Great article for those that like to get on their treadmills and ellipticals and just do the "fat burning zone" workouts..
The Rule of the Game: Intensity
If you're looking to shed stubborn pounds, the rule of the game is to increase the intensity of your workouts. I want you to be working out at 85 percent of your maximum heart rate (MHR). However, you may have heard the "fat-burning zone" theory that encourages you to work out at just 70 to 75 percent of your MHR. The outdated assumption is that your body is drawing predominately on fat calories for energy — WRONG! It's completely misleading and it's time to lay the "fat-burning zone" myth to rest.
During physical training, your body has three possible sources of energy: carbs, fat, and protein. Protein is a last resort — of the three energy sources, your body is the most reluctant to draw on your protein stores.
Whether your body takes energy from glucose, which it gets from the breakdown of carbs, or fat depends on the intensity of your workout. Training at a high level of intensity forces your body to draw on carb calories for energy — they are a more efficient source of energy, and your body goes for its premium fuel when you're working hard. If you are training at a low level of intensity, your body doesn't need to be as efficient, so it will draw on a higher percentage of fat calories for fuel.
Sounds like low-intensity training would be more effective when it comes to losing fat, right? Wrong. These physiological facts are the ones that spawned the mistaken belief that low-intensity activity is better than high-intensity activity when it comes to burning fat and losing weight. These days we know that even though the ratio of fat-to-carb calories might be higher during low-intensity exercise, fewer calories are used up overall. High-intensity exercise burns the biggest number of calories.