Metabolic Rate

Metabolism

Metabolism is a catch-all term that refers to the totality of the biochemical reactions that provide energy and molecules to an organism. These reactions also require a sustained energy input by acquiring and synthesizing energy.

Some examples of metabolic functions are respiration, photosynthesis, thermoregulation, cell growth, nutrient transport, nerve function, muscle function or defense mechanisms.

Thermoregulation

A particularly important component of metabolism is thermoregulation, or how an organism keeps its body temperature constant.

Animals are further divided into two categories, depending on their ability to thermoregulate independently or not: endotherms and ectotherms.

Metabolic Rate Definition

Since the metabolic needs of organisms are constantly fluctuating throughout the day and seasons, measuring metabolism can feel clumsy and inadequate. It is even more difficult to compare the metabolism of two different organisms.

For example, the metabolism of a beaver building a den is different from a beaver resting, which is much different than the metabolism of a deciduous tree in the winter.

Instead, it is more adequate to speak of the amount of energy expenditure occurring over a period of time to perform metabolic tasks, or metabolic rate. The definition of metabolic rate is shown below.

Basal Metabolic Rate (BMR) Definition

An elevated metabolic rate is characterized by high activity and increased energy expenditure. The more active an organism is, the quicker its respiration, the quicker its heart pumps, the faster its muscles twitch.

Metabolic rate doesn't only vary depending on the daily activity. Metabolic rates vary across species.

• Slow-moving organisms, such as pandas and sloths, also have less energy expenditure and a slower metabolic rate compared to predators such as lions and wolves.

Occasionally, an organism needs to rest. I am sure you do too! Yet, even at rest, your body must perform basic metabolic tasks to keep you alive. In biology, this is called the basal metabolic rate, or BMR. The definition of BMR is as follows:

In ectotherms and plants, this is called the standard metabolic rate, or SMR. A key reason for basal metabolic rate (BMR) or standard metabolic rate (SMR) is to allow the scientist to compare metabolic rates across species.

Organisms can have shifting metabolic rates depending on the activities performed that day, but metabolic rates can also shift through the year. Endotherms can enter a state of torpor.

In the winter, endotherms enter hibernation, a prolonged state of torpor to escape the cold rigors of winter. Alternatively, some endotherms may enter estivation, which is a prolonged state of torpor to persist through torrid or arid summers.

Adjacent to torpor and hibernation, dormancy is a state of low-energy expenditure and low metabolic rate in plants during winter.

Resting Metabolic Rate (RMR)

As an alternative to the basal metabolic rate, a second way to quantify metabolic rate is used.

At rest is different in RMR compared to Basal Metabolic Rate (BMR). In RMR, a fully resting organism is also considered to be sleeping and fasting.

Kleiber’s Law

In 1932, Max Kleiber observed and researched the relationship between metabolic rate and weight (Fig. 1).

Logically speaking, this makes sense. If an organism has more moving parts than another, then its body must perform more metabolic maintenance and energy expenditure because it has more parts!

Max Kleiber was able to quantify an equation correlating metabolic rate and weight, known as Kleiber’s Law, by plotting the two variables on a graph.

Figure 1 - An original graph from Max Kleiber's research showing a linear graphical relationship between body weight and heat production (i.e., metabolic rate).

Kleiber discovered that metabolic rate is proportional to weight by a power of ¾.

Metabolic Rates in Ecology

In ecology, metabolic rates are used to compare food chains and food webs.

Observing metabolic rates can help quantify the dynamics between populations in an ecosystem. More precisely, the energy expenditures are fluxes and the organisms themselves are energy reservoirs.

An energy value can thus be assigned to populations, communities, or ecosystems. To do so, ecologists use an energy pyramid (Fig. 2).

Looking closer at the pyramid, we can also observe that with every increase in trophic level, there is a reduction in the size of the energy reservoir. Generally, as energy expenditure moves energy up the food chain, energy is lost by a ratio of 10 to 1. Also note, the weight of the organisms at the top of the pyramids tends to be higher than the weight of the organisms below it.

Basal metabolic rate (BMR) formulas

As humans, knowing our basal metabolic rates is important to ensure we have a diet that is balanced with our energy expenditure.If we can quantify our metabolic rate, and the amount of energy in our food, then we can ensure our energy inputs match our energy outputs.

To count the number of joules or calories our bodies need to function (i.e., BMR), we use the variables of weight, height, age, and sex. The basal metabolic rate (BMR), the energy expenditure of an organism at rest, is used to quantify metabolic rate because metabolism is the most consistent at rest.

