Beneficiaries often tell me their goals and what they’d like to achieve through exercise, and we work together to prescribe an individualised programme tailored to meet their needs. If losing fat is their primary aim, once they’ve seen their programme, one response I frequently hear is “but what about the fat-burning zone? Gym equipment often tells me to work in this zone to burn fat.”

The lure of some kind of zone that burns fat sounds tempting, but it’s also misleading if you’re not aware of what it actually means. In fact, it’s one of the biggest misconceptions in health and fitness.

So what is this mysterious fat-burning zone? Well, it is a theory that says while you exercise you should raise and maintain your heart rate to between roughly 50 and 70% of its maximum intensity (the greatest amount of times your heart can safely beat in one minute). While working at this intensity, the body is using most of its energy from fat stores, which is the optimal level for fat burning.

But there’s much more to weight loss than working in the fat-burning zone.

How do you work out your fat burning zone?

Working out your own target heart rate zone sounds complicated, but all it means is to track your heart rate during work outs and then keep within a set intensity range that corresponds to your fitness goal. So first of all you’ll have to work out your personal heart rate. You can do this with a heart monitor, or some exercise machines will pick up your heart rate automatically. They’ll then calculate your maximum heart rate.

Machines usually uses a common formula to work out predicted levels, of 220 minus your age. So for example, the machine would tell a 30-year-old they have a maximum heart rate of 190 beats per minute. (There are limitations to this formula as the hear rate will vary between individuals. To calculate your true heart rate maximum, you would need to complete a maximal fitness test.)

Once you’ve got that figure, you can work out the fat burning zone, which would be between 50 and 70% of your maximum heart rate.

It requires a bit of maths, but it’s not complicated. You simply multiply your maximum heart rate by 0.5 (which is 50%) and then multiply it by 0.7 (70%). This would then give you the range to work between.

So our 30-year-old with a maximum heart rate of 190, if we multiply this by 0.5, we get 95 beats per minute (bpm) and by 0.7, we get 133bpm.

Therefore they should work out between 95 and 133bpm to work in the supposed ‘fat burning zone’.

Why do we burn fat at this level of exercise intensity?

To understand the fat-burning zone, we need to understand the biology. Your heart rate or exercise intensity level determine which energy systems your body uses during a workout.

Your body primarily fuels itself when exercising by burning a mixture of stored fat and carbohydrates (energy you get from food). The less active you are at a particular moment the greater percentage of that fuel mix comes from fats. As you increase your exercise intensity the percentage of carbs in that fuel mix also increases.

Fat is burned at lower levels as fat is burned aerobically (with oxygen). When you work aerobically, larger amounts of oxygen are delivered to the muscles, making it easier for the body to oxidise or burn fat as an energy source. The cardio zone and higher heart rate zones require instant fuel that your body more easily converts from glycogen.

Glycogen (carbohydrates) is a less dense form of energy storage compared to fats (While one gram of carbohydrate contains four calories of energy, one gram of fat contains nine calorie)s. As such, glycogen is your body’s first source of energy during higher intensity activity.  Since high-intensity anaerobic (without oxygen) workouts require more energy more quickly, your body taps into its glycogen rather than fat for fuel. Your body will only start tapping into more of its next fuel source, fats, when you start to run out of glycogen.

Fats takes longer to burn and use as an energy source and there is bigger demand for energy at higher levels therefore we use carbohydrates as our body produces energy from this faster.

A good analogy to remember is this: the faster you go, the higher gear you use in your car, the greater the fuel source needed. Fats alone can’t meet the demands of the energy.

In simple terms when you exercise at a higher intensities your body will use an energy source that is the most accessible, when you exercise at a lower intensity  ie 50-70%, your body doesn’t need energy as instantly and instead will sources its energy from primarily fat. Your body uses fat as its primary energy source when exercising at lower intensities. Whereas when you’re working out at a higher intensity, your body uses carbohydrates as its primary energy source.

Now we understand what’s going on in our body, we can understand why working at lower levels of activity (ie 50-70% of our maximum heart rate) your body is happy to utilise fats, as there is no rush to provide energy.

Don’t fall for the misconception

It would be easy to read this as working out at a lower exercise intensity is the best exercise for fat loss. But that’s not necessarily the case. Which is why you shouldn’t fall for the allure of the fat-burning zone without understanding it.

Quite simply, working at a higher exercise intensity means you’ll burn more calories, even if the percentage of fat burned is less. Working at a higher intensity will almost always lead to greater caloric expenditure because the body will burn both carbs and fat.

When working at lower intensities, you have to exercise for longer to burn the same number of calories you would at higher intensities.

(It should also be noted that unused carbohydrates can also be converted to fat as well. Otherwise we could eat as much chocolate as we liked without gaining any weight, which, sadly, is not the case. Both carbohydrate and fat burnt both contribute to total calorie loss.)

But lower intensity workouts still have their place

Lower-intensity workouts are great for beginners; if you struggle to sustain a faster pace, reducing your speed may mean you can exercise for longer so you end up burning more calories and fat. Also for those with particular health conditions, such as respiratory or cardiac issues, working at a lower rate allows them to exercise more safely. It’s a preference, finding what works for you.

High intensity interval training

This training method of alternating interval periods of high intensity with lower intensity exercise is a great option because your body will alternate between energy sources, using both fats and carbohydrates as an energy fuel. Interval training has been shown to increase fitness and burn more calories than doing one steady source of activity.

This also means you can exercise for shorter periods of time, making it easier to fit exercise into busier lives. Going for a half hour run every day may sound like a big commitment, so how much better would it be if I suggested completing five sets of sprinting for a minute and jogging for a minute?

Resistance training

Contrary to common belief, taking part in resistance workouts won’t necessarily make you big and bulky, but it will help you with your weight loss journey.

This is because how many calories we burn is controlled by our metabolic rate, which is in turn controlled by our thyroid and is largely to do with our muscle mass. Therefore the more muscle we have, the more calories we’ll burn during exercise and at rest.

So if you’re looking for fat loss, make sure you include activities that use either weight machines, resistance bands or your own body weight.

In summary

There is no hack to the system when trying to lose body fat and energy storage can only be reduced when we’re using more energy than we’re consuming. It is not the percentage of fat burned that dictates how effective your exercise choice is for fat burning, but rather it is the number of calories burned.

Therefore the fat-burning zone may not be the best intensity for you to work out if you want to lose body fat. Instead you should try burning more calories by perhaps mixing higher level activities, interval training and resistance work.