Fat burning vs. burn more fat

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Many times when I talk with a gym member and ask them what they are doing for their cardio exercise, I will get an answer like, “I do 30 minutes on the treadmill 2 or 3 times a week … and I’m doing the fat burning program!” A common misconception that many people have when doing their cardio work is the difference between having a greater fat burning percentage while doing the work versus actually burning more fat. Let’s set the record straight.

There is something called the respiratory exchange ratio (RER). The RER is the ratio of carbon dioxide produced relative to the amount of oxygen that is consumed. The RER is a marker for the proportion of fat or carbohydrate used for fuel at different intensities during steady state exercise. Steady state exercise is where the relative intensity stays generally the same for an extended amount of time. At rest, the average RER is 0.75, and this means the body is burning approximately 85% fat and 15% carbohydrate. As the intensity increases, so does the RER. A higher RER means that a larger percentage of the fuel is coming from carbohydrate and less from fat. The link that has been incorrectly made is that low intensity exercise is the best for burning fat.

Here’s the take home message, in case the science has bored you. A greater percentage of fat being used as fuel does NOT mean you burn more fat! What this means is that when you train at a higher intensity, the fat burning percentage may be lower, but the amount of fat you actually burn is greater. Not to get all numbers and science-y on you again, but here’s an example: A person weighing 130 pounds runs at 5.0 mph for 20 minutes, they burn about 157 calories. If fuel from fat is down to 50%, they burn about 9 grams of fat! Same person walking at 3.5 mph for 20 minutes burns about 74 calories, and with 75% of fuel from fat, ends up burning about 6 grams. While you may be saying to yourself, “there’s not a lot of difference here!”, the fact is more fat was burnt at the higher intensity.

If you are new to exercise or unaccustomed to greater intensity in your cardio, this doesn’t mean you should go kill yourself with your cardio tomorrow at the gym. If your goal is to burn fat, you should attempt to gradually increase your intensity levels. The higher you can sustain a working steady state intensity, the more fat you will burn. Let’s not forget, as you get in greater physical condition, your body becomes much better at utilizing fat as a fuel source and you are a more effective fat burning machine, even at a rest!