Recognise the image above? You may have seen that or something similar on certain cardio machine before. Basically, this is saying that there are certain levels of cardio intensity we should be aiming for depending on our goal. For example, if we want to improve fitness, we should aim to get our heart rate somewhere in the range of 60-90% of our maximum heart rate (MHR roughly equates to 220 minus our age), but if we want to lose weight, we should aim for 50-70% MHR; which is on the lighter intensity side of things. Hooray! That's good news, right? We lose weight with easier cardio… Well, sorry to burst that bubble, but that's not quite true, and here's why.
The truth about the fat burning zone
At any intensity level, you will be burning a combination of carbs and fats, and the truth is, you do burn more fat at lighter intensities; that's because carbs are saved for the hard stuff. There is a way to calculate exactly what you're burning, but it's not easy. You need a special machine which measures and analyses the breath you inhale and exhale whilst exercising; and this gives you data from which you can then calculate not only how many calories you're burning, but also the ratio of fats and carbs burnt in those calories.
So yes, at a lower intensity level you will burn more fat, however you will not burn as many calories as you would at a higher intensity, and that's what matters most for fat loss. High intensity cardio also has an after-burn effect. This is because your metabolism increases, and this can last anywhere between a few hours to a whole day before returning to baseline. In this after burn, your body also uses fat as its predominant energy source, all while you're resting. So next time your trainer suggests a HIIT cardio session, think of the after burn!*Disclaimer: Individual results vary based on agreed goals. Click here for details.