The Fat Burning Zone

By: Brian Betancourt M.S
Edited By: Rebecca Raskin

The possibility of weight loss draws many people to exercise. One thing that many fitness professionals talk about utilizing, in regards to weight loss, is a concept called the “fat burning zone.” The “fat burning zone” is a concept that comes from good old exercise science. The idea behind the “fat burning zone” is based on substrate availability (carbs, fats, and proteins) and the intensity of exercise. Low intensity and long duration of physical activity is associated with Fats being the primary source of fuel for the body, thus if you engage in this type of exercise you’d assume that more fat will be burned. High intensity and short duration are associated with Carbs being the primary source of fuel for the body during exercise, thus you’d assume that less fat would be burned. The funny thing is, that the “fat burning zone” burns less fat than higher intensity exercise. If you are confused right now, don’t worry, I have a simple explanation just for you. But first, here are some important concepts to note:

  • Weight loss is a matter of being in a caloric deficit; a 3500 calorie deficit a week is about 1 pound of weight loss a week.
  • Although the body likes to use certain substrates at certain intensities, try to think calories only.
  • A person’s size and cardio-respiratory fitness level will determine how many calories a person can utilize at rest and during exercise.

If we consider all of those points, we would want to create the highest caloric deficit. So here is an example of why the concept of the fat burning zone doesn’t necessarily get you the best results. Jon is a 23 year old man. He is 5′ 5″ and weighs 225 pounds and has a cardio-respiratory fitness level of poor (38.10 ml ∙ kg ∙ min¬-1). Now we are going to clone him so we have two Jons. Each Jon will undergo different cardio workouts. Jon 1 will walk for 30 minutes at a moderate pace (think 2.5-3.5 mph on a treadmill) so he will burn about 156-197 calories. Let’s say about 60% percent of those calories used came from fat; that means he utilized 94-118 calories from fat alone. Jon 2 will perform four sets of three sprints with a 2-3 minute break between sets. It took Jon about 15-20 minutes to complete his workout. Jon expends 277 calories only using about 45% from fat. Now here you may be thinking 45% is less than 60% that Jon 1 used. You’re not wrong, but 60% of 156 to 197 calories is about 94 to 118 calories. Well, 45% of 277 calories is 125 calories that come from fat. Now, all these numbers are greatly generalized, because diet and training history have a huge impact on substrate utilization. Short term, the diet would affect the performance of the activities, however long term eating habits and training history effect substrate utilization because of the way the body adapts to those changes. Now you know that the “fat burning zone” may not burn as much fat as you originally thought. In the next article, I will go into the “why?” So stay tuned!

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