What is metabolic rate?

The chemical reactions that take place in our cells release energy, and this energy is ultimately derived from the breakdown of ­energy nutrients namely carbohydrate, protein, fat and alcohol. Over the course of the day almost all of the energy released will be converted to heat and lost from the body. Metabolism refers to the sum of the energy (calories) generated in our body and lost as heat. To go a little further, metabolic rate is the amount of heat we produce within a specified period of time, such as over an hour or a day.

 

If energy expenditure is measured over an hour’s time, it only estimates the expenditure during that hour and cannot be confidently extrapolated to longer periods of time. For instance, if energy expenditure is measured for one hour after lunch or during a morning exercise session, surely it would be greater than when you are sleeping. On the contrary, if energy expenditure is expressed over a day’s time, it will not indicate periods within the day when the metabolic rate was higher, such as in more active times of the day, or lower, as in less active times of the day or when sleeping.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How do we measure metabolic rate?

Our body works very hard to maintain its temperature at around 37°C (98.6°F). This means that excess heat generated by chemical reactions in cells must be dissipated. Because this dissipated heat is a direct indicator of our metabolism, we can use an insulated chamber sensitive to temperature change to determine how much heat we produce (energy expenditure). This method of estimating metabolic rate is often referred to as direct calorimetry. Calorimetry literally means “heat measurement.” However, since the operational expense for this scientific tool is overwhelming, facilities designed to perform direct calorimetry may be found at only a handful of universities and research institutions.

 

One alternative method can be employed to assess metabolic rate called indirect calorimetry. Because ATP is generated from the combustion of energy molecules which requires O2 and produces CO2 it is possible to estimate energy expenditure based upon these gages (see Photo 8.1). Representative chemical reactions for the combustion of carbohydrates, protein, and fat are shown below. You see that O2 is used as a reactant for each reaction while CO2 is a product. Utilizing mathematic equations we can estimate the amount of heat produced in a given period of time based upon the amount of O2 inhaled or the amount of CO2 expired. As it turns out, indirect calorimetry is not only a very accurate indicator of metabolism, but it also gives us an idea of the mixture of energy substances our body is using during that time.

 

Carbohydrate

C6H12O6 + 6 O2 -à 6 CO2 + 6 H2O

 

Triglyceride (fat)

2 C57H110O6 + 163 O2 -à 114 CO2 + 110 H2O

 

Protein

C72H112N2O22S + 77 O2 -à 63 CO2 + 38 H2O + SO3 + 9 CO(NH2)2

 

Based on the amount of O2 used during a period of time researchers can estimate the amount of energy used or more commonly calories burned. For instance, we can use 4.8 calories burned per liter of O2 used to estimate calorie needs. If a man uses 20 liters of O2 an hour (360 liters/day) this would translate to around 96 calories/hour or 2300 calories daily.

 

How can we know what our body is using for energy?

Based on the chemical reactions shown above, we can calculate what researchers call the respiratory exchange ratio (RER) (or respiratory quotient (RQ)) for a given time period. RER is equal to the amount of CO2 exhaled divided by the amount of O2 inhaled.

 

RER  =  CO2 / O2

 

• RER of glucose 6 CO2 /6 O2 = 1.0

• RER for the triglyceride 114 CO2 / 163 O2 = 0.70

• RER for the protein 63 CO2 / 77 O2 = 0.82

   

If we measure a person’s gases during a period of time we can calculate a few things. For example, say that during one hour a person consumed 15 L O2 and expired 12 L of CO2; we can first calculate their RQ for that hour:

RQ = 12/15 = 0.80

 

We can find the RQ 0.80 on RQ Table and follow it over to the calorie source columns. At an RQ of 0.80 this individual would be using approximately 33 percent carbohydrates and 66 percent fat to fuel his or her metabolism. We will assume that the contribution from amino acids toward energy production during that time is minimal. This is a fair assumption for a healthy person not engaged in prolonged fasting or endurance exercise during this time. Furthermore, we can estimate metabolic rate by multiplying the amount of O2 consumed (15 L) by the Caloric Value for 1 L O2 for an RQ = 0.80. Their metabolic rate would be:

 

15 × 4.801 = 72 Cal/hour

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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 laying down after a good night 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.

   

 

 

 

 

 

 

 

 

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). 

 

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 where 1 pound = 0.454 kilograms

              Ht = height = centimeters where 1 inch = 2.54 centimeters

 

 

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 various 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. Interestingly, 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.

 

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 how these two tissues will have a major impact on RMR. This is especially true since they are the 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. Simply 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.

 

 

 

How does age impact RMR?

RMR is highest during infancy when considered as calories per lb (or kg) of body weight. At this stage resting metabolism not only reflects normal life-sustaining operations of the infant but also must power the building of new tissue. The same can be said for growth spurts in children and teens. Conversely, as we age, our basal metabolism seems to slow down. Some researchers have estimated the slowdown to be on the order of 2-3% in each decade. This downward progression of RMR in later life can be attributed to the loss of fat-free mass due to physical inactivity. However, while researchers agree that some of this is related to declining hormones, much of it is related to changing body composition. As we age we become less active and thus lose muscle mass and gain fat mass. In fact, when older individuals are placed on an exercise program that includes resistance exercise for muscle development they tend to gain muscle and increase their RMR.

 

 

Can we determine RMR based on muscle mass?

The equation above is appropriate for inactive adults. However, for leaner, muscle muscular people such as athletes and fitness enthusiasts estimating RMR based on body composition is more appropriate. The equation below is the Cunningham equation and uses fat free mass (FFM) to estimate RMR.

 

RMR   =  500 +  22 (fat-free mass)

 

Estimating FFM is simple once %BF has been determined (below). Begin by calculating Fat Mass (FM) which is body weight time %BF. Then subtract FM from body weight to determine FFM. Assuming our example man (82 kg, 180 cm) from above is also an athlete with 15% body fat mass (FM) let’s use the Cunningham equation to estimate his RMR.

 

Step 1. Determine %FFM:       100%   -   15%   =    85% FFM

 

Step 2. Determine FFM:          82kg   x   .85      =    70 kg FFM

 

Step 3. Determine RMR          500   +   22 (70)  =    2040 calories

 

You see that the estimate of RMR for our example man is as an athlete (using the Cunningham equation) is much higher than his estimated RMR is he was not an athlete (using the Mifflin-St. Jeor equation). The difference is largely skeletal muscle mass and condition.

 

What is physical activity and how do we estimate it?

Physical activity is the energy used by skeletal muscle activity. Simply stated, the more we contract our skeletal muscle the more calories will be used to power this activity. Physical activity includes everything from every day basic tasks such as showering, loading the dishwasher and driving to work to exercise running, swimming and dancing.

 

You can use the physical activity factors (PAF) presented below to generally estimate of total calories burned daily. Let’s apply these factors to estimate total calories burned daily for our example man as either an inactive person and as an athlete training most days of the week.

 

 TEE    =    RMR  x  PAF (Daily Activities + Exercise)

 

Inactive (PAF = 1.2):    1775 calories  x  1.2                 =     2130 calories

                       

Athlete (PAF 1.725):    2040 calories   x  1.725  =     3520 calories