Your body's most efficient energy source is fat.
Just one pound of stored fat can provide about 3,600 calories of energy, which is far more than most people actually burn in a single day. In comparison, a pound of storage protein or carbohydrate provides less than half that much energy.
Since it's so efficient to use as energy, your body relies primarily on fat during rest and during relatively slow and easy physical activity. But once you begin moving quickly, your body begins to burn more carbohydrates, since fat doesn't provide energy as quickly as carbohydrates. When you need to get from point A to point B quickly (like a running race), or need to hoist a heavy object overhead (like weight training), your body needs more immediate energy, and that's where carbohydrates come in. They don't provide as much energy, but they certainly provide it far faster than fat.
So when you move from a standstill to a walk to a jog to an all-out sprint, your body begins to tap into carbohydrates more and more, while gradually reducing reliance on fat as a fuel.
Of course, it's important to remember that as you move faster, you're burning more overall calories, too -- so while the percentage of fat used as a fuel is decreasing, the total fat calories you burn might still be increasing since your overall calorie burn is significantly increasing. And this is where the "maximum fat-burning zone" comes in.
If you burn 200 calories per hour while walking, and get 60 percent of those calories from fat, then you burn 120 fat calories per hour. But if you burn 600 calories per hour while jogging, and only burn 40 percent fat during that time, you burn 240 calories of fat per hour, twice as many as when you were walking. You're burning more overall fat calories, but using less fat as a percentage of your overall fuel utilization. Using this concept, you can approximate the point at which fat burning peaks during exercise -- aka, your "maximum fat-burning zone."
The maximum fat-burning zone typically occurs at 45-65 percent of your maximum heart rate, and that is the calculation ordinarily used by personal trainers or gym machines. They'll take the number 220, subtract your age to find your maximum heart rate, and then take 45-65 percent of that number to find your maximum fat-burning zone.
But the result you get from this method is highly variable and tends to be inaccurate, primarily because your maximum heart rate is highly variable, and that 220 equation doesn't pinpoint it very well. So here is a better way to find your maximum fat-burning zone:
Warm up on a bicycle for 10 minutes. An indoor bicycle is better, since there's no traffic, hills, etc.
For example, if your average heart rate was 160, 160-20 is 140, 140+3 is 143, 140-3 is 137, and so your maximum fat-burning zone is when you have a heart rate of 137-143 beats per minute.
Compared to the results that I have obtained from hundreds of individuals in a professional exercise physiology lab with all sorts of gas masks and gadgets, this method obtains very similar results.
Finally, remember that the maximum fat-burning doesn't necessarily burn a high number of calories; and if you do all your exercise in that zone, you won't necessarily develop strong lungs or muscles, or much fitness or athleticism. As a matter of fact, because they burn so many calories and boost your metabolism so much, hard cardio bursts and weight training help you lose fat much faster than exercising in your maximum fat-burning zone.
So your ideal workout program should combine cardiovascular exercise in your maximum fat-burning zone (for example, in the morning or on easier, recovery days) with a combination of resistance training and cardio intervals that go above the fat-burning zone (for example, on afternoons or alternate days). Here is a sample workout week that incorporates the fat-burning zone:
With the workout above, you give your body a chance to burn fat fast with the resistance training and cardio intervals, but you also get to utilize easier days to burn fat in your maximum fat-burning zone, without quite as much strain on the body.
So now that you know how to find your maximum fat-burning zone, it's time to head to the gym and do your test!
For more by Ben Greenfield, click here.
For more on fitness and exercise, click here.
Ben Greenfield is a fitness and triathlon expert and host of the Get-Fit Guy podcast on the Quick and Dirty Tips network. His book, Get-Fit Guy's Guide to Achieving Your Ideal Body -- A Workout Plan for Your Unique Shape, will be published by St. Martin's Press in May 2012.
Follow Ben Greenfield on Twitter: www.twitter.com/GetFitGuy