First, let’s look at the concepts of resting heart rate and your maximum heart rate. As convenient as it would be, there is not an arbitrary number that applies to everyone. Your resting heart rate is the speed at which your heart beats when you’re resting, which in this context doesn’t just apply to complete inactivity. You might be doing some non-strenuous work, reading a book, having breakfast, or lounging in front of the TV. On average, your normal resting heart rate would be between 60-100 beats per minute, however age, gender and levels of activity have an effect on your resting heart rate. 
I am sure we are all aware from our school days that your maximum heart rate (MHR) is calculated by the formula max HR = 220 – age. Now how can that help you with your workouts? Well, the number obtained can give you a range at which to workout and there are four different training zones to consider.
Low intensity, (fat-burning zone) is 50-70% of your MHR, it’s an activity like light cardio or a warm-up.
Moderate intensity is 70% to 80% of your MHR, it’s an activity you are doing that still allows you to talk.
High intensity is 80% to 90% of your MHR, this exercise pushes you out of your comfort zone and pushes you to your anaerobic threshold.
Maximum intensity is 90% to 100% of your MHR, exercises that most highly trained athletes can’t maintain for more than a few minutes.
So which heart rate zone is optimal for burning fat?
On the surface, it might seem that the fat burning zone is the answer – I mean, it is literally in the name! However, the human body is a little more complicated than that. 
The two main sources of fuel in your body are glycogen (carbohydrate stores) and fat, which break down to form glucose. To break down either of them into glucose, the body requires oxygen, and to distribute more oxygen throughout the body, our heart pumps faster. By nature, glycogen is less dense and easier to break down – each gram of carbohydrate contains 4 calories, while every gram of fat contains 9. That means that when the body needs quick access to energy, it naturally opts to break down glycogen. Since high-intensity exercise requires more fuel quickly, the body tends to use the carb storage instead of tapping into body fat.
The low-impact exercise that takes you into the fat burning zone, uses fat as fuel, because even though it takes longer to break down, it is ultimately a more dense, and therefore more sustainable source of energy. Unfortunately, the rate of fat breakdown during 70-80% of MHR activity is very slow, and to achieve any notable results, you would have to engage with that activity for much longer.
While it’s true that high-impact activity uses glycogen first for “fast energy”, it actually depletes the glycogen stores quickly enough before switching to using fat as its source of fuel. That means that during aerobic activities (such as jogging, cycling, dancing etc.) the calories you burn are split evenly between your fat stores and glycogen. You are not likely to burn more fat than glycogen, however you will be burning more calories overall (compared to low impact exercise).
The added bonus of exercising within your aerobic zone is that it makes your heart healthier and more resilient, which is why cardio exercises are often recommended as a staple activity that improves your overall quality of life.
If you cross the line into the anaerobic zone, you will likely find it difficult to perform any activity for more than a minute, since your glycogen stores would be depleted faster than they can be replenished. It’s also hard for your body to distribute the necessary amounts of oxygen required to transform the fat and glycogen into glucose. However, anaerobic exercise has its benefits. It certainly burns calories at a rapid rate and also triggers a temporary increase in your metabolic rate, meaning that even after you slow down, your body continues to burn more calories than it would otherwise. (3)
So, when we perform low intensity exercises our body uses fat for fuel. As the exercise intensity increases, fat can’t be metabolised fast enough to meet the energy demand, so the body will use carbohydrates.
The way in which our body burns fuel (our metabolism) depends not only on training zone, but we need to also consider the duration of the activity, what we eat (diet and even what we have eaten before we have exercised), our sex (male/femle), how many times we work out and how long we have been exercising (4,5). So, as we can see there is no one answer for everyone, but these zones allow us to consider what fuel source we are using during exercise and also throughout the day.
- Resting heart rate chart: Factors that influence heart rate in elderly by Dr. Victor Marchione, 2018 https://www.belmarrahealth.com/resting-heart-rate-chart-factors-influence-heart-rate-elderly/
- Your Fat-Burning Zone and Weight Loss by Paige Waehner, 2020 https://www.verywellfit.com/the-truth-about-the-fat-burning-zone-1231545
- Quantifying Differences in the “Fat Burning” Zone and the Aerobic Zone: Implications For Training by Carey, Daniel G, Journal of Strength and Conditioning Research: October 2009 – Volume 23 – Issue 7 – p 2090-2095 https://journals.lww.com/nsca-jscr/Fulltext/2009/10000/Quantifying_Differences_in_the__Fat_Burning__Zone.25.aspx
- Determination of the Exercise Intensity That Elicits Maximal Fat Oxidation by Juul Achten 1, Michael Gleeson, Asker E Jeukendrup. Med Sci Sports Exerc. 2002 Jan;34(1):92-7. https://pubmed.ncbi.nlm.nih.gov/11782653/
- Optimizing Fat Oxidation Through Exercise and Diet by Juul Achten 1, Asker E Jeukendrup. Nutrition Jul-Aug 2004;20(7-8):716-27.