Although we think of our metabolism as how quickly we metabolize food or burn calories, it’s much more than that. In fact, when you hear someone say they have a “fast” or “slow” metabolismthey really refer to their metabolic rate.
An individual’s Basal Metabolic Rate (BMR) is the number of calories an individual needs to perform basic physiological functions (again: breathing, digestion, blood circulation, etc.). Their Resting Metabolic Rate (RMR), or Resting Energy Expenditure (REE), is the actual number of calories they burn at rest. A person with a “fast” metabolism has a higher REE than a person with a “slow” metabolism.
Now, before you get discouraged, it’s important to point out that your metabolic rate is influenced by a number of factors— some of which are fixed (e.g. genetics, biological sex, age) and many of which can be optimized (e.g. mitochondrial health, muscle mass, adiposity, insulin sensitivity, thyroid hormone health, nutrition, meal times, stress levels, sleep health, physical activity, etc.).