Basal metabolic rate, or BMR, is the amount of energy an organism uses when it’s at rest. Learn how to calculate the BMR of humans, as well as describe how animal BMR is calculated.
Basal metabolic rate, or BMR, is the measurement of an organism’s energy expenditure when at rest. In other words, when you’re at rest, not digesting any food and at a comfortable temperature, BMR is the amount of energy it takes for your body to maintain life. About 70% of the energy we use every day is due to our organs functioning to keep us alive and healthy. The remaining 30% is broken down into digestion of food (10%) and energy needed for activity (20%).
Knowing your BMR can be useful if you’re trying to gain or lose weight. Since BMR is a calculation of how much energy it takes to maintain life, consuming more kilocalories in a day than your BMR calls for can cause you to gain weight, while eating fewer kilocalories than your BMR requires is likely to cause you to lose weight. However, these numbers are also dependent upon how active you are. Since exercising burns calories, if you are very active, you will need more calories to sustain life than someone who is not active.
Humans vs. Other Animals
A human’s BMR is dependent upon several things, and one of the most notable is temperature. Mammals are endotherms, which means that we (since humans are mammals) maintain a constant body temperature, no matter what temperature it is outside. When we’re cold, we shiver to get warm again, and when we’re hot, we sweat to cool off. This regulation of temperature takes energy.Other organisms, known as ectotherms, allow their body temperature to fluctuate with their environment.
When it’s hot outside, these organisms are warm and active, and when it’s cold outside, these organisms are cold and sluggish. A few examples of ectotherms are reptiles, fish, and insects. An advantage of being an ectotherm is it takes less energy to exist, but their BMR also changes depending on the temperature outside.
Calculating BMR in Humans
In 1919, James Arthur Harris and Francis Gano Benedict published an equation that estimates an individual’s BMR. (We now call it the Harris-Benedict equation.) This equation was later revised in 1984 to be more accurate.
A new equation, called the Mifflin-St. Jeor equation, was introduced in 1990 and is still in use today. This equation states:P = (10.0*m/1 kg) + (6.
25*h/1 cm) – (5.00*a/1 year) + sIn the equation, P is total heat production when completely at rest, m is the weight of the individual in kilograms (kg), h is the height of the individual in centimeters (cm), and a is the age of the individual in years. s is a constant, which is +5 for males, and -161 for females.You should note that P is listed as being total heat production.
This equation gives an estimation of an individual’s BMR in kilocalories (kcal) per day. You’ve probably noticed that the side label that appears on food lists the number of calories per serving size. These calories, which you might think of as the stuff that makes up food, are actually units of energy.
One calorie is the amount of energy it takes to raise 1 gram of water by 1° Celsius. Therefore, the number on the food labels you see is actually listing kilocalories, not calories.So let’s say we have a woman who is 30 years old, 5 feet 3 inches tall (or 63 inches) and weighs 130 pounds.
As you may have noticed, we have to convert her height and weight into centimeters and kilograms. Since there are 2.5 centimeters in one inch and 2.2 pounds in one kilogram, that means that our volunteer’s height is 157.5 cm and about 59 kg.
If we plug in all our numbers, we get the following equation:P = (10*59 kg) + (6.25*157.5 cm) – (5.0*30 years) + (-161)P = 590 + 984.375 – 150 – 161P = 1263.
4 kcal/dayThis means that this woman needs to eat about 1263 kcal a day in order to maintain her current weight. Therefore, if this woman ate more than 1263 kcal in a day, she would gain weight, but if she ate less than 1263 kcal in a single day, she would lose weight. This is quite a bit different from the 2,000 calorie diet we always hear about.
How is BMR Measured in Animals?
In the 1930s, a scientist named Dr. Max Kleiber discovered an interesting relationship between animals and BMR, now described as the mouse-elephant curve. Most times, when we want to know the BMR of an animal, we use a measurement technique called respirometry.
Using a respirometer, we can measure the amount of oxygen an organism is consuming, which is directly proportional to their BMR.But Kleiber’s Law states that the BMR of an organism is approximately their mass (in kg) raised to 3/4 power, or:BMR = M^0.75This means that a 2 gram mouse should have a basal metabolic rate of about 0.
009, and a 2 kilogram elephant should have a BMR of about 1.68. Furthermore, Dr. Kleiber discovered that this formula works for both endotherms and ectotherms alike.
In 1962, scientists learned a hard lesson about measuring a BMR in organisms. A group of scientists at the University of Oklahoma were studying the effects of LSD on the brain.
Cats were injected with the drug to determine the harmful effects to mammals, which would hold true for humans which took the drug. The next step was to try the experiment on a larger mammal, which ended up being an elephant named Tusko. The scientists used Kleiber’s Law to adjust the dosage of LSD that was appropriate for Tusko’s much larger mass. Understandably, they were shocked when Tusko collapsed and died almost immediately after receiving his LSD injection.So what went wrong? Tusko’s death proved that there is more to an organism’s metabolic rate than just its mass. Pound for pound, a mouse uses a lot more oxygen than an elephant.
This is why simply scaling the dose of LSD from a cat to an animal Tusko’s size without taking into account their mass-specific metabolic rate ended with scientists overdosing Tusko by over 30 times a safe amount.This is why scientists now report the oxygen consumption of an organism as oxygen consumed per body mass. An organism’s mass-specific metabolic rate is thus more accurately described by the following equation:BMR = M^-0.25, where M is the organism’s mass
The basal metabolic rate of an organism is the measurement of how much energy is used by that organism when at rest. This illustrates how much energy the organism needs just to keep living.
In humans, BMR is currently calculated using the Mifflin-St. Jeor equation. Animals, on the other hand, are typically placed in a respirometer to measure their oxygen consumption in order to calculate their BMR.
The Hard Work of Rest
Everyone knows that our bodies require calories to burn energy. Burn enough calories and we can lose weight, burn too little and we gain.
What many do not realize is that each person has the specific amount of calories our bodies need to function, called basal metabolic rate or BMR. Knowing your BMR can play a major role in helping you to customize your own health plan.
If you can answer the title question, you’re probably also prepared to:
- Compare the BMR of humans to that of other organisms
- Calculate BMR for both people and animals
- Indicate how and why the animal BMR formula was modified