Posts Tagged ‘fat burning’

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?




When someone gives you good advice, you should probably take it.

As you know, I am training for this absolutely crazy race in January and I’m trying to cover 50 miles in 24 hours. In order to train for this, I am increasing my weekly long runs by a few miles each week until I can run 25-30 miles at a time. Right now, I’m at 10. So, since this is the first time I have ever done anything like this, I sought out the advice of my friend who’s done several 50 mile+ races already.  I was wondering how often she eats/drinks during a run, since I never run long enough to need additional sustenance. She wisely advised me to take in some carbohydrates about every hour or so. Did I listen?…

Let me give you a little inside look as to why she would give me this advice. When we exercise, our bodies either use carbohydrates or fats for energy. Carbohydrates, or glucose, are the easy energy. Your body is great at breaking down simple sugars which are stored both in your muscles and in your blood stream. The harder you work out, the more your body relies on sugar for energy. Because everything in life is a trade-off, these easy to break down, readily available sugars do have a draw back; you can only store so much and you don’t get much bang for your buck.  Sugar is metabolically expensive and heavy to store, so your body would much rather use fats for fuel. Fat gives you a great bang for your buck as far as energy production. Even the leanest person literally has days worth of stored energy from fat.  But, because fat is so valuable to your body (all that energy!), it takes a lot to convince your body to use it up. Fats take a long time to be released from adipose tissue (stored fat), then they need to be taken up by the muscle and oxidized, or broken down before they can be used. It’s a long, slow process, but one that is vital to long distance running. Plus, in order to use fats, you have to be exercising at a light-to-moderate intensity. Fats just don’t break down quickly enough to give you energy for intense exercise.  However, the longer you train, the better your body becomes at breaking down fats and using them for energy.

If you are like me and just starting out, your body is still used to using carbs for energy and you can run out quickly! If you don’t supplement your running with some kind of carbohydrate intake, your blood glucose will drop quickly and the longer you exercise, the worse you feel. Which is exactly what happened to me this weekend. I knew I should have taken some kind of sports drink of power gel or something, but no, I figured I could handle it. After running just over an hour, I felt AWFUL!!! My body had run out of carbs to burn and since I now was relying on fats for energy, I had to slow waaaayy down. By the time 75 minutes rolled around, I was nearly walking and was definitely second-guessing my ability to complete this crazy race in just a few months. Not easily deterred, I swallowed my pride and realized had I just listened to my friend’s advice, I probably would have been fine. Sunday night I went out for another go; this time I brought my friend Powerade with me, and what do you know… I ran a great 85 minutes! Hooray!

So, lesson learned. Someone who has completed more endurance runs than I have probably knows a whole lot more about them than I do!

Stay tuned for next week! I’ll talk to you a little more about what fuels we use when exercising and why all those cardio machines and personal trainers keep talking about your “fat burning zone”


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