Heart Rate (HR) is the number of times the heart beats per minute. This is usually taken by pressing on the front, left side of the neck, or the inner wrist, and counting the number of beats in a certain time. i.e. If you counted 10 beats, starting at zero, in 10 seconds your HR would be 60 (10*60 sec(1min)/10). When doing an aerobic work out your Max Heart Rate (MHR) needs to be determined. Your MHR is defined as 220-Your Age. A 20-Year old's MHR would be 200, while a 50-Year old's would be 170. While performing aerobic exercise your HR should stay in the range of MHR*.50 to MHR*.80. This is your Training Zone. So a 20-year old should keep their HR between 100 and 160. Keeping the HR above this zone will not provide additional aerobic benefit, in fact, it may provide no aerobic benefit at all. For best results, aerobic and FAT burning, keep your HR in the aerobic zone for at least 12 minutes. The longer and more frequently you do this the more improvement you'll see. http://www.faqs.org/faqs/misc-fitness/part1/

Connecting heart rate to fat loss is popular, but I ignore it myself for the most part. Your body will use about equal amounts of fat and carbohydrates normally. As your activity levels go up, that will shift to more and more carbohydrates being used. If you continue exercising, your body will eventually run out of available carbohydrates to burn, and switch over to fat. This is called hitting the wall, and the second wind. It is also why many people like to work out in the morning on an empty stomach, or sometime after a carbohydrate free breakfast. (With time to digest of course.) Again I am not too worried about heart rate. I prefer what is called high intensity interval training, (HIIT) As an example you can jog at an easy pace for about a minute, then sprint for 15 seconds, then repeat about 5 or 6 times. You slowly build up to about 10 of these cycles, then switch to 1 minute jog followed by 30 second sprints. The nice thing about HIIT is that unlike regular cardio, your metabolism is ramped up for the rest of the day. (Yes, cardio has a lesser effect.) Also you will find your endurance will increase, and going up stairs will become nothing.

Your muscles are fueled by calories and oxygen. Your heart is a muscle. Your heart is a muscle that circulates calories and oxygen to the other muscles. The harder you work your muscles the more fuel and oxygen is needed and the faster the heart pumps. Just like the rest of the muscles in your body, the stronger the heart, the more efficient it becomes in circulating the blood. When you hear someone talking about a cardio workout, they are addressing a workout that targets the heart muscles. The desired outcome is the heart muscle will be strenghtened and therefore become more efficient at circulating blood in times of rest and normal activities. Now concerning how to lose fat. Too lose fat you must burn more calories than you take in. It's that simple. If you keep the same caloric intake as you always do, but burn more calories by doing more exercising, you will lose fat. How fast you lose weight depends on how many more calories you burn than you take in. It's not advisable, but if you stopped eating and exercised allot more, you'd waste away quickly. IF you simply reduce your caloric intake and increase your exercise routine, you will lose weight. Fat cannot be reduced in just one part of your body. The calories stored as fat will slowly be recalled and burned by the muscles when needed. Good luck.