i thought this article was very interesting. especially the last 2 lines.
NBC.com - The Biggest Loser
Fat Burning
How Does Your Body Burn Off Weight?
By Michael Scholtz, BiggestLoserClub.com Fitness Expert Q: How do I know if I’m in my fat burning zone? A: Fat burning is not the issue. It is actually calorie burning that you need to be concerned about. While much has been made about the existence of a magical “fat burning zone,” it is actually one of the more persistent myths in the area of exercise and weight loss.
The myth stems from the fact that lower intensity exercise does burn a higher percentage of calories from fat than from carbohydrates. That sounds good on the surface. After all, it offers up images of fat melting off your body! What could be better?
The first hole in the “fat burning zone” theory is that your body does not care how you burn the calories or what fuel was consumed in burning them. The body gains or loses weight according to how much of an overall calorie surplus or deficit exists over time. In other words, to lose weight, you must eat fewer calories than you expend.
Therefore, the question must be raised: Do you somehow burn MORE calories by deliberately slowing down to stay in the infamous “fat burning zone”? Of course, the answer is “NO”! You burn more calories by exercising a bit longer or a bit harder, not by slowing down. The key is exercising at a pace that is comfortably challenging and, over time, having that pace increase as your fitness increases. As this happens you will burn more and more calories per minute.
The second hole in the theory is that even if it WERE true that it was more important to burn fat calories than total calories, the “fat burning zone” proponents would still be off the mark.
It seems preposterous to even make this point, seeing as it is not true that burning more fat calories matters at all. But, it is a fun way to look at the issue. So here goes.
You see, when you slow down to burn more fat you might burn 50% of your calories from fat vs. perhaps 30% if you exercise at a higher rate. However, you also burn fewer TOTAL calories; therefore you burn 50% of a LOWER number!
For a side by side comparison, look at the below estimates for 30 min of exercise for a 200 lb person:
Low intensity exercise % Fat burned 50
Total calories burned 120 Fat calories burned 60 High intensity exercise % Fat burned 30
Total calories burned 200 Fat calories burned 60 
... due February 13, 2009
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