Many people think that cardiovascular exercise (a.k.a. cardio) is the most critical component in losing fat. While it is definately an important factor in losing fat, weight training is better. Here's why.
Cardiovascular exercise is the process of exercising your heart. These types of exercises are designed to increase your heart rate. By doing this you are exercising your heart. Your heart is a muscle that pumps blood throughout your body. Without your heart pumping blood, you will die, so obviously you heart is an important muscle. By doing cardio you increase the strength and efficiency of your heart. By doing this, your heart is better able to deliver blood throughout your body with less effort. The stronger your heart becomes the more effort you can get out of it. This means that as you do cardio, you will develop a stronger heart and as a result you will be able to run faster, for longer periods of time, because your heart is able to handle a better load. Doing cardio also burns calories. If your heart is pumping faster than normal, this means that your body has to burn more calories in order to provide the energy needed to keep your heart pumping at an accelerated rate. This means that, the more cardio you do, the more excess calories you burn. If you burn more calories, you are going to lose fat. There is a downside to this, however.
When you do cardio, your body is burning calories to provide the energy to keep you moving. The problem is, it is not getting those calories just from your fat stores. It is also burning your muscles in order to get those calories. This means that the more cardio you are doing, the more muscle you are burning! This is a bad thing because muscle is metabolically active. Your body burns calories just to maintain the muscles - even if you are doing nothing. By doing to much cardio, you burn muscle, which means that you are actually decreasing your body's ability to burn calories!
This is why you must do weight training when trying to lose fat. By weight training, you are increasing your muscle mass. This has the side effect of offsetting some of the muscle loss you suffer as a result of cardio exercise. Plus, by increasing your muscle mass, you are permanently increasing your metabolism. The more muscle you have, the more energy it takes your body to maintain it. With an increased metabolism, you burn more calories.
Cardio burns calories, but it only provides a short term benefit. Studies have shown that cardio only causes an increase in your metabolism for an hour or two after you complete the exercise. Weight training causes in increase in your metabolism for two days after you work out. You might burn more calories in the short-term by doing a session of cardio, but in the long-term you will burn more calories as a result of weight training.
Now, don't be afraid to do cardio either. Cardio is a good thing, but balance it out with weight training. Combine 3 sessions of cardio a week with a complete weight training program and you get the best of both worlds - heart health improvement, increased muscle mass, more calories burned, and a higher metabolism.