Metabolism: Bust the Myths (Part 1)

Reyna F

I often get asked “what is my metabolism?” or “how do I boost my metabolism?”  Depending upon the person, they can get the short, simplified answer or the long explanation.  You are the lucky ones who are getting the long, scientific answer.  It is so long that I split this into two parts. Part 1, which you are reading, defines metabolism and how you can determine your metabolism. Part 2 will explain what will and what will not improve metabolism.  So continue reading to learn more.

What is Metabolism?              

Metabolism is the sum total of all chemical reactions involved in maintaining the living state of the cells, and thus the organism. Metabolism is divided into two categories: catabolism or the break down of molecules to obtain energy; and anabolism or the synthesis of all compounds needed by the cells (examples are DNA, RNA, and protein synthesis).

Difference Between BMR and RMR

BMR and RMR are estimates of how many calories you would burn if you were to do nothing but rest for 24 hours. It is the minimal rate of energy consumption necessary to support all cellular functions and accounts for 50-75% of total energy expenditure in humans. They represent the minimum amount of energy required to keep your body functioning, including your heart beating, lungs breathing, and body temperature normal.

·        BMR stands for Basal Metabolic Rate, and is synonymous with Basal Energy Expenditure or BEE. BMR measurements are typically taken in a darkened room upon waking after 8 hours of sleep; 12 hours of fasting to ensure that the digestive system is inactive; and with the subject resting in a reclining position. BMR is measured by indirect calorimetry after the subject has fasted ~12 h (usually overnight), then rested motionless in a supine position 20-30 min in a thermally neutral environment. The subject is instructed to remain still for the next 30-40 min while the indirect calorimetry measurements are taken. The procedure is not only time-consuming but requires extensive subject cooperation as well as accurate and precise flow and concentration measurements, using sophisticated flow and gas analyzers.

• RMR stands for Resting Metabolic Rate, and is synonymous with Resting Energy Expenditure or REE. RMR measurements are typically taken under less restricted conditions than BMR, and do not require that the subject spend the night sleeping in the test facility prior to testing. An example of a device used to determine RMR is “BodyGem.”  

Several factors impact metabolism:

• Body size and composition. The bodies of people who are larger or have more muscle burn up more calories, even at rest.
• Gender. Men generally have less body fat and more muscle than women of the same age and weight, thereby burning more calories.
• Age. As one get older, the amount of muscle tends to decrease and fat accounts for a greater percentage of weight, slowing down metabolism.
• Heredity. Some people are born with faster metabolisms, while others naturally have slower metabolisms.
• Hormones such as thyroid and insulin.
• Medication
• Stress
• Sleep
• Thermogenesis or the Thermic Effect of Food (TEF).  Digesting, absorbing, transporting and storing of food consumed. This accounts for about 10 percent of the calories used each day. For the most part, the body's energy requirement to process food stays relatively steady and isn't easily changed.
• Calorie expenditure. Lifestyle, exercise, movement.

Calculating BMR/RMR

There are several calculations and apps for determining metabolism.  As you see below, there are quite a few.  Which is the best? Most programs and apps use the Harris-Benedict equation.  But Mufflin is a bit more accurate for average adults.  Two formulas that consider lean body mass (LBM) include the Katch-McArdle equation (BMR) and the Cunningham equation (RMR). These formulas should be used for athletic adults where muscle mass is significantly higher than average.

Online: www.caloriesperhour.com

BMR and RMR Equations

The Harris-Benedict equation for BMR:

• For men: (13.75 x w) + (5 x h) - (6.76 x a) + 66
• For women: (9.56 x w) + (1.85 x h) - (4.68 x a) + 655

The Mufflin equation for RMR:

• For men: (10 x w) + (6.25 x h) - (5 x a) + 5
• For women: (10 x w) + (6.25 x h) - (5 x a) - 161

Where:

w = weight in kg

h = height in cm

a = age

Katch-McArdle (BMR):  370 + (21.6 x LBM)

• To calculate LBM, multiple total body weight by body-fat percentage. If one has 10% body fat and weighs 80 kg, his LBM would be 72 kg (i.e., body weight of 80 kg – 10% body fat of 8 kg = 72 kg of LBM). Therefore, using the Katch-McArdle equation: 370 + (21.6 x 72) = 1,925

Note that this result is significantly higher than the non-LBM formulas (Harris-Benedict and Mifflin).

Cunningham (RMR) - 500 + (22 x LBM)

• This formula is similar to Katch-McArdle, but provides a slightly higher estimate.

500 + (22 x 72) = 2,084

Using the Calculations

As BMR and RMR only represent resting energy expenditure, an adjustment must be made to reflect activity level. This is done by multiplying BMR or RMR by an activity factor. Note that the following activity factors also take into account the thermic effect of food:

Activity Factor	Category	Definition
1.2	Sedentary	Little or no exercise and desk job
1.375	Lightly Active	Light exercise or sports 1-3 days a week
1.55	Moderately Active	Moderate exercise or sports 3-5 days a week
1.725	Very Active	Hard exercise or sports 6-7 days a week
1.9	Extremely Active	Hard daily exercise or sports and physical job

Use of these activity factors produces a very rough estimate, and there are many different opinions on what these activity factors should be.

Now that you estimated your metabolism, what can be done to change it?  Stay tuned for Part 2 of Metabolism: Bust the Myth.

-Reyna F 

Blogmaster's Note:  For more information, contact Reyna Franco (Reyna@reynafranco.com). She is a registered dietitian, certified personal trainer, certified specialist in sports dietetics and NY Flyers member.  Her office is located on the upper west side and she counsels individuals and groups on sports and medical nutrition.  Her website which is in the process of being upgraded is www.reynafranco.com.