Step on any piece of cardiovascular equipment, and once you have figured how to turn it on, you will most likely be asked to choose the type of program you want to perform. A standard choice has always been something called the "fat burning zone." You may also be presented with a colorful diagram showing you the spectrum of intensity where your specific fat burning zone is, giving you the impression that fat is only burned when you are exercising in this magical zone.
The concept behind these fat burning zones is that by staying at a particular intensity, usually a lower intensity, you burn more calories from fat. Most cardio machines will even conveniently help you carefully dial into your supposed fat burning zone.
Although there is some truth in the general principle, what people don't understand is that it is the total number of calories burned that is more important, no matter what source they come from. On a very general or basic level weight loss is a balance of calories in versus calories out, so theoretically the more calories you burn the more weight you will lose. It really doesn't matter if those calories come from fat or carbs.
Typically fat burning zones tend to use steady-state, lower intensities over longer durations. Yet many recent studies have shown that high intensities performed using intervals over shorter durations actually burn both more overall calories as well as a higher amount form fat.
Here's a quick math example to show how this works. Let's say subject A exercises for 60mins in their so-called "fat burning zone" and burn a total of 200cals. Now let's say 40% of those came from fat stores, this equals 80 "fat" calories. At the same time subject B exercises using a high intensity interval program for only 30mins but burns 300cals and let's say only 30% of these come from fat. This not only equals a total of 90 fat calories burned in half the time, but also one and a half times the total calories burned. This leads to a much more efficient workout both in terms of time and caloric burn.
Ultimately these machine fat burning zones are just another way for the manufacturer to promote their product. Typically you will burn both more total calories using a higher intensity over a shorter duration. So forget the fat burning zones and concentrate on overall caloric burn!