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  • Writer's pictureTimothy P. Smith MS, RD, LDN

What is "Metabolic Testing"?

It was 2015; I was sitting in Nutritional Biochemistry, discussing future practices in the field of nutrition. We debated prospective ideas such as "designing nutrition plans based on a person's genome", or "using a blood sample to determine a person's specific food sensitivities".

However, the future practice that caught my attention, most, was something called "indirect calorimetry". This is a method in which a machine can precisely measure how fast or slow a person's metabolic rate ("metabolism") is. I thought "No way. Is that even possible for a private practice?"

However, the future practice that caught my attention, most, was something called 'indirect calorimetry'.

At this stage of my career, I had spent time in various hospitals & professional environments; traditionally, caloric (energy) needs have always been estimated using predictive formulae such as Harris-Benedict, Penn State, or Mifflin St Jeor.

In my opinion though, these formulae were overly simplified, and left out a number of important considerations.

So, while this technique was ubiquitous amongst healthcare professionals, the ambiguity of these equations bothered me. "Sure, we can factor for age, gender, height, weight, and activity level, but what about body fat percentage? Or sleep schedule? Or occupation-related activity/activities of daily living?"


Since starting Noble Nutrition, only 7 years after our class discussion, I've actually been able to acquire a Korr ReeVue Metabolic Analyzer, and I have to say it has been an amazing supplement to my existing nutrition programs.

Before we get into the benefits of metabolic testing, let's cover what metabolic testing entails: First, the person must fast for at least 4-5 hours before testing; no food, caffeine, or exercise, as we know these can measurably increase metabolic rate. Therefore, this fasting period ensures we get a true measure of your "Resting Metabolic Rate" or "RMR" (RMR is defined as "the number of calories your body expends in a 24 hour period without doing anything, other than normal physiological processes.")

Once the individual is fasted, we can get together (in-office) and start by calibrating the Korr, then hooking up the breathing filter/Metabreather hose.

There is also an included nose plug which must be placed before starting the analysis.


So at this point:

Fasted? Check.

Machine calibrated? Check.

Nose plugged? Check.

All systems are go; at this point, we can start breathing into the Metabreather hose until the test concludes (which usually takes approximately 10 minutes.)


While we wait for the test to finish, let's quickly discuss how metabolic testing actually works.

"Metabolism" describes all of the chemical reactions that occur in our bodies to keep us alive. There are billions of these reactions happening in your body every second of every day, and they all transfer energy in some manner. As such, we need to consume a certain amount of energy (which we measure using "calories") to ensure metabolism can continue to occur normally & that you can stay alive.

Similar to how a wood fire consumes oxygen and leaves ash behind, most processes of metabolism involve the consumption of oxygen, and the production of carbon dioxide as a waste/byproduct. So, if we can measure the amount of oxygen a person consumes, as well as how much carbon dioxide they produce, we can determine how "fast" or "slow" his/her metabolism is operating.

This is how metabolic testing works. As you breathe into the device, we can precisely measure how much oxygen you consume at rest, which gives us an indirect measure of how rapidly these processes of metabolism are taking place.


Now that we're done breathing into the Korr, we can begin factoring for variables such as height, weight, occupational activity, workout schedule, activities of daily living, and even sleep schedule & number of steps per day.

Once we include all of these components, we end up with your actual "Total Daily Energy Expenditure" ("TDEE"). This is defined as "the total number of calories a person burns each day, with all activity included." This is what we will use to develop your program, and figure out how many calories you actually require each day, based on your individual lifestyle.


The benefit of indirect calorimetry metabolic testing, compared to the predictive formulae we used to rely on, is that we can precisely measure how fast or slow an individual's metabolism is at rest, instead of estimating based on a few physical features. We can also include a number of crucial lifestyle components, and adjust the intensity/duration of each. Eliminating these variables drastically increases the precision with which we can make nutrition-related recommendations, and can allow you to hit your goals faster.

If you are interested in having us test your metabolism, please feel free to get in touch with us directly using the contact information on the Noble Nutrition website. Text, call, or email; whichever you prefer.

As always, thank you for reading,

Tim Smith MS, RD, LDN

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