How to find your resting metabolic rate—and why it matters.

Your body burns calories all the time, even during the most relaxing savasana at the end of yoga or while binging your latest Netflix obsession. Understanding the rate of calories burned at rest, or your resting metabolic rate, can help you track your overall wellness and make choices that support your health goals. Improving your resting metabolic rate (RMR) means your body burns calories more efficiently, a sign of heightened physical fitness. 

For most people, RMR represents about 80% of the total energy we burn, along with about 10% on digestion and 10% on physical activity. If you tend to manage a few workout classes a week, chances are that you would fall into this 80/10/10 split of calorie expenditure. For competitive athletes, such as NFL linesman Devon Kennard, it's more likely to be a 60/10/30 split, with up to 30% of daily energy spent on exercise.

You can find your RMR in a few different ways, but accuracy varies depending on the type of test. We will review RMR calculation methods so that you can start using this important metric to guide your nutrition and exercise plans, even if you haven't trained since your days on the high school soccer team. We'll also explore the fitness journey of NFL player Devon Kennard, a Fitnescity client who uses body composition and RMR to unlock strength training as the secret to reaching his performance goals.

Method 1: Mifflin-St Jeor Formula 

Scrunchies and spandex pants aren't the only noteworthy inventions to come from the 1990s. The Mifflin-St. Jeor Formula for estimating RMR was published in the American Journal of Clinical Nutrition at the start of the decade. It uses the following equations to estimate a person’s RMR:

• Men: (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5

• Women: (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161

Although Mifflin-St. Jeor improved on the 1919 Harris and Benedict RMR formula, it still reports a margin of error of 10%, as per the Journal of the American Dietary Associations. While this may seem miniscule, a 10% error in calculating calories could lead to a decent amount of over or under eating over time. 

This calculation also has issues beyond its significant margin of error. It doesn’t account for the many factors that can affect your individual RMR, including but not limited to:

• Ethnic background

• Environmental factors such as smoking, climate, and air quality

• Lean body mass

Building muscle increases RMR as the percentage of body fat goes down. Reviewing your RMR periodically can illustrate this process in real-time as you tighten and tone your muscles. 

In the case of Devon –a 30-year-old male, who is 6'3" (190 cm) tall and weighs 261 lbs (118.5 kg)–  if we plug in his information to the Mifflin-St. Jeor equation, we would estimate his RMR as follows: 

Devon’s estimated RMR = (10 x 118.5) + (6.25 x 190) - (5 x 30) + 5

Devon’s estimated RMR = 2227.5 calories

With this number of 2227 calories in mind, let’s look into the other Methods for determining a person’s resting metabolic rate.

Method 2: Machine-Based Estimates

While Method 1 can calculate an estimated RMR, medical-grade body composition test such as DEXA or BOD POD can predict your RMR using the information gathered during your body composition assessment. These screenings provide a detailed measurement of your body fat and lean muscle mass, which can provide an RMR precition. You'll get a wealth of valuable information to build a smart nutrition and fitness plan for a healthy lifestyle.

With BOD POD, the less invasive option, you'll be weighed several times in a special chamber. This test relies on volume displacement to determine the percentages of muscle and fat, while DEXA uses X-rays for this purpose. Although DEXA is slightly more invasive, it provides more detailed information about body fat and non-fat mass distribution. 

While body composition assessments such as the DEXA and BOD POD use more data points to predict RMR than the Miffler St. Joeur equation, these machines still cannot account account for factors like health history and environmental exposures. 

When we take a look at Devon Kennard’s DEXA scan results, we can see that the machine gave a prediction for his RMR as well. The DEXA machine used not only the Miffin-St. Jeor equation, but also factored in his Fat Free Mass. With this information, the DEXA predicted Devon’s RMR to be 2,402 calories. That is 175 calories more than we had calculated using Method 1. 

Next, we will look into a method that is dedicated to calculating a person’s RMR, and uses designated machines to do so. 

