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 

Understanding energy use and the fat burning zone

Fahey, Insel and Roth, Fit and Well

Have you resolved to lose weight this year and get rid of some excess body fat?  Maybe you made the same resolution last year, but are determined that this year's outcome will be different. Perhaps you're a workout veteran who is now finding it difficult to shed those last few stubborn pounds. Whatever the case, I think it's safe to say that we are all familiar with how diet and exercise directly impact our body composition.  This is the ratio of body fat to lean body weight (including water).The illustration on the left shows the body composition of a fit 20-24 year old male and female.

Knowing our body composition is a useful measure of fitness. It can be determined in a number of ways. The most common method being bio-electrical impedance. You can read more about the different measuring methods and tools here.  In order to improve the odds of achieving weight loss this year, it's important to understand how our weight and body composition are influenced by our energy expenditure.  So let's take a closer look.

Energy equation - Generally speaking, too much or too little body fat results from an energy imbalance (1). You may be familiar with the energy equation:

energy in = energy out 

Image from The Science of Sport, Tucker PhD & Dugas PhD

The way the body regulates weight is actually quite complex, but the simple picture is that when a person's food intake (energy in) is in balance with their calorie expenditure (energy out) their body weight usually remains stable. If food intake is less than the calories expended, weight loss usually occurs. Conversely, if more calories are taken in than are used up, the body stores these excess calories as fat, and a person typically gains weight.

A few interesting fat facts: Body fat isn't all fat. It's composed of a mixture of fat (87%), some protein and water. A pound of body fat = 3500 calories. A healthy adult can have an ample reserve of fat stores at anywhere from 50,000 to 200,000 calories (1). Fat is stored by the body as fuel for energy. Our bodies need essential fat. The health benefits of eating the right types of dietary fats were discussed in previous posts, The Skinny on Fat, More Skinny on Fat and Understanding the Omegas. It's only when there is an excess of fat that health problems can result. This brings us to how our body composition is influenced by the body's energy needs. 

Energy In 
Food and beverages provide the energy in. The amount of energy provided depends on the type and amount of things a person eats and drinks. The 3 major macronutrients found in food that provide energy are: carbohydrates, fats and proteins. Heat is given off when the body breaks down or metabolizes food. This is measured in kcalories, or what we more commonly refer to as calories. Scientists have determined that:

• 1 gram of carbohydrate = 4 kcalories 
• 1 gram of fat = 9 kcalories 
• 1 gram of protein = 4 kcalories 
• 1 gram of alchohol = 7 kcalories

Knowing the calorie content of the macronutirents is helpful  in determining the energy value of foods. For example, "a food containing 12 grams of carbohydrates, 5 grams of fat and 8 grams of protein will provide 48 carbohydrate calories, 45 fat calories and 32 protein calories for a total of 125 calories (1)".  Keep in mind that the efficiency of our digestion system also influences the amount of energy yielded by the macronutrients and absorbed by the body. This is why a high fiber meal has less energy absorbed than a meal containing less fiber. Fiber causes food to be moved through the intestines more quickly resulting in less absorption. You can read more about the value of eating nutrient dense vs. energy dense foods here. For now, suffice to say that in order for weight loss to be successful, it is important to consider not only how much food we are consuming, but the types of food we are eating as well.

Energy Out affected by 3 factors:

Basal Metabolic Rate- The "energy out" part of the energy balance equation occurs whenever the body uses calories for energy. The good news is we expend energy constantly just staying alive. This is called our basal metabolism. "About two-thirds of the energy the average person expends in a day" is used to maintain life. Everyone has their own unique basal metabolic rate (BMR) and the BMR can fluctuate in an individual depending on their physical condition or circumstances (1). The chart below lists the factors that influence BMR.

Whitney and Rolfes, Understanding Nutrition, 11th ed.

 Thermic effect of food - As mentioned earlier, when the body digests and metabolizes food, energy is required and heat is generated. This is what is called the thermic effect of food (TEF) and can be estimated at "10% of energy intake" (1). According to Whitney and Rolfes, however, "for most purposes the TEF can be ignored when estimating energy expenditure..." (1). 

Physical Activity - Energy output is also affected by our level of physical activity, and this is one area where we can affect a change in the composition of our bodies. By increasing physical activity we accelerate the amount of energy needed to support the work of the muscles and other body systems. Different kinds of physical activities, as well as the frequency, intensity and duration of the activity will influence energy demands.  In addition, body mass and body weight play a role in the amount of energy needed.  The chart below lists a few activities and their corresponding number of calories burned per minute while we are engaging in them.

ACE Fitness

Let's consider walking, for example.  This chart from the American Council on Exercise (ACE) doesn't specifically mention at what level of intensity the walking is being performed, but let's assume a normal pace. Walking at a normal pace, a person who weighs 120 lbs. will use approximately 6.5 calories per minute. Walking for 20 minutes uses a total of 130 calories. Now, let's put this into perspective, and the point I'm trying to make isn't to belittle the positive effect of burning 130 calories. However, what if after their walk this person rewards themselves with a Starbucks 16 ounce strawberry smoothie made with 2% milk that has 280 calories? (4) This person would now have to walk an additional 23 minutes to burn off this beverage and keep the energy equation in balance. These kinds of energy use imbalances are common and occur daily. Over time they can begin to add up to those extra pounds we struggle to get rid of each year.

So what's all this have to do with the fat burning zone?  We'll delve into this in more detail when I post about the exciting world of physical activity, the energy systems and what this means to us in terms of burning calories no matter what zone we are in. In the meantime, keep up the good work with those resolutions and keep making fitness a priority in your lives!


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) Starbucks nutrition info http://www.starbucks.com/menu/drinks/vivanno-smoothies/strawberry-vivanno-smoothie