What Is Metabolism And How Does It Work?


Though used to explain everything from the ability to consume large quantities of food without gaining weight to the main reason for being overweight/obese, ‘metabolism’ is often misunderstood and is quite complicated. Metabolism is actually the collection of chemical reactions that takes place in all of your body’s cells, those responsible for converting the food you eat into fuel to power everything you do. Each chemical reaction is coordinated with other bodily functions. Typically, thousands of metabolic reactions happen simultaneously to keep your cells working.  Learning the facts goes a long way toward helping you understand the terminology and sort the facts from the myths surrounding metabolism.

Definition of metabolism?

By definition, metabolism describes the sum total of the chemical processes and reactions that occur in living cells/an organism, necessary to support life. Put more simply, metabolism is the amount of energy (measured in calories) that your body expends or burns just to maintain itself during every activity, such as sleeping, working, doing housework and eating. Research has shown that even though your baseline metabolism is determined at birth many factors affect it. Having a “fast” metabolism simply means that your body utilizes calories more quickly and efficiently. Though a complicated chemical process, many people think of metabolism in its simplest form: something that influences how easily our bodies gain or lose weight.

How does it relate to resting metabolic rate (RMR) or basal metabolic rate (BMR) and what is the difference between RMR and BMR?

Metabolism covers a broad range of chemical reactions however, the number of calories you “burn”, even at rest, is based, for the most part, on your resting metabolic rate (RMR). RMR is used interchangeably with basal metabolic rate (BMR) but, technically, it is not the same thing. RMR is measured under less restrictive conditions than BMR. Measuring your BMR involves a 12 hour fast and spending the night in a testing facility to ensure eight hours of rest before testing. Most calorie counters/calculators use RMR because the conditions upon which it is taken reflect a more normal situation in day to day activities. Both BMR and RMR can be measured by a gas analysis process through direct or indirect calorimetry or estimated with an equation using age, gender, height and weight.

About 70% of an individual’s caloric/energy expenditure is devoted to maintaining the life processes of the organs of your body.  Interestingly, your liver uses the most energy to sustain itself followed by your brain. Together, these two organs are responsible for nearly 50% of your body’s basal energy expenditure.

What factors influence your metabolism?

Your metabolic rate is affected by many factors, including genetics. Your body size and composition or the ratio of muscle/lean body mass to fat affects metabolism. A large woman has a higher metabolism than a small woman and men have a 10 to 20% higher metabolism than women because they have significantly more muscle tissue. Muscle requires more calories than fat to maintain itself. Thus, in general, muscular individuals with a low body fat percentage tend to have a higher metabolism than their less muscular counterparts.

Two individuals of the same height and weight may have very different metabolic rates. If person #1 is sedentary and person #2 regularly engages in cardiovascular exercise and resistance training, such as lifting weights, person #2 will have a lower percentage of body fat and more muscle mass than person #1. Thus, the second person will have a higher metabolism than the first person, meaning his body requires more calories to for everyday maintenance.

Other factors, besides gender, muscle mass and genetics influence your metabolism, such as age. As you age, your body becomes less efficient at burning calories partially due to a loss of lean muscle mass. Body temperature (fever) and stress may slightly, but temporarily, increase metabolism.  The hormone thyroxine (the thyroid gland hormone), aids in metabolism regulation. Even the amount of calories you eat or your energy intake and medications can play a role in metabolic rate.


  • American Council on Exercise (ACE). BMR vs. RMR: http://www.acefitness.org/blog/616/bmr-versus-rmr/
  • David Frankenfield et al. “Comparison of Predictive Equations for Resting Metabolic Rate in Healthy Nonobese and Obese Adults: A Systematic Review”.  JADA 2005 105 (5): 775–789. Abstract available: http://www.adajournal.org/article/PIIS0002822305001495/abstract
  • St-Onge MP and D Gallagher. “Body Composition Changes with Aging: the cause or the result of alterations in metabolic rate and macronutrient oxidation?” Nutrition 2010 Feb;26(2):152-5. Full text available: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2880224/?tool=pubmed
  • Whitman, Stacy “The Truth about Metabolism.” Shape. September 2003



Leave a Reply

Your email address will not be published. Required fields are marked *