How much energy does your body use each day? Your body uses energy to perform the vital bodily functions that keep you alive and for any physical activities you engage in. Learn about the methods available to determine energy use by the body.
Your body is a lot like your car. When your car needs energy, you feed it gas; when you need energy, you feed your body food. The energy-yielding foods - namely fats, carbohydrates and proteins - are then used by your body to make it run, just like the gas in your car is used to make it run.
It's easy to keep track of how much gas you pump into your car and how many calories you feed your body, but when it comes to calculating how much energy your car or your body uses, things get a bit more complicated. For example, your car will use more gas, or energy, when you are driving down the road compared to when you are idling in front of a stop sign. Likewise, some cars guzzle up more gas than others simply because they have bigger engines or less efficient systems.
Just like your car, your body's energy use is influenced by a number of factors, such as your body size, gender, age and activity level. In this lesson, we will consider these factors as we look at methods used to determine the energy use by the body.
Your body uses a certain amount of energy to carry out the vital functions essential to your survival. This would include things like breathing, circulating your blood and the countless cellular processes that go on all day long. We use the term basal metabolic rate (BMR) to describe the rate at which energy is used for your vital functions and is measured in calories.
So, if we want to determine the amount of energy used by your body, we must first figure out what your BMR is. But, there's a catch. You see, BMR varies from person to person based on the factors we mentioned earlier, like your height and weight, if you are male or female, and your age.
So we need a way to calculate your specific rate. One method is to use the Harris-Benedict equation, which is a formula that estimates your BMR and then applies an activity factor to determine your energy use.
Adult male: BMR = 66 + (6.23 x body weight in pounds) + (12.7 x height in inches) - (6.8 x age in years)
Adult female: BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)
When we look at this equation we notice that there is a separate calculation for men and women. This is because men tend to have a higher BMR than women because their bodies contain more lean muscle, and muscle requires more energy to maintain than other tissues, such as fat.
Let's plug in some numbers for a somewhat typical male and see what we get. Let's say our male subject weighs 200 pounds, and let's make him 70 inches tall and 30 years old. When we plug his statistics into our equation we see that the estimated BMR of our subject is 1,997. So we now know that our subject uses this many calories to run his vital body functions.
With this information we are ready for the second part of the Harris-Benedict equation, which asks us to take your estimated BMR value and multiply it by an activity factor. To do this you must determine how physically active you are during the day. You then match your activity level to a chart that assigns a numerical physical activity value for different activity levels. For example, if you participate in little or no exercise during the day, you would be considered sedentary and would multiply your BMR times 1.2. The resulting number is the energy your body uses during the day, and also the amount of calories you could consume to maintain your current weight.
To illustrate how this works, let's go back to our male subject. We estimated his BMR to be 1,997. If he leads a sedentary life, then we take this number times 1.2 and end up with about 2,396, which is the energy he uses in a day as well as the number of calories he would want to consume to maintain his weight. So what if he boosts his activity level and becomes very active? Well, then we would take his BMR times 1.725 and we find that he would need about 3,445 calories per day to match his energy use per day.
There are other methods for determining energy use by the body, but they are admittedly more complex. The first we will consider is called direct calorimetry, which is a process of measuring heat production to determine energy used by the body. This procedure requires an air-tight chamber surrounded by water. Heat that emanates from the body of a person exercising inside the chamber increases the temperature of the surrounding water, which can then be measured. I am sure you are seeing some downfalls to this method, not the least of which is the fact that the subject must climb inside a scary machine for the test to work. So, let's take a look at another method.
I think most people would be happier having their energy use checked by simply lying down and breathing into a special gas mask. This is referred to as indirect calorimetry, which is a process of calculating heat production to determine energy used by the body by first measuring oxygen consumption and carbon dioxide production. It might help you to recall these terms if you keep in mind that the word 'calor' is Latin for 'heat.' With direct calorimetry we directly measured the heat given off, and with indirect, we indirectly measure the heat production by first measuring the gases. From these gas measurements a lab tech can determine the percentage of carbohydrates and fatty acids being burned and then estimate the energy being used.
Your basal metabolic rate (BMR) is the rate at which energy is used for your vital functions.
To determine the amount of energy used by your body, you must first figure out what your BMR is. BMR varies based on height, weight, gender and age. So we need to do some calculations with the help of the Harris-Benedict equation, which is a formula that estimates your BMR and then applies an activity factor to determine your energy use.
There are other, more complex methods for determining energy use by the body. For example, direct calorimetry is a process of measuring heat production to determine energy used by the body that involves being inside an air-tight chamber. We can also use indirect calorimetry, which is a process of calculating heat production to determine energy used by the body by first measuring oxygen consumption and carbon dioxide production.
After you've completed this lesson, you should be able to:
- Explain how to use basal metabolic rate and the Harris-Benedict equation to determine daily caloric needs
- Describe the purpose and difference in direct calorimetry and indirect calorimetry