Fat Burning, Fitness, Nutrition

Fact or Myth: The Fat Burning Zone?

Posted by Dr. Del | March 23, 2014

Fact or Myth:  If you want to burn fat during your workout, you should keep your heart rate within a certain zone (The Fat Burning Zone)?  Just recently I heard a personal trainer said this to a client on the treadmill.  Like most things, if you hear it often enough you start to believe it whether it’s true or not.  So is such advice fact or myth?  Let’s look at the science of fat burning.

Fat burning and exercise
During exercise you use both fat and carbohydrates for energy, with these two fuels providing that energy on a sliding scale. At a very low intensity (e.g., walking), fat accounts for most of the energy expenditure. As exercise intensity increases the body uses more carbohydrates for energy.

If you exercise long enough (1.5–2 hours), your muscle carbohydrate (glycogen) content and blood glucose concentration become low. This metabolic state presents a threat to the muscles’ survival, since carbohydrates are muscles’ preferred fuel. When carbohydrates are not available, the muscles are forced to rely on fat as fuel.

Since more fat is used at low exercise intensities, people often assume that low- intensity exercise is best for burning fat, an idea that has given birth to the “fat-burning zone.”

However, while only a small amount of fat is used when exercising just below the lactate threshold, the rate of caloric expenditure and the total number of calories expended are much greater than they are when exercising at a lower intensity, so the total amount of fat used is also greater. What matters is the rate of energy expenditure, rather than simply the percentage of energy expenditure derived from fat.

Since you use only carbohydrates when exercising at a high intensity, does that mean that if you run fast or take a high-intensity indoor cycling class, you won’t get rid of that flabby belly? Of course not. Despite what most people think, you don’t even have to use fat during exercise to lose fat from your waistline. Have you ever seen a fat sprinter? Sprinters primarily train anaerobically, never using fat during their workouts. Yet they’re still very lean. Carbohydrates are actually the muscles’ preferred fuel during exercise.

During a low intensity workout, most of the fat that is used in combination with carbohydrates is in the form of intramuscular triglycerides (tiny droplets of fat within your muscles). This is because during exercise, when you need to regenerate ATP quickly for muscle contraction, it is more efficient to use fat that is physically closer to the mitochondria. To use adipose fat (the fat on your waistline and thighs), the free fatty acids would first have to be transported to the mitochondria, where they could be oxidized. Adipose fat is burned during the post-workout hours, while you’re sitting at your desk or on your couch.  And research shows that you burn more adipose fat after an intense workout.

Fat burning and high intensity training
Not only does high-intensity exercise burn more calories in the same amount of time as low-intensity exercise (because the rate of caloric expenditure is greater), but high-intensity exercise also causes a greater elevation in the post-workout metabolic rate. For example, one study observed that triathletes who cycled at 75% VO2max for 20 minutes burned more calories after their workout than they did after cycling at 50% VO2max for 30 or 60 minutes.

Another study compared metabolic rates following two equal calorie-burning workouts—a short-duration, high-intensity workout (51 minutes at 75% VO2max) and a long-duration, low-intensity workout (78 minutes at 50% VO2max)—found that the high-intensity workout resulted in a higher post-workout metabolic rate than the low-intensity workout.

A great way to perform high-intensity exercise and decrease your body fat percentage is through interval training, which breaks up the work with periods of rest. Not only does interval training allow you to improve your fitness quickly; it is also more effective than continuous exercise for burning lots of calories during exercise and increasing your post-workout metabolic rate, which increase fat burning of adipose fat—the kind that sits around your waist.

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Myth busted
So if you want to burn more fat during your workouts, should you keep your intensity within the “fat-burning zone?”  This is pure MYTH, so the next time you hear someone say this, just refer them to this article because they obviously need a basic physiology lesson.

Until next time, be well, do good, and live inspired with passion and purpose!
Photo: freedigitalphotos.net
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About Dr. Del

Dr. Del Millers is the founder of TheBestYouAcademy.com and author of eight books on mind-body health and nutrition. A PhD Nutritionist with a Masters degree in psychology, Dr. Del teaches motivation and high performance strategies to busy entrepreneurs and professionals, so that they can make a difference in their lives tomorrow in just ten minutes today. Dr. Del has appeared on FOX Television (Good Day LA) and E-Entertainment TV (DR 90210), magazines, and newspapers throughout the United States and Australia (LA Sports & Fitness, Australian Ironman, Health & Fitness, Stuff, Fighting Fat and others). Dr. Del's books include: Fitness & Spirituality, Dr. Del’s 10-Minute Meals, and Dr. Del’s Rapid Fatloss series of books.