Fat Burning Zone

You may have heard this advice sometime, “If you want to burn fat, you need to be exercising in you fat-burning zone.” Even the cardio machines at the gym have a nice little graph of where your heart rate should be if you want to burn fat versus where your heart rate should be if you’d like a cardiovascular workout.  So what is the “fat-burning zone” and does it actually help you burn more fat?

The Physiology

Your body has two main sources of energy; carbohydrates and fat. You are constantly going through metabolic processes that break down these macronutrients, producing something called ATP, which is the main source of energy in the body.  To use carbohydrates, your body breaks down the carbs into their simplest form, glucose, and goes through a 10 step process called glycolysis. Glucose is available in your bloodstream after you’ve eaten, is stored in your muscles and produced by the liver. The other way you can produce ATP is by breaking down fat and you use a simple form here as well (the fatty acid). However, in order to use fat, it must be broken down in your adipose tissue (those are your fat cells stored all around your body), travel through the blood stream, be taken up by your muscles and then go through two cycles before you get ATP. It’s a much lengthier process, but the end result is much more ATP.  Because this process is so lengthy, however, you can’t produce energy very quickly. That means that if you are exercising intensely, you will rely on carbs for energy rather than fats.

The bottom line: the harder you are exercising, the more you use carbohydrates, and the lower your intensity, the more you rely on fats.

So what’s the claim?

The idea behind working in your “fat-burning zone” is that if you lower your intensity, you will burn a larger percentage of calories from fat, instead of carbohydrates. Which is true, sort of. The thing you don’t hear much about is that your ultimate fat burning zone is sitting completely still. When you are sedentary, you don’t need energy very quickly, so almost all your energy is coming from fat. Well great! Why don’t we all lose weight sitting around then?  The problem is that you burn very few calories while sitting, so who cares if they all come from fat? The same principle applies to exercise.

Practical Application

Exercising in your “fat-burning zone” usually entails a heart rate of approximately 120-140, however you will burn significantly fewer calories in your exercise session than if you were able to maintain a heart rate of 150-160. Yes, more of them may be derived from fat, but because your total number is so much less, you won’t come out ahead. If burning fat is what you’re after, you can either work out at a higher intensity for 20-30 minutes, or, if you prefer a lower intensity, then you need to increase your time to 60 minutes plus.

How intensely do you like to exercise? Do you prefer longer and slower or shorter and more intense?



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