We have an obsession with Calories. We fear them, yet crave them. We want to watch them, count them, track them, calculate them, avoid them, and believe we can somehow burn them!?
What is a Calorie?
Energy is needed to make the body work, but we cannot directly measure this 'energy' in the body. However, we can attempt to measure the heat that is released when energy is used. Calories help us with this conundrum.
A Calorie is a measure of heat energy.
More specifically, a Calorie is the approximate amount of heat (or energy) needed to raise the temperature of 1 kilogram of water by 1 degree (º) Celsius. A Calorie is actually a kilocalorie (kcal), or 1,000 calories (little 'c'). They are heat units within our metric system, and are called 'kilojoules' in the International System of Units.
Think about it this way.
If you had one kilogram (2.2 pounds) of water in container, the amount of heat energy that would be required to raise the temperature of that water from 14.5º to 15.5º Celsius would equal 1 Calorie. We could get this heat energy from anywhere, and not just from food. To know how much energy is stored in a substance, we usually can ignite it, and see how much heat it gives off. We can then see if this heat is enough to raise our 2.2. pounds of water 1º Celsius (see 'Energy In' section below).
So, Do We Burn Calories?
As you can clearly see now, we do NOT 'burn' calories — which is like saying we burn other units of heat or energy, such as Celsiuses, Fahrenheits, or kilowatts. I guess I can see where the idea comes from, though, as Calories deal with heat energy stored or released. But, we do not burn them.
If you are still intrigued with Calories, and how they are measured in our food and from physical activity, I have provided some more in-depth, yet basic information to clarify some common misperceptions.
Measuring Calories in Food (Energy In)
To measure Calories in food, we take a sample and place it in a 'bomb calorimeter', like the one to the right. The food sample is ignited, and blown up in a bomb-like fashion. The heat (energy) from the explosion warms up the water around it. The change in temperature is measured by the thermometer.
So, if we had 2.2 pounds of water in our container here, and it was heated up 10º Celsius by our exploding food sample, then we would say the food sample (or non-food sample) contained 10 Calories.
NOTE: Our stomachs are not bomb calorimeters. Of course, I have eaten at some restaurants that make me think it is one. However, when we chemically breakdown food (carbohydrate, fat, protein), heat is released in a more controlled, non-bomb-like fashion. This bomb calorimeter just allows us to put a number on the amount of heat energy stored in the selected food sample.
Empty Calories
Now that you know how Calories are measured in food – 'empty calories' describe foods that have a high amount of heat producing energy stored in them (i.e. Calories), but little to no nutritional value. This type of food is what most Americans eat in the Western diet, such as highly processed foods, and contributes to many of our current health problems. The opposite of an empty calorie food would be a 'nutrient dense' food, which has very few Calories, but packed with nutritional value. These are the foods that we should be consuming the majority of our Calories from.
For example: You would have to eat 21.5 cups of baby spinach to equal 1 Twinkie cake! Calories are the same (approximately 150 Calories), but are drastically different in their nutritional value.
Measuring Calories During Exercise (Energy Out)
Do not worry, we do not have to use a bomb-calorimeter to measure Calories during exercise, but we can simply measure the heat energy given off during exercise (then convert to Calories). This process of heat production in humans is called 'Thermogenesis'.
Body Heat
We are always producing heat from the chemical reactions in the body. About 60% of the heat energy we give off every day happens at rest, which is called 'resting metabolism' or 'resting energy expenditure'. The more active we are, and/or the harder we work, the more heat energy we give off.
Direct Calorimetry
The ideal way to measure the body's heat energy is in a 'Direct Calorimeter'. As you can see in the image, we have someone exercise in an air-tight box. We see how much the heat energy produced from their bodies heat up the water in the water filled copper coils above their head, abd then calculate the number of Calories.
Indirect Calorimetry
Direct calorimetry is expensive, and there are not many places that have one. So, we use 'Indirect Calorimetry', which is an estimation of energy expenditure using assumptions and calculations of how much carbon dioxide we produce, and how much oxygen we consume during an activity. Although highly sensitive to measurement error, these calculations can give us an estimation of how much heat (energy) is coming from carbohydrate and/or fat.
Of course, we can use a lot of energy that does not require oxygen (i.e. anaerobic) to produce energy, such as weight training, running sprints, plyometrics, and so forth. Thus, the energy expenditure from such activities cannot be estimated with this method.
Other Methods
There are other, more easily accessible methods out there. Many of these 'fitness trackers' are accelermometers that we wear on our arm, wrist or waist. They try to predict how much heat energy we give off during physical activities (in Calories). Some examples are the Nike+ FuelBand, Fitbit, BodyMedia armband, and JawBone. Research is still determining the accuracy of these fitness trackers, comparing them to 'indirect calorimetry', which as shown, is based on several assumptions and limitations.
For example, in our own lab we found that the Nike+ FuelBand was comparable to indirect calorimetry estimates of Calories expended during aerobic activity (treadmill) at low to moderate intensities (2-5mph), but overestimated at higher intensities (8 mph). Also, our current research is showing that the BodyMedia armband overestimates energy expenditure during resistance exercise for both men and women, with the greatest overestimations in women. In short, we must be careful to assume that these monitors are accurate. Rather, we should base our decisions on what the research says.
If you are interested in learning more about these fitness trackers, and what the research says about them, let me know.