Is the fat-burning zone really just a myth?

Yes and no. The idea of the fat-burning zone was developed based on the fact that during certain intensities, the body utilizes more fuel from fat than it does from carbohydrates. Unfortunately, this is the case at LOWER levels of intensity. We could even say that we are burning a higher percentage of fat (than carbohydrates) when we are resting. However, the ultimate goal is to burn more calories, so crank up the intensity as high as you can tolerate!
Dr. Michael Roizen, MD
Internal Medicine

No and Yes (primarily yes, it is a myth). Technically, the charts you see at the gym and on exercise machines are correct: With a low heart rate, your body likes to fuel activity by using fat. When you work harder and your heart rate climbs, your body prefers the quicker energy hit of any sugar lying around in your system.

But that doesn't mean that keeping your heart rate low for 30 minutes will make you slimmer than if you had pumped things up for 30. Quite the opposite! You incinerate more calories per minute -- that's what melts pounds -- when your heart rate is high.  So, up the intensity and keep that heart rate high for at least 30 minutes and if you can (and your doctor gives you the A-OK) go as fast as you can for at least a minute every 10 minutes during your work out.
There is such a thing as a fat-burning zone, however it is not what most people think it is. The fat-burning zone myth suggests that slower, or less intense activity, will burn more fat because the body would burn fat as fuel as opposed to other energy sources stored within our muscles. However, the truth is that less intense activity will burn less overall calories and result in less weight loss. Weight loss comes down to calories. Therefore, the more calories you can burn in the time you have to exercise, the better. So don't get stuck in the myth that less intense exercise or activity burns more fat, because it really doesn't. You will burn more calories doing higher intensity activity - and as a result burn more overall fat.
Dr. Mike Clark, DPT
Yes and no. There is a fat-burning zone; however, it is not what most people think it is. We do have an energy zone where the body uses mostly fat for fuel, however this zone requires a very low activity level, therefore it usually only occurs when you are sleeping. This “zone” burns very little overall calories – so even though you might be using more fat for fuel, you are not burning many calories. The “myth” behind the fat burning zone is that low level activity, such as light walking, uses more fuel from fat than carbohydrates in your body. However, this is not the case. Even with light exercise, your body uses a fairly equal combination of fat and carbohydrate to fuel the exercise. However, you are not burning many calories during this light activity and therefore, you may not reach your daily caloric deficit needed to lose fat. Zone training is an excellent method of cardiovascular training to burn calories and reach your fitness and performance goals, however, in order to burn fat – you need to burn more calories each day than you eat. Talk with a fitness professional about different cardio training zones and what zones you should be training in to reach your fat loss goals. 

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.