Have you ever heard of the mythical ‘fat-burning zone’? This is supposedly the heart rate range where your body will utilize fat as its main energy source. I think this question often comes from these little graphs you see on treadmills or the walls of the cardio section in your gym:
In this post, we’ll talk about why certain heart-rate ranges are considered to be in this fat-burning zone, as well as how much of an effect it can have on your workouts.
Getting in the Zone
The idea of a fat-burning zone comes from something called the “Respiratory Exchange Ratio” (RER). The ratio is between the amount of CO2 exhaled : O2 inhaled in a breath. This ratio can help to indicate whether your body is using mostly fat or carbohydrates as a fuel source. At rest, the ratio is ~0.7, which indicates a high use of fat. While exercising, the amount of CO2 exhaled tends to increase, which can raise the ratio to over 1. Exhaling more CO2 than you’re taking in O2 is a good indicator of high intensity exercise! A higher ratio indicates more use of carbohydrates as a fuel source.
However, most of us don’t have a sport science lab with technicians handy everytime we want to go on a run, so these principals were applied to heart rate ranges: the higher your heart rate is, the more intense your workout is, the higher your RER is, the more carbohydrates you’re using as fuel. This chart gives an idea of the conversions:
How it Applies to You!
So it looks like you’ll get the highest amount of fat burn at the lower heart rate percentages. Here is the thing about ‘fat-burning zones’ that I find a bit funny: The less intense your activity, the more you will use fat as the fuel source. So, technically, if you want to burn the MOST fat as a percentage, the best activity for you to do is to sit on the couch and do nothing.
Doesn’t that sound silly? Yes, you are using the most fat, percentage wise, but you’re burning next to no calories. Here’s a more detailed write-up on this point:
“Bear in mind that a 30-minute low-intensity workout does not burn nearly as many overall calories as a 30-minute high-intensity one. So if we do a workout that burns 100 calories at 65% MHR, that’s 40 calories of carbs and 60 of fat. (Roughly; protein does play a part, but such a small one it is typically discounted) However, if we do a workout that burns 200 calories in the same time at 80% MHR, that’s 130 from carbs and 70 from fat.
As well, we shouldn’t think of using fat as a fuel source as being the only way to lose body fat…If you burn carbohydrates to fuel your muscles in a high intensity workout, carbohydrates you take in after that will go towards refilling your muscle stores rather than being converted to fat. It’s just a different path to get to the same end.”
Ideally, the fat-burning zones would be the heart-rate ranges where you are burning the optimal ratio of fat to total calories. This can take a lot of mental gymnastics, especially since it can be difficult to calculate out the total number of calories you’re burning on the treadmill. (Generally the calorie counters on the various pieces of equipment are not super accurate)
So what’s a person to do? My suggestion: If you’re doing cardio to train for a 5k, 10k, Mud Run, or any kind of event, use the heart rate ranges that challenge you athletically. If you’re a serious weight-lifter, there isn’t much need to do intense cardio, as it can impede recovery between strength-training. Stick to low-intensity. If you’re just looking to burn a few extra calories, you can feel free to run the whole gamut! Long-duration, low-intensity or short-duration high-intensity or somewhere in between, whatever you prefer! Just remember that the bottom line is total calories burned.
As well, keep in mind that the best way to get into a calorie deficit is by reduction in intake of calories!