What's the deal with the Fat Burning Zone?
Well, as I wrote in the this post, it's a heart rate zone that utilizes a fuel mix consisting of a majority percentage (50-60%) of fat.
As some comments suggested, 30 minutes of exercise in the lower fat-burning zone will burn an equal amount or slightly more fat than a 30 minute workout in the upper cardio zone, but the 30 minute workout in the cardio zone will burn more overall calories. In terms of weight loss, burning more calories is best.
While training in the upper end of the cardio zone burns more calories, it is a training zone beginners, unconditioned people, and people who carry excessive weight should generally avoid. Working out at 70-85% of one's maximum heart rate is strenuous work. When an unconditioned person attempts to continue working out at this level of intensity, they are likely to feel exhausted, overtrained, and unsuccessful. They are likely unable to maintain high intensity for a long workout. They may become more susceptible to injury. In these cases, they are more likely to drop out of the program from burnout. Additionally, people who suffer from a variety or conditions like hypertension, obesity or morbid obesity, or heart disease, might be advised against this level of exertion by their doctors. In these cases, they might instead be advised to build their endurance through longer workouts (up to 45 minutes!) in the "fat burning zone." In this way, they burn a high percentage of fat calories safely and effectively, and may actually burn the same number of calories as a "cardio" workout of half the duration! Most importantly, they will feel successful. Finally, as a trainer, I advise people to pursue activities they enjoy, and some people are simply not interested in high intensity work, no matter how much MORE fat or more calories they can burn.
Though the number of calories burned in a 30 minute workout in the cardio zone might look impressive on paper, the reality is that high intensity workouts strain the heart, muscles, joints, and tendons, and may not be the best workout for everyone, regarless of caloric burn! As I once read with respect to exercise and intensity, "Make haste. Slowly."
As a trainer, I strive to give my clients a roadmap to safe, effective, and enjoyable weight loss and fitness. I cannot imagine a circumstance in which I would recommend that a conditioned athlete take down the intesity of their workouts to shed some fat! I might instead look at their nutrition or strength training plans.
Yet, when an unconditioned client begins their journey to safe weight loss, I usually begin by suggesting that their aerobic workouts remain in the "fat burning" zone. It's a concept that's easy to understand for many people. It's a location on a chart. I use the term for lack of a better term, and it's an effective tool for illustrating to people how their body changes as the intensity of their work changes.
The titles of "fat burning" and "cardio" zones are simply helpful misnomers. They describe, roughly, how the body is consuming fuel. Posting these guidelines was simply a statement of facts.