What is the fat burning zone?

The fat burning zone is the heart rate zone at which you supposedly burn most fat. Typically heart rate zone for fat burning is quoted as being between 60% and 70% of your maximum heart rate. So to work out fat your burning zone you’d calculate you maximum heart rate using the formula: maximum heart rate = 205 – 0.5* your age. The fat burning zone is then between 0.6 * max heart rate and 0.7 * max heart rate. Alternately you can find a few fat burning zone calculators on the web.

The trouble is, fat burning zone is a myth.

Exploring the fat burning zone myth

The concept of the fat burning zone is based on an assumption and some solid science. Firstly the science. It is a fact that at lower intensities our bodies primarily use fat to fuel exercise.  However the fat burning zone makes two assumptions:

1. that the maximum heart rate is the same for everyone of a given age. In reality maximum heart rates vary considerably;
2. that the heart rate measured during exercise can be used to predict the oxygen consumption, which isn’t always true.

So how do we address these assumptions? Well firstly you can actually determine your maximum heart rate on your own using your heart rate monitor.The second assumption doesn’t matter, because the science on which the fat burning zone is based has been misinterpreted.

While it is true that at low intensities your body burns the highest percentage of fat, it does not reflect the best way to burn fat and hence lose weight quickly. Consider this example. Lets assume that you burn 100 kcal per mile  that you run. If you slowly run 4 miles then you’ll burn 400 kcal in theory around 65% of these calories will be fat. Now consider what happens if you doubled you running speed and ran 8 miles in the same time. Now you’ve burned 800 kcal but only 35% of those calories will come from fat. Easy to see why many suggest we focus on low intensity exercise to burn fat.

Lets now consider what that means in terms of calories of fat burnt though. In the first case 65% of 400 kcal is 260 kcal of fat and in the second case 35% of 800 kcal is 280 kcal. So the high intensity exercise burned more calories, more of which came from fat and it has a number of other benefits too (such as even more calories burned as a result of EPOC, but that’s the topic of a future post).