I’m sure you can imagine how frequently I’m asked how to burn off and lose more fat. A client recently emailed me and asked about fat burning and cardio exercise. “A trainer recently told my mother she should not allow her heart rate to ever go over 100 during cardio,” wrote my client. “Is that correct?”

I have to make the assumption that this personal trainer was specifically referring to keeping my client’s mother in the “fat-burning zone.”

Well, there’s a simple answer to the question of when is fat burned: Your body is burning fat almost all of the time! The details are so much more complex, but let’s put this into a simple perspective. All of your body’s fuel systems are active at a given time, but the level of involvement depends on the intensity of your exercise workout.

In a fed state (after you’ve eaten and your body is not in starvation mode) during mild to moderate exercise, you’ll get your energy from approximately 50% carbohydrates in the food you eat, and 50% from your body’s stored fat. As the exercise intensity increases, the ratio moves more towards burning carbohydrates. At a very high intensity, your body could be running up 80% off of carbohydrates.

In contrast, when your body is at rest, you burn the highest ratio of body fat!

Does this mean if you run or sprint you’ll burn less fat? Technically, as a ratio, yes. During a high-intensity workout, more carbohydrates than body fat are burned. But here’s the important point to remember: That workout alters your body’s chemistry for the next several hours so that when you’re at rest, you’re burning much more fat than if you hadn’t exercised.

Here’s a quote from a relevant study (I’ve bolded a very important line):

Low intensity aerobic exercise does not heavily tax any of the fuel systems, but does rely on approximately fifty percent of fat as a fuel source. Don’t get excited yet! Training the phosphagen, glycolytic and oxidative system at a moderately high intensity increases the post exercise oxygen deficit. This stimulates the metabolism for several hours after exercise, which equates to a larger percentage of fat burned overall. If your goal is to improve your sport, train the fuel system that best resembles your sport. If your goal is fat loss, train all three systems equally. If the goal is cardiovascular improvement, train all three, but spend more time training at a moderately high intensity level. I wish you well in your training.
– Williams, Melvin H. 1995. Nutrition for Fitness and Sport Dubuque, IA. WC Brown.

I hope this clarifies the fat-burning question a bit. Stay well, and thank you for your support!