Resting Metabolic Rate: Understanding Energy Expenditure

Weight management is a multi-billion-dollar industry that brings has brought out a lot of fad diets, supplements, “secret tricks”, and conflicting do’s and don’ts. Trying to get down to the brass tacks on what works and what doesn’t can be a daunting task leaving individuals confused and demotivated.  Most of us get the idea that losing weight means burning more calories than we eat, but how do we really know where to set our calories at?

If we added up an individual’s total energy expenditure for a day, approximately 70-75% of the calories burned would come from their resting metabolic rate (RMR) which is the amount of energy their body requires to maintain basic physiological function and support their lean mass.  That means only 25% of daily energy expenditure comes from daily activity (exercise activity, non-exercise activity and the thermic effect of food).   Research has shown that increasing RMR is a major factor in fat loss and long term weight management.  Increasing RMR can be done by increasing activity levels – specifically resistance training to increase/ maintain lean muscle mass, and adequate nutritional intake. High intensity interval training (HIIT) has also shown promising effects for increasing RMR.

One of the biggest mistakes often made in pursuit of weight loss is extreme caloric restriction.  Without enough energy to support your body’s basic physiological function and lean mass, eventually muscle tissue is broken down through catabolism to provide that energy.  This negatively affects RMR as well as body composition.  If most the weight lost is muscle then body fat percentage might not even change, even though the total mass is lower.  In extreme cases such as functional hypothalamic amenorrhea (FHA), the lack of energy causes normal hormone cycles to become disrupted potentially causing numerous secondary complications.

There are a lot of equations or calculators that can estimate your RMR, and for a lot of individuals it will be close – within 100-200 kcal (not enough variation to cause weight loss/gain).  Athletic populations are often underestimated in these calculations because their activity levels and lean mass drive energy expenditure up.  As previously mentioned, severe dietary restrictions can reduce RMR, meaning the calculations would overestimate that individual’s expenditure.  Having an actual measurement of RMR using open circuit spirometry becomes a very useful tool for planning dietary intake.

At SSMI we consider your RMR to be a vital statistic that can be tracked throughout your lifetime.  We perform the test using our Metamax 3b open circuit spirometry to measure your oxygen consumption  at rest. This simple yet powerful 10-20 minute breath test determines how much energy (calories) you burn in a day at rest.  You simply sit back, relax and breathe.  During the test, the air you breathe in and out is analyzed for how much oxygen your body uses to maintain basic life functions.  Once completed, you use your RMR calories to provide a foundation for your calories burned during a day.  You then add how many calories you burn during activities of daily living (ADLs) and exercise calories to get a total calorie expenditure for the day.  Let us know when you would like to schedule your next metabolic test!

Alex Edwards, CEP

Exercise Physiologist

Disclaimer:  Articles are based on real cases seen at Scottsdale Sports Medicine. The information on this site is not intended or implied to be a substitute for professional medical advice, diagnosis or treatment. All content, including text, graphics, images and information, contained on or available through this web site is for general information purposes only. Please consult your medical professional for individualized healthcare.