For anyone embarking on a new fitness plan whether it is to lose fat, gain muscle, or both it is useful to understand your Basal or Resting Metabolic Rate. What is your Basal / Resting Metabolic Rate (BMR / RMR)?

Basically your BMR/RMR is how many calories your body burns doing nothing, meaning how much energy your body requires to just live by breathing and keeping your heart beating. You can do a Google search for BMR and find numerous sites that have a BMR Calculator you can use to calculate your metabolic rate. There are some slight differences between BMR and RMR testing, but for the purpose of a calculator they are essentially the same.

Some calculators will use the Harris-Benedict Equation created in 1919. Wheras the Mifflin St Jeor Equation created in 1990 has been shown to be slightly more accurate, both equations are described below (Source: WikiPedia)

The Harris-Benedict Equation:

  • for men, P = \left ( \frac {13.397 m} {1 ~ \mbox {kg}} + \frac {4.799 h} {1 ~ \mbox {cm}} - \frac {5.677 a} {1 ~ \mbox {year}} + 88.362 \right ) \frac {\mbox {kcal}} {\mbox {day}}
  • for women, P = \left ( \frac {9.247 m} {1 ~ \mbox {kg}} + \frac {3.098 h} {1 ~ \mbox {cm}} - \frac {4.330 a} {1 ~ \mbox {year}} + 447.593 \right ) \frac {\mbox {kcal}} {\mbox {day}}

Here is a site that uses the Harris-Benedict Equation

 

The Mifflin St Jeor Equation:

  • P = \left ( \frac {10.0 m} {1 ~ \mbox {kg}} + \frac {6.25 h} {1 ~ \mbox {cm}} - \frac {5.0 a} {1 ~ \mbox {year}} + s \right ) \frac {\mbox {kcal}} {\mbox {day}}, where s is +5 for males and −161 for females.

Here is a site that uses the Mifflin St Jeor Equation

Both equations will use your height, weight, and age to predict your BMR. Both equations will give you different numbers, for example my BMR per Harris-Benedict is 1615 cals per day and my BMR per Mifflin St Jeor is 1563. Not too much difference so you can’t go wrong with either equation, however, most people don’t know that these equations can be up to 25% inaccurate depending on the person. Fortunately there is a more accurate test.

Indirect Calorimetry

Our bodies consume a fixed amount of oxygen for every calorie we burn. By measuring the amount of oxygen you consume and comparing it to the carbon dioxide you produce can more accurately determine your metabolic rate. This test is typically done in a doctors office, performance facility, or university setting due to the cost of the machine used to measure O2 and CO2. I mentioned earlier that there are some differences between BMR and RMR, those differences are how each indirect calorimetry test is performed. BMR testing is typically more stringent with longer fasting, special facilities, and a more controlled testing setting. Whereas a RMR test has a relaxed fasting time and takes less time to perform. BMR will be slightly more accurate than RMR due to the controlled testing, but the equipment is generally the same.

The RMR test is typically performed with the subject breathing through a tube connected to an indirect calorimetry machine in a resting position for 10-15 minutes. The machine measures the amount of O2 you are inhaling and CO2 you are exhaling and calculates an accurate metabolic rate.

I had my RMR tested and was surprised at the results compared to the equation approach:

Mifflin St Jeor            :  1563 Calories

Harris-Benedict         :  1615 Calories

Indirect Calorimetry :   2020 Calories

These results show that my metabolism is faster than the average population as compared to the equation method, meaning that depending on what my fitness goals are I need to alter my calorie input differently to account for a faster metabolism. It’s important to also note that these numbers do not account for activity calories or exercise calories. Activity calories are the calories spent walking, digesting, talking, thinking, etc and exercise calories are those calories spent doing some physical exercise such as running, weight lifting, etc. Activity calories could account for another 200-300 calories and exercise calories could add on another 400-1000 calories spent.

In the event I want to put on more muscle I now know that I need to eat more than approximately 2400 calories in order to ensure I have enough nutrients to pack on some muscle, whereas if I am interested in losing some fat while retaining muscle mass I now know that I can’t dip too far under 2000 calories depending on my physical exercise otherwise I might find myself into too much of  a calorie deficit forcing my body to use muscle for energy.

For people just starting out on their fitness journey leveraging the equation method to determine your metabolic rate should be just fine, but if you are well on your way and need to refine your goals I would recommend that you get an indirect calorimetry test.