What are the major factors that contribute to our metabolism or energy expenditure?
Since all bodily operations and activities burn calories we can categorize them to determine the number of calories we expend daily. Classically researchers defined for principal factors that contributed to our total calories burned daily which are:
- Basal Metabolism – Calories burned by basic bodily operations and measured in a laboratory setting while laying down after a good nights sleep and fasted for at least 12 hours.
- Physical Activity – Calories burned performing all physical movement.
- Thermal Effect of Food – Calories burned to digest food and process nutrients internally.
- Active Thermogenesis – Changes in calories burn due to changes in environmental temperature.
| Total Energy Expenditure (Calories Burned) | = | Basal Metabolic Rate | + | Physical Activity | + | Thermal Effect of Food | + | Active Thermogenesis |
A simpler and more common way to estimate the total number of calories we burn daily is to use Resting Metabolic Rate (RMR), which includes the thermal effect of food and to not address adaptive thermogenesis since for most people it really isn’t a factor. By doing so you focus on the number of calories your body burned in a resting (not moving) state and the number of additional calories you burn when you are moving (physical activity).
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TEE = RMR x Physical Activity Factor (Daily Activities + Exercise) |
What is “resting” metabolic rate?
Resting metabolic rate is the number of calories your body burns while not moving (rest) to function normal and to keep you alive and well. This includes the beating of the heart, breathing, making urine, thinking, and making new molecules and cells. For instance, every second our body generates two million new red blood cells. RMR tends to account for 50 to 75% of total calories burned daily. That means that physical activity contributes between 25 to 50% depending on how active someone is throughout the day and the amount and type of exercise they do.
How do we estimate RMR?
RMR can be estimated using equations. One of the most common ways to assess RMR is the Mifflin-St-Jeor equation. The Mifflin St Jeor equation for RMR is:
- For men: (10 x Wt) + (6.25 x Ht) – (5 x Age) + 5
- For women: (10 x Wt) + (6.25 x Ht) – (5 x Age) – 161
Note: Wt = weight in kilograms (1 lb = 0.454 kg) and Ht = height in centimeters (1 in = 2.54 cm)
Example RMR for a 35 year old man who weighs 180 lbs (82 kg) and is 5 ft and 11 inches tall (180 cm) using Mifflin- St. Jeor equation:
- RMR = (10 x 82) + (6.25 x 180) – 5 x 35) + 5
- RMR = 1775 calories
How much does different tissue contribute to RMR?
Looking specifically at basal metabolism occurring within different tissues in the body we find that the most metabolically active tissue (calories expended/gram tissue) are the vital organs namely the heart, kidneys, lungs, pancreas, brain, and liver. While only making up roughly 10 percent of our body weight, these organs accounts for as much as 50 to 60 percent of our RMR. Meanwhile, the retina of the eye is the most metabolically active tissue (per gram of tissue). Interestingly, the energy expenditure of the heart, lungs, kidneys, brain and liver is estimated to be 15-40 times greater than muscle and 50-100 times greater than fat tissue on a lb to lb basis.
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Skeletal muscle tends to makes up about 40 percent of an adult’s body weight and is not as metabolically active as the organs just mentioned when we are not moving. Skeletal muscle energy expenditure contributes about 25 percent to our RMR. However, keep in mind that this expenditure takes place when skeletal muscle is not working! In fact, researchers have estimated that the metabolic rate of muscle is about 4½ to 7 calories per pound (muscle) per day or about 10 to 15 calories per kilogram. On the other hand, fat tissue contributes relatively little to our RMR unless a person has a lot of body fat and then it makes a relatively greater contribution. |
How important is body composition to resting metabolic rate?
Since skeletal muscle and body fat typically make up more than half of our body weight it is easy to understand why these two tissues will have a major impact on RMR and daily metabolism. This is especially true since they are the tissues that are most easily manipulated. You can voluntarily gain or lose fat and muscle but you can grow more brain or heart. In fact, the ratio of skeletal muscle to body fat is the best predictor of a person’s RMR for a given body weight. For example, we would expect an athletic, muscular 200-pound man (91 kg) with 12 percent body fat to have a higher RMR than a different man who weighs the same but has 25 percent body fat. Simple put the more muscular man has a higher muscle to fat ratio, and thus a higher RMR. On a per-weight basis RMR is typically higher in males than in females because men tend to have a higher skeletal muscle to body fat ratio.
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