The word "metabolism" gets used to explain everything from weight gain to fatigue to why your friend can eat anything and stay thin. Most of that usage is imprecise. Here's what metabolism actually is.
The Actual Definition
Metabolism is the sum total of all chemical reactions in living cells that convert food and stored energy into forms the body can use. It encompasses:
Catabolism — breaking down complex molecules into simpler ones, releasing energy. Digesting food, breaking down fat stores, converting glycogen to glucose.
Anabolism — building complex molecules from simpler ones, consuming energy. Building muscle, synthesising hormones, repairing cells.
Every second of every day, billions of metabolic reactions are happening in your cells — far beyond just "burning calories."
What People Usually Mean by "Metabolism"
When people say someone has a "fast metabolism" or want to "boost their metabolism," they're usually talking about one specific component: resting metabolic rate (RMR) or basal metabolic rate (BMR).
BMR is the number of calories your body needs to maintain basic functions at complete rest — breathing, circulation, cell repair, temperature regulation, brain activity. It accounts for 60-70% of your total daily energy expenditure.
See the full breakdown of how BMR is calculated and what it means in what is BMR.
What Actually Determines Your Metabolic Rate
Lean muscle mass is the biggest variable