One way that we can lose weight really fast, is by doing high intensity training. Most of us do not really focus on our heart rates because we can tell if are heart is about to explode. But if you are keeping your rate up too high for a long period of time, generally past 45 minutes, you can start to burn muscle not just fat. It is important to at least know your target heart rate, which can be found in the website below. High intensity training for about thirty minutes on the other hand, is not so much a bad thing. It can help burn a lot of calories and fat, in a short amount of time.
But just be careful, once you start going further in time it can have negative effects. Lets say you are doing cardio for an hour. Usually on that time moderate intensity is the best for you, sometimes going a little more intense is okay. The longer you are training, the more experienced you can get, the easier it is to tell just from how your body feels, where your heart rate should be. That is why it is important to always keep exercising, and gain knowledge on fitness.
Heart rate:
http://www.heart.org/HEARTORG/HealthyLiving/PhysicalActivity/FitnessBasics/Target-Heart-Rates_UCM_434341_Article.jsp#