I had the same questions, and read:


Perhaps no other training "technique" is more gimmicky and misleading than the "fat burning" and "cardio" modes on the control panels of exercise equipment. They are based on the biology that at lower levels of exertion a higher percentage of fat is burned compared to carbohydrate. That's because:

1. Fat is denser fuel than carbohydrate (9 calories per gram vs. 4 calories).

2. It takes more oxygen to burn fat than carbohydrate because fat is denser.

3. At lower levels of exertion, you presumably breathe in and deliver more oxygen to the muscles to burn fat.
All of the above may be true given the right circumstances, but there are problems with it when it comes to real-world exercise scenarios. First off, lots of fat is burned at all intensities within the aerobic training zone. Secondly, the terminology "fat burning" and "cardio" can confuse individuals into thinking that fat is burned only during exercise in "fat burning" mode and that no fat is burned in "cardio" mode. The fact is that you burn fat during both modes. But the major problem is that the fat-burning mode is typically too slow a workout for many people to maximize benefits. In fact, at the end of a fat-burning workout, you could end up burning fewer calories and less total fat than during a cardio-mode workout. Here's an example of what I mean.

Suppose a 150-pound moderately fit man walks on the treadmill for 60 minutes at 3.0 mph ("fat burning" mode). That's 300 calories for a 150-pound man (a 150-pound man burns 100 calories per mile whether he walks or runs). Since this man is moderately fit, he will burn approximately 60% of the calories from fat (180 calories) and 40% from carbohydrate (120 calories).

Now let's say the same 150-pound man walks on the treadmill for 60 minutes at 4.0 mph ("cardio" mode). That's 400 calories burned, with approximately 50% of the calories from fat (200 calories) and 50% from carbohydrate (200 calories). The percent of fat burned may be less at 4.0 mph than 3.0 mph because the exertion is higher and so theoretically less oxygen is delivered to the muscles.

If you examine the example carefully, you will notice that at the slower fat-burning mode the man does indeed burn a higher percentage of fat compared to cardio mode (60% v. 50%), but in cardio mode, he burns more total calories (400 v. 300) and more total fat (200 calories v. 180 calories). My suggestion is to ignore the fat-burning mode (unless you want a less intense workout). You're not going to burn more fat in this mode than in cardio mode, and it could end up being an inefficient use of your time. I suggest training as hard as you comfortably can without risking injury so that you maximize the calorie and fat burn and the overall cardiorespiratory training effect.

That answer your questions?