Method 3: Metabolic Cart Reading

Just as machines such as DEXA and BOD POD provide reliable information for body composition, Metabolic Cart is designated to analyze the body’s expired gas exchange. To measure a person’s RMR using a metabolic cart, a person would simply relax and breathe into a special mouthpiece or canopy hood for about 20 minutes. We can close our eyes and pretend we're on the pristine white sands of the Bahamas, surrounded by shimmering aqua waves (queue meditation music). As you let your mind wander, the device measures how much oxygen you breathe in and CO2 you expel, precisely calculating your calorie burn at rest.

When our fit friend from the Arizona Cardinals recently did his RMR at our Scottsdale location, the metabolic cart measured his actual RMR to be 2707 calories. As a reminder:

Miffin-St. Jeor estimated RMR = 2227.5 calories

DEXA scan predicted RMR = 2,402 calories

Metabolic Cart actual measurement = 2,707 calories

As you can see, Devon's estimated RMR is nearly 500 lower than his actual RMR, as measured with gas exchange. As an athlete, Devon will track his RMR over time to achieve muscle gain and maintain lean mass. His test results will enable him to break down the calories needed to burn to gain, lose, or maintain his current weight, which, in turn, informs his fitness goals. 

If you want to drop 20 pounds before your best friend’s wedding, you know your calorie burn needs to be in the weight loss zone. On the other hand, if you're training for a triathlon, stay out of the weight loss zone since you need calories to build muscle mass.

You don't have to be training for the next Avengers sequel to start a weight-lifting regimen. Studies say strength training is the most effective way to boost your RMR. For example, 2015 research published in the European Journal of Clinical Nutrition found that lifting weights three times a week for nine months raised RMR by 5%. Why does it work? Every pound of muscle burns about three times more daily calories than a pound of fat, speeding up the chemical processes that take place in each and every one of your cells. 

Many fitness experts also recommend high-intensity interval training to target a higher metabolic rate. With HIIT, you exercise at full intensity, followed by a brief cool-down before returning to full speed. These workouts can temporarily increase RMR for up to two full days, which gives you a calorie burn boost. 

You also need to consume enough protein to build lean muscle. Be sure to stop by the seafood counter of your favorite grocer, as salmon, tuna, and shrimp are all protein-rich choices. You can also emphasize eggs, chicken breast, lean beef, Greek yogurt, soybeans, cottage cheese, beans, and whey. These foods contain the amino acids required for metabolic functions. Without a rich protein resource, your body begins to break down muscle tissue.

You might not consider yourself an athlete, especially if your brother runs marathons or your best friend plays roller derby. But don't sell yourself short--anyone who is physically active can take steps to make the most of performance and build long-term physical fitness. 

Devon's example also shows why you can't necessarily rely on body mass index, or BMI, as a measure of a healthy weight. According to BMI, the football player is 60 pounds above the maximum weight for his height. However, body composition reflects Devon's status as an elite athlete, revealing a body fat percentage better than 95% of men his age.

Getting started or feel like you’ve hit a plateau? Consider doing an RMR test to see if you might be under or overeating based on your goals. It's the most accurate assessment of your body composition and gives you an ideal starting point to make smart wellness goals. 

References

https://blog.nasm.org/nutrition/resting-metabolic-rate-how-to-calculate-and-improve-yours 

https://pubmed.ncbi.nlm.nih.gov/15883556/ 

https://pubmed.ncbi.nlm.nih.gov/2305711/ 

https://www.everydayhealth.com/news/essential-facts-about-your-metabolism-weight-loss/ 

https://pubmed.ncbi.nlm.nih.gov/25293431/ 

https://www.vox.com/2016/5/18/11685254/metabolism-definition-booster-weight-loss 

https://www.verywellfit.com/metabolism-facts-101-3495605 

https://www.health.com/weight-loss/metabolism-facts 

https://www.wellandgood.com/resting-metabolic-rate/