You hear it shouted in the gym, "feel the burn! Only 10 more reps!" What does it actually mean and should we really feel the burn? Is that the fat we are burning?What is this "burn" that everyone is talking about? Well, its your muscles inability to keep up with increased energy demands. The body typically produces energy through aerobic metabolism (using oxygen.) This is what is happening right now as you are sitting reading this. If you start to exercise or higher intensity activity, your body will start demanding more energy. When your body starts demanding more energy than it can produce, your anerobic system kicks in. Your anerobic system allows you to produce energy without oxygen. However, there is a cost...you can build up metabolic byproducts, one of which is lactate (and pyruvate). Now, its a bit of a misconception that the lactate/lactic acid/pyruvate is what causes muscle fatigue and burning, but it is a way to measure how much your anerobic system is working and how efficient your aerobic system is at getting rid of it. Your aerobic system uses the metabolic byproducts from the anerobic system as fuel for recovery from exercise. This is why you often see professional hockey players doing post game interviews while they are riding a stationary bike. This uses their aerobic system to "flush" out some of the lactate!
Now the irony is that most people looking to "burn fat" during their workouts. If you are working out at a high intensity, you are using the anerobic system (no oxygen). If you are using the anerobic system you ARE NOT USING FAT AS FUEL! The anerobic system uses sugar as fuel. The, in the body, this sugar is "blood sugar" or stored as glycogen. Only the aerobic system uses fat as fuel. So if you are doing high intensity exercises like spin, step or kettlebell classes, you are probably burning more sugar as fuel than fat. The best aerobic (using oxygen) exercises are long duration low intensity exercises such as long walks/jogs etc.I'm not saying that you should not do high intensity workouts. I'm sure you have seen many people be quite successful at losing fat by doing high intensity exercises. This happens because you will recover from exercise using your aerobic system (using oxygen), and your aerobic system is the one that burns fat as fuel!