There are two different variations of the general formula for metabolic rate: the revised Harris-Benedict BMR formula and the Mifflin-St-Jeor BMR formula.

Revised Harris-Benedict BMR Formula:

BMR_Male = 13.4·Weight (kg) + 4.8·Height (cm) - 5.68·Age (yrs) + 88 kcal/day

BMR_Female = 9.25·Weight (kg) + 3.1·Height (cm) - 4.33·Age (yrs) + 447.6 kcal/day

Mifflin-St-Jeor BMR Formula:

BMR = 10·Weight (kg) + 6.25·Height (cm) - 5· Age (yrs) + Sex-Based Constant

Where,

• The Sex-Based Constant is 5 kcal/day for males and -161 kcal/day for females.

Alternatively, there is a formula to calculate basal metabolic rate (BMR) which factors in body fat percentage. If you compare two individuals with the same weight, height, and age, the previous two BMR equations will yield the same results.

Yet if one individual is a football player and the other is a banker, we can correctly conjure their BMR should really be different. Here is an alternative way to calculate basal metabolic rate (BMR), called the Katch-McArdle BMR formula. It uses body fat percentage and weight as the variables. Notice that sex is not a variable.

Katch-McArdle BMR Formula:

BMR = 370 + 21.6·Weight (kg) · (1 - Body Fat %)

Average Basal Metabolic Rate (BMR)

The range of basal metabolic rate (BMR) values for adult human males is 1600 to 2000 kcal/day, and for human females is 1400 to 1800 kcal/day. Equipped with this knowledge, try calculating your own basal metabolic rate! What value did you get?

Knowing your basal metabolic rate (BMR), you can now look at a food calorie chart or the food nutrition table printed on the packaging of the food you purchase.

Basal Metabolic Rate (BMR) Exercises

To better understand the range of values obtained from the basal metabolic rate (BMR) equations, here are a few exercises. You will use the Revised Harris-Benedict BMR formula and the Mifflin-St-Jeor BMR formula, and compare BMR from different individuals.

1. Calculate your own Basal Metabolic Rate (BMR), using both BMR equations. Are the BMR values similar?
2. Calculate the Basal Metabolic Rate (BMR) of a friend whose weight is 20 kg less than you. Use both BMR equations. How is your friend's BMR different from your BMR?
3. Calculate the Basal Metabolic Rate (BMR) of a friend whose weight is 20 kg more than you. Use both BMR equations. How is your friend's BMR different from your BMR?
4. Calculate the Basal Metabolic Rate (BMR) of a friend whose height is 30 cm less than you. Use both BMR equations. How is your friend's BMR different from your BMR?
5. Calculate the Basal Metabolic Rate (BMR) of a friend whose height is 30 cm more than you. Use both BMR equations. How is your friend's BMR different from your BMR?
6. Calculate the Basal Metabolic Rate (BMR) of a friend whose age is 10 years less than you. Use both BMR equations. How is your friend's BMR different from your BMR?
7. Calculate the Basal Metabolic Rate (BMR) of a friend whose age is 10 years more than you. Use both BMR equations. How is your friend's BMR different from your BMR?
8. Calculate the Basal Metabolic Rate (BMR) of a friend whose sex is different from yours. Use both BMR equations. How is your friend's BMR different from your BMR?
9. Imagine two individuals of the same sex, whose weight is the same at 80 kg, both 172 cm tall, and both 33 years old. One is an Olympic distance runner, and the other is a computer programmer. Both Basal Metabolic Rate (BMR) equations will yield the same result. Do you think this is accurate? How should the two individuals' basal metabolic rates compare?
10. Calculate the Basal Metabolic Rate (BMR) of your parents, or other family member or close friend. Use both BMR equations. How does their BMR compare to your BMR? Should both of you have a similar diet? In reality, do both of you have the same energy expenditure?
11. Use your favorite search engine to look up the weight, height, and age of your favorite celebrities or athletes. Calculate their Basal Metabolic Rate (BMR). Do you think the option you chose has a higher than average or lower than average energy expenditure?

Answers

1. BMR values will vary, but the resulting numbers should be comparable.
2. Friend's BMR < your BMR.
3. Friend's BMR > your BMR.
4. Friend's BMR < your BMR.
5. Friend's BMR > your BMR.
6. Friend's BMR > your BMR.
7. Friend's BMR < your BMR.
8. Answer's will vary. A female friend's BMR will be lower than a male friend's BMR.
9. No, an Olympic distance runner should have a higher BMR.
10. Answers will vary depending on the individuals' BMR.
11. Answers will vary depending on the individuals' BMR.