While we think of our metabolism as how quickly we metabolize food or burn calories, it’s so much more than that. In fact, when you hear someone saying they have a “fast” or “slow” metabolism, they’re really referring to their metabolic rate.
An individual’s basal metabolic rate (BMR) is the number of calories an individual needs to carry out basic physiological functions (again: breathing, digestion, blood circulation, etc.). Their resting metabolic rate (RMR), aka 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 someone 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, timing of meals, stress levels, sleep health, physical activity, and more).