The Fat Burning Zone Myth

The Fat Burning Zone Myth

For a long time the fitness industry talked about people getting in to the ‘Fat Burning Zone’. Posters on gym walls, training zones on cardio machines and trainers all pushed the benefits of working out in this zone to shred some fat. The legacy of this continues today. However, the ‘Fat Burning Zone’ is one of the most often misunderstood things in fitness. So looking to exercise in this zone isn’t necessarily the best way to lose fat.

Woman measuring her waist

What is the Fat Burning Zone?

The Fat Burning Zone refers to exercising a lower intensity (around 55-65% of your Max Heart Rate). At this level of intensity your body is producing energy aerobically (meaning with oxygen) where it draws most of the fuel that it uses from fat. So, yes the Fat Burning Zone does exists. It refers to the exercise intensity at which the body produces the largest proportion of its energy from burning fat.

How do you work out your exercise intensity?

This is usually expressed as a percentage of your Maximum Heart Rate (MHR). Simply work out your maximum heart rate and measuring your heart rate during exercise to know what percentage (and therefore intensity) you are working at. Traditionally you would take your pluse. Today it is more common is measuring it through various gadgets such as watches, heart rate monitors or activity trackers that are available.

First you need to work out what is your Maximum Heart Rate (MHR). This is done by simply taking your age away from 220. So the MHR of a 20 year old would be 200, while the MHR of a 45 year old would be 175. Of course this is an average and the exact number will vary from person to person but this reasonably accurate guide.

A bit of maths will tells you that if you are looking to work in the so-called ‘Fat Burning Zone’ (55-65% of your MHR), that would be a heart rate of 110-130 for our 20 year old and 96-114 for our 45 year old.

For a more simplified version, you can use a Rate of Perceived Exertion (RPE).  Here you rate how hard you are working during exercise on a 1 to 10 scale. One is doing virtually nothing and 10 is exercising as hard as you possible can. On this RPE scale you’d be looking at 4-6 out of 10 to be in the Fat Burning Zone.

Understanding the Body’s Other Energy Systems

As I said before when you exercise at a lower intensity fat is the main use to provide you with energy.  However, when you exercise at higher intensity levels (more than 65% of your MHR) your body can no longer produce the fuel it needs from burning fat.  So another energy system kicks in and your body begins producing energy anaerobically (meaning without oxygen). This is a quicker process for producing the fuel that your muscles need and is done by burning glycogen (sugar) stored in your body.

While you are no longer burning fat to fuel your muscles when you exercise at higher intensities, you do burn more calories overall1 if you exercise for the same length of time. Or to put it another way, you can burn the same amount of calories in a much shorter time when exercising at high intensity.

Plus there is a bonus.  When exercising at a lower intensity, you only burn calories during the exercise and in a very brief recovery period afterwards.  If you exercise at a higher intensity you body has to replenish its glycogen stores.  So you continue to burn calories throughout the following 24 hours to fully replenish these energy stores. In fact research has shown that up to 95% of the calories burned by high intensity exercise are in the 24 hours following the exercise2.

Why is High Intensity Exercise More Effective at Burning Fat?

You need to think about total number of calories burnt during exercise rather than simply burning fat.  To lose weight you need to be creating a calorie deficit – simply using more calories each day than you consume.

So as explained above, higher intensity exercise will burn more calories per minute of exercise than at low intensities.

Secondly, consider the duration of the exercise. At a high intensity, not only are you burning more calories overall but this can be done in a much shorter time. That has to be good news when we all have busy lives and struggle to find the time to for longer workouts.

Thirdly, remember that ‘after-burn’ as your body continues to burn calories to replenish your glycogen store. Yes, you are still burning calories, and your body (working aerobically again) can revert to drawing these from your fat stores

Additionally there is a further bonus. Training at higher intensities builds additional muscle in the body. As muscle needs fuel all the time, that increases your metabolic rate everyday. So you body will be burning more calories every day before taking in to account any exercise you do.

These are the reasons that High Intensity Interval Training (HIIT) is become so popular these days.

Man running up steps

What is HIIT Training

HIIT means High Intensity Interval Training. HIIT workouts are all based around short high intensity activity burst (20-60 secs) with lower intensity recovery periods. The ‘rest’ periods allow you to sustain the intensity for the work part of the interval and recover sufficiently to repeat it again (and again). The key to HIIT is to be working as hard as you can during the activity periods. The frequent ‘rests’ are needed to ensure this can be done. Find out more about HIIT Training in my blog.

Issues with Training at Higher Intensities

The combination of shorter workouts and ‘rest’periods might make you wonder ‘How it can HIIT be more effective when less time is spent exercising?‘. As explained earlier, you are working harder while exercising. Plus your body recovers over the following 24 hours so overall the impact on calories burnt is greater.

A second issue is that those just starting to exercise may not able to work at such high intensities repeatedly.  This is very much a case of building up the intensity, duration and number of intervals completed. So that could be starting with shorter bursts of high intensity and longer recovery periods, and fewer repetitions of these.

The third issue is that working out at very high intensities is hard (very hard). And not a lot of fun while you are doing it!  So it just isn’t for everyone. It needs a certain amount of determination and mental toughness to keep working very hard, or someone there to motivate you through it.

Should I stop doing lower intensity exercise?

There isn’t a straightforward answer here.

As I’ve explained there is a more effective way of burning calories through shorted periods of higher intensity exercise.  So it would be a good idea to include some higher intensity training in your programme.

However, you probably only want to do this type of sessions 2 or 3 times per week. Allowing a day or two between HIIT sessions will enable your body to recover.  If you want to be active every day, some moderate intensity activity on the days inbetween would be fine.

Personal preference is also important.  If you prefer doing lower intensity activity or can’t face pushing yourself hard enough to reach higher intensities, then stick with the lower intensity activity.  You are much more likely to stick at activity you enjoy than something you fine too hard or unpleasant. Just be aware that you will need to spend longer exercising at lower intensities to achieve the same number of calories burnt.

The Importance of Nutrition in Burning Fat

If you are looking to lose weight, burning additional calories is only a part of the equation.  There is an old saying “you can’t outrun a bad diet”.

Quite simply, losing weight and reducing fat is all about creating a calorie deficit. Increasing the amount of exercise you do (whatever form) increases your calorie expenditure. But, you also need to be aware of the amount of calories you consume through what you eat and drink. So exercise regularly, but also look at what you are eating and drinking if you really want to lose fat.

References

  1. https://www.verywellfit.com/the-fat-burning-zone-3119977
  2. https://www.builtlean.com/2011/06/29/afterburn-effect-of-exercise-qa-with-dr-christopher-scott-phd/