Am I burning fat? Aerobic exercise & energy use

Now that we have a general understanding of the energy balance equation which was discussed in last week's post found here, let's focus on how this all relates to something commonly referred to as the fat burning zone. You may have seen this zone, along with the cardio zone, illustrated on the treadmills, elipticals or aerobic exercise heart rate charts at your fitness center or gym. These zones are based on aerobic training principles and in theory are true, but in application can be somewhat misleading.

Basic Principles of Fitness Training-

If one of your fitness goals is to lose weight, you might be thinking that the fat burning cardio workout listed on your treadmill is the way to go. However, before we look at the fat burning zone specifically, I want to highlight a few basic principles of fitness training first. In order to put together an effective exercise program we need to address:

1. Overload - Overload is the amount of stress or demand that we put on our body during exercise. In order to improve our level of fitness we need to challenge our bodies during our workouts by increasing our level of effort. The amount of overload effects the level of intensity of our workouts and this in turn influences the improvements in our fitness level. If we work out too easily we are likely to see little change in our fitness level, whereas too much overload can lead to injury or burn out.(3)
2. Progression - The human body has the amazing capacity to adapt to the demands of exercise by improving its functioning. Because of this ability it is essential to make changes to your workout after you notice that the current level you're working at is becoming too easy. This isn't always a simple assessment, nor is it something many people want to do.  We get comfortable with a routine, so it is easy to become complacent.  This is why many people plateau, or find themselves at a point where they see little to no change in their fitness level (3).

Now that we understand two core principles of fitness training, we need to know how to apply them. When putting together a workout we now know that we must consider the amount of overload needed to maintain or improve our particular level of fitness and for a specific fitness component.  In this post, for example, we're looking at improving our cardiovascular fitness level and body composition by doing aerobic exercise. In order to address the progressive overload principles we must consider 4 areas that are represented by the acronym FITT, which stands for:

• Frequency (how often we perform the activity)
• Intensity (how hard we are challenged by the activity)
• Type (mode of activity)
• Time (how long/duration of the activity) (4)

I prefer the acronym FITTER - which adds Enjoyment and Rest into consideration as well. Adherence to an activity is more likely if it is enjoyed. Arguably more important is the balance between exercise and rest. Too much of a good thing is possible with exercise.  Having said that, over training is something I rarely see.  In fact, the opposite is more often the case.

So, at this point you might be wondering what all this has to do with the fat burning zone. Well, what the fat burning zone is referring to is the level of intensity, along with the amount of time an aerobic activity is performed at, and the macronutrient (fats, carbs, proteins) energy sources used by the body to accomplish the task. By the way, if you're curious to know what exactly constitutes an aerobic workout you can read more here, and if you'd like to catch up on our discussion of energy sources from last week you can find that here.  Now, this bring us to the three energy systems utilized by the body during physical activity. 

Physical Activity and the Three Energy Systems-

As discussed last week, in order to function our bodies need energy in the form of food. We get this fuel from the carbs, fats and proteins found in what we eat and drink. However, the amounts of  these macronutrients the body uses for energy vary depending on the type of activity we're performing.

1. For immediate, explosive energy needs the body uses the immediate energy system. This system fuels activities that last for about 10 seconds or less.
2. For high intensity acitivities lasting for about 10 seconds to 2 minutes the body uses the nonoxidative (anaerobic) energy system. 
3. For any activities lasting longer than 2 minutes the oxidative (aerobic) energy system kicks in. (3)
   

It's important to note that the body typically uses all three energy systems when we exercise, or in activities of daily living (3).  "The intensity and duration of the activity determines which system predominates" (3). For example, if you're running late and walking quickly to catch the bus you're using the oxidative energy system. But if the bus pulls up and you need to sprint to catch it, the other systems become important because the oxidative energy system isn't able to supply energy fast enough to sustain this high-intensity effort.

Modified Borg Rate of Perceived Exertion Scale
Image from ACE

Fat Burning Zone and Exercise Intensity-

So, back to the fat burning zone... The wisdom behind this zone is that if you're exercising in a target heart rate range (read more about this here) of between 50-60% of your maximum heart rate (MHR) (4), or a level 3-4 on the on the modified Borg Rate of Perceived Exertion (RPE) Scale shown at left, (more about that here) you will be utilizing more of the oxidative system, which draws more energy from the body's fat stores.  While this is true, it's just part of the picture.

 Let's look at an example taken from research done at the University of Wisconsin at LaCrosse.  "A 160 pound male walks at a pace of 3.5 miles per hour. At this pace 40% of his energy is supplied from fat sources and 60% from carbohydrate sources.  On another day the same subject runs for 30 minutes at a pace of 6.5 miles per hour.  At this faster pace (higher intensity exercise), 25% of his energy is supplied from fat sources and 75% from carbohydrate sources" (7). Clearly, this would seem to validate the theory that lower intensity exercise utilizes a greater percentage of fat for energy.

However... Please hang in there for a little math, because this will distinctly illustrate the key point I'm attempting to make. "At the 3.5 miles per hour pace the subject burned a total of 240 calories during his 30 minute walk. He therefore burned 96 total fat calories (40% of 240 calories = 96 calories). At the 6.5 miles per hour pace, the same subject burned a total of 450 calories during his 30 minute run. Therefore, he burned 112 total fat calories (25% of 450 calories = 112 calories), which exceeds the number of fat calories burned during the lower intensity exercise session" (7). Not only did this study subject ultimately burn more fat calories, but he was able to burn more total calories overall working out at the higher intensity level for the same amount of time.

The bottom line-

Remember the energy balance equation? Calories in = calories out. When it comes to changes in body composition and weight loss, clearly higher intensity exercise is a better zone to shoot for because it burns more calories than the lower intensity so-called "fat burning zone". That being said, it is important to keep in mind your level of fitness. Obviously, if you're just beginning an exercise program you want to start at a lower intensity and build up gradually. This is where the progression and overload we talked about earlier comes into practice.  With all this talk about burning calories, let's not forget the impact that a healthy nutrition plan has on the "calories in" part of the energy balance equation. Armed with these fitness tools and knowledge, you're closer to achieving your weight loss goals and better health. 


Sources
(1) Whitney, E., Rolfes, S., Understanding Nutrition 11th ed., Thomson Wadsworth
(2) http://en.wikipedia.org/wiki/Metabolism
(3) Fahey, T., Insel, P., Roth, W., Fit & Well: Core Concepts in Physical Fitness and Wellness, 6th ed.
(4) IDEA Fit - http://www.ideafit.com/fitness-library/comparing-intensity-monitoring-methods-0
(5) American Council on Exercise (ACE) -http://www.acefitness.org/
(6) American Heart Association on body composition -http://www.americanheart.org/presenter.jhtml?identifier=4489
(7) Wescott, W.,  What is Fat Burning Zone?
(8) Hoeger, W. K., Hoeger, S. A., Lifetime Physical Fitness and Wellness: A Personalized Program