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