exercise longer to burn the same amount of calories you would at higher intensities.
In a high-intensity workout, although your body uses your glycogen stores first for ‘fast energy’, it depletes the glycogen stores rapidly enough to force your body to tap on the fat storage. This means that high-intensity workouts are more efficient in burning way more total calories – both glycogen and fat calories. Ultimately, the total number of calories you burn leads to the most weight (and fat) loss.
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 heart rate of 170. This means that on average, the maximum number of heartbeats per minute is 170 for this person.
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 exercise intensity: 50 – 70% of your maximum heart rate
- Vigorous exercise intensity: 70 – 85% of your maximum heart rate
Do remember not to rush into achieving a vigorous exercise intensity. If you’re just beginning an exercise routine, aim for the lower end of your target heart rate zone.
Finally, to know whether or not you’re in your target heart rate zone, you can either use an activity tracker or measure it yourself using the following steps:
- Briefly stop your exercise
- Take your pulse for 15 seconds by placing two fingers on your wrist, nearer to the thumb.
- Multiply this number by 4 to calculate your heart beats per minute.
The 4 training zones
Working out with a heart rate monitor helps you to gauge the specific zones in which your body is working and how your body benefits from different intensities of exercise. Each of the 4 main training zones can be predicted by your heart rate:
Your warm-up zone is where you prepare your cardio-respiratory system, muscles and joints to exercise harder. Here, you are functioning at 60 – 70% of your maximum heart rate. It is a comfortable pace where you feel as though you can go on for a long time.
Just beyond the warm-up zone is the so-called fat burning zone where you are working out at about 70 – 80% of your maximum heart rate. It is still a comfortable rate but you might sweat more and breathe harder than usual. Although you may burn more fat than glycogen at this zone, the absolute amount of fat burnt is much less than the subsequent stages.
Still in the comfortable zone is the aerobic zone. Your heart rate is at 81 – 93% of your maximum heart rate. You will be able to talk but only in short phrases. The calories you burn here split evenly between your fat stores and glycogen. Although you will not burn more fat calories than glycogen, you will be burning more calories overall. (Plus, the aerobic zone makes your heart pump hard, which is great to keep your heart healthy!)
Finally, you will be at 94 – 100% of your maximum heart rate in the anaerobic territory. You are panting and unable to talk. It is hard work and nearly impossible to spend more than a minute here as your glycogen stores are depleted faster than they can be replenished. Anaerobic intervals widen your fat and aerobic zones and zap tons of calories. This is where the afterburn (temporary increase in metabolism) kicks in. Also known as excess post-exercise oxygen consumption (EPOC), your body continues to burn more calories even after a high-intensity workout, as compared to a low-intensity exercise.
The intensity of your activity determines how much your heart rate increases. For example, a running heart rate should be between 50% and 85% of your maximum heart rate. You can then adjust your pace based on what you’re aiming for in your run. If you notice that your heart rate is going below this, you can increase your pace to improve your workout results. Or if your heart rate reaches its maximum, it would be better to slow down so that you are able to finish your run.
A high-intensity workout reaps many benefits of burning total calories efficiently both during and after exercising, and keeping your heart healthy. But if you prefer a low-intensity workout, it would require you to devote a longer amount of time to burn the same amount of calories!
Article reviewed by Dr Ong Hean Yee, cardiologist at Mount Elizabeth Novena Hospital
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