The myth of the fat burning heart rate zone

Myths Under the Microscope is a new feature of the blog which reviews some of the more well-known theories, assumptions and misconceptions about health and dieting. I’ll be looking at these ideas objectively, seeking out the evidence and sharing my conclusions on whether they are good or bad advice – and providing you with the sources I’ve used so you can verify for yourself.

This week: the fat burning heart rate zone

You may have heard about heart rate zones while exercising, and saw the colorful posters splashed around every gym showing which zone you need to optimize your workout.  I’m sure you’ve heard one or two of these statements before too:

  • “You should be in the ‘fat burning’ zone to get the most effective workout for weight loss”
  • “You’re just burning the food in your stomach if you work too hard, you need to slow down to start burning fat”
  • “Lower intensity is the way to go to lose weight, higher intensity just wastes your muscles away”

So you’re telling me, that I can do easier workouts and lose MORE fat?  Sounds too good to be true, right?  But, what does the “fat burning heart rate zone” mean?  And is it true?

heart rate

First things first: What are heart rate zones?

When you exercise, your heart rate will increase depending on how much you are physically exerting yourself – the harder you work out, the higher your heart rate will be.  Your heart rate is measured in Beats Per Minute (BPM), and there are two numbers you need to know to figure out your heart rate zones:

  1. Resting heart rate – this is the lowest BPM that your heart will reach when you’re at rest. The best time to measure this is first thing in the morning after waking.  Typically this will be 60-100 beats per minute in adults, or 40-60 in athletes.  Mine sits at ~60 beats per minute.
  2. Maximum heart rate – this is the highest BPM that your heart will reach during physical exertion. The only accurate way to measure your maximum heart rate is though a treadmill stress test, but there is a calculation to give it your best guess: take 220 minus your age, so mine is 220-32 = 188.

There are 5 heart rate training zones, shown in the graphic below.

Polar heart Rate Zones
Photo from polar.com

So, what’s the difference between the zones?

The main difference in the zones is the way in which your body uses energy.  In the lower zones, you are burning more fat than carbohydrates, and in the higher zones, you are burning more carbohydrates than fat.  What’s more, some of the carbohydrates being burned are stored in your muscles, this is called glycogen – so it’s a no-brainer that you want to get rid of fat and not muscle.

But – who figured out how your body is using energy in this way?!?

I’ve found lots of articles and blogs which state this as a fact, but I’m finding very little in terms of real data to back it up.  There has been a lot of studies done, results of which all seem to contradict each other too!  For example:

  • This study on trained athletes showed that the maximum fat oxidation (fat burning) rate was at 74% of maximum heart rate, which would be in the ‘aerobic’ zone of the heart rate chart (one level above the traditional ‘fat burning’ zone).  It also confirmed that exercise above this zone (in the ‘anaerobic’ zone) shows a marked drop in fat oxidation, so this agrees with the rules above.
  • Compare the results above to this study on overweight, sedentary women, where there was no significant differences between the amount of weight lost in groups who did lower intensity exercise and those who did higher intensity exercise, when all groups were under the same controlled diet.  However, those who performed lower intensity workouts could exercise for longer.  This implies that it’s the act of exercising rather than how you exercise that’s the most important part.

Heart Rate Blog

The BIGGEST flaw with heart rate zones

I’ve read a lot of articles and scholarly papers while researching this blog post, and I do think your body uses energy in different ways whilst at different heart rate zones.  There’s certainly enough studies out there which prove that your body does do different things depending on heart rate.  However, none of this matters for losing weight.  At the end of the day, you will burn calories when you exercise, and it’s the DURATION of this exercise that gives you the most benefit.

Don’t get me wrong, I think heart rate monitoring is an excellent tool for athletes to monitor progress, or to improve your performance in any endurance sport.  For weight loss, however?  I don’t think it’s necessary to worry about which zone you’re burning fat in.

To explain what I mean, let’s imagine the following were true:

  • In the fat burning heart rate zone, I burn 55% fat and 45% carbohydrates
  • In the moderate / aerobic zone that I burn 40% fat and 60% carbohydrates
  • In the hard / anaerobic zone I burn 35% fat and 65% carbohydrates

Assuming I exercise for a solid 60 minutes in all these zones, I can use this online calorie calculator to work out how many calories I’ll burn, using an equation derived from this paper.

1 hour in ‘fat burning’ heart rate zone:

  • 65% max heart rate (122.2bpm)
  • 372 calories burned – 204.6 calories from fat

1 hour in ‘moderate’ heart rate zone:

  • 75% max heart rate (141bpm)
  • 494 calories – 197.6 calories from fat

1 hour in ‘hard’ heart rate zone:

  • 85% max heart rate (159.8bpm)
  • 615 calories – 215.25 calories from fat

Now, I’ve made up these percentages, but you can see the point that I’m making, which is that the fat burning heart rate zone isn’t important for weight loss.  What matters is how long you want to exercise for – if you struggle to fit long workouts in, then shorter, more intense workouts are better.  If you don’t like the stress of high-impact, then be prepared to exercise for longer periods of time to get the same calorie burn.

Let’s look at a real life example.  I’ve taken the heart rate data from a faster, tempo run of mine (~10 minute miles), and an easy walk/run (~14 minute miles) – one is quite firmly in the higher heart rate zone (pink) and the other is mostly in a mix of moderate and fat burning zones (yellow and green):

Comparison of heart rate during tempo run vs. walk/run
Comparison of heart rate during tempo run vs. walk/run

In the tempo run, I burned 396 calories in 32 minutes, average ~12.5 calories per minute.  In the walk/run, I burned 407 calories in 44 minutes, average ~9 calories per minute.  In other words, both were equally effective at burning calories – it’s just that the walk/run took a lot longer to reach the same level of calorie burn.

One other (very important!) thing to take into consideration is that higher intensity exercise will keep burning calories for longer than lower intensity alternatives – it’s called the ‘afterburn’ effect.  This study showed that after subjects rode a stationary bicycle at high intensity, they continued to burn calories for up to 14 hours afterwards.  Compare this to the same study with lower intensity effort which saw no additional calorie burn at all.  In other words, the graph you see above doesn’t tell the whole story, there’s actually bonus calories burned if you work out to a higher heart rate zone!

‘Fat Burning’ heart rate zone – the myth under the microscope

It’s a very widely used misconception that you should exercise at lower intensity to burn more fat – but what people don’t tell you is that although you will burn a higher PERCENTAGE of fat compared to carbohydrates, you’ll actually have to exercise for a lot longer to see the same results as higher intensity exercise.  And what’s more, higher intensity work will keep burning calories for hours after you finish working out.  Isn’t your body amazing?!

My advice?  If your main goal is to lose weight, just do whatever exercise you know that you can stick to.  If you absolutely hate doing high intensity intervals, then do something that’s more moderately paced.  If you hate working out for long periods of time, then find a good high-impact routine that you can fit into your schedule.  Don’t worry about the heart rates, just do what you love – the fact that you’re out there exercising is what’s important.  Follow your heart!

Pam x

Let me know what you think about ‘myths under the microscope’ in the comments!  Do you agree with me or have a different point of view?  Do you have another diet myth you want investigated?  

 

Sources:

http://www.shapesense.com/fitness-exercise/calculators/heart-rate-based-calorie-burn-calculator.shtml

http://www.ncbi.nlm.nih.gov/pubmed/15966347

http://archpsyc.jamanetwork.com/data/Journals/JAMA/4895/JOC30009.pdf

https://www.researchgate.net/profile/Michael_Gleeson/publication/11574960_Determination_of_the_exercise_intensity_that_elicits_maximal_fat_oxidation/links/09e4150c59a6ad913d000000.pdf

http://journals.lww.com/acsm-msse/Abstract/publishahead/A_45_Minute_Vigorous_Exercise_Bout_Increases.98999.aspx

http://www.heart.org/HEARTORG/HealthyLiving/PhysicalActivity/FitnessBasics/Target-Heart-Rates_UCM_434341_Article.jsp#.VtzSFF9Viko

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