The Fat Burning Zone Myth
Over the past several years there has been a lot of attention given to the concept of "The Fat Burning Zone" of cardiovascular exercise. The reason for this is likely because so many people have fitness goals based around weight loss, fat loss, and improving body composition. If you are like many people with those goals and you hear about the fat burning zone of course you are going to be intrigued because it has a catchy name and it falls in line with your fitness goals. Most people want to exercise in a manner that is best for reducing fat.
When talking about the fat burning zone it's important to have a baseline understanding of the energy systems and calorie utilization during exercise. In very basic terms the more intense the exercise is the higher the percentage of total calories burned that are coming from carbohydrates will be. The lower the intensity of exercise is the higher the percentage of total calories burned that are coming from fats will be. This means that sprinting as fast as you can maximizes the amount of contribution coming from carbohydrate energy sources while sleeping maximizes the amount of contribution coming from fat energy sources.
In general many fitness professionals will recommend cardiovascular exercise (running, biking, stair stepping, rowing etc.) at about 70-80% of your maximum heart rate. This is considered an ideal range of exercise intensity to increase cardiovascular fitness which has been reinforced by numerous research studies. The fat burning zone is identified as 50-70% of maximum heart rate. Again, the fat burning zone theory suggests that cardiovascular exercise performed at lower intensities uses a larger percentage of fat calories than cardiovascular exercise that is performed at higher intensities. As we learned that part of the theory is actually true but it is very misleading if you don't look at the entire picture.
To better understand the entire picture lets look at a real world example of this concept. A 160-pound man walks for 30 minutes at a pace of 3.5 miles per hour. When walking at this slower and less intense pace 40% of his energy supply is coming from fat sources and 60% is coming from carbohydrate sources. The next day the same man runs for 30 minutes at a pace of 6.5 miles per hour. At this faster and more intense pace 25% of his energy supply is coming from fat while 75% of his energy supply is coming from carbohydrate sources. If we stop there the fat burning zone theory seems to hold up because the lower intensity exercise uses a higher percent of calories from fat for energy supply. However, you need to know the rest of the story.
At a pace of 3.5 miles per hour the man will have burned a total of 240 calories in his 30 minutes of exercise. This means that he burned 96 total fat calories (40% of his total of 240 calories burned). At a pace of 6.5 miles per hour the man will have burned a total of 450 calories during his 30 minutes of exercise. This means that he burned 112 calories from fat (25% of his total of 450 calories burned). As you can see the even though a lesser percentage of calories burned were from fat the total amount of calories burned at the higher intensity was much higher so the total amount of calories burned from fat stores was higher than at the lower intensity. So when you multiply the percentage of fat used by the total amount of calories burned, you always utilize more fat calories when you are exercising at a higher level of intensity than when exercising at a lower level of intensity.
Now that you understand that the fat burning zone is misleading lets dive a little bit deeper into how your body actually loses fat. The amount of fat you lose from your body fat stores is not determined by the type of calories you are burning but rather by the total amount of calories that are burned. The only thing that matters when it comes to body fat reduction is the difference in the amount of calories you consume and the amount of calories you burn. One pound of fat is 3500 calories so when you've burned 3500 more calories than you have eaten in any given period of time you will lose 1 pound of fat.
Knowing this and seeing from our example it's obvious that you burn calories faster during higher levels of exercise intensity than during lower levels of exercise intensity. Once you look at the full picture you can see that "The Fat Burning Zone" is not a fat burning advantage at all. In fact, it's a distinct disadvantage to be exercising at a lower intensity if your goal is the reduce the amount of fat on your body in the most efficient way possible.
In general the same heart rate zone that promotes cardiovascular fitness (70-80% of maximum heart rate) is ideal for the purpose of fat loss as well. Unless intend to exercise for a very long period of time on any given day, training at lower intensities reduces both the cardiovascular conditioning benefits and the rate of fat loss.
If you are starting a new exercise program I would suggest having an exercise professional give you a personal recommendation and provide you with supervision. You can try a FREE workout with Reformed Fitness by clicking HERE but generally speaking you want to begin a new workout program by starting slowly (60-70% of maximum heart rate). As your fitness level improves you will feel comfortable to increase the intensity of your exercise and you will experience more fitness benefits. If the cardiovascular equipment you are using has a fat burning zone feature, feel free to ignore those guidelines and train at a more effective exercise intensity. If you don't understand all of the information around heart rate that's okay, simply perform cardiovascular exercise at an effort level that enables you to converse in short sentences and you should be training at the right level for both improved cardiovascular conditioning and fat loss.
Xavier Robinson, B.S. Kinesiology, ASCM certified exercise physiologist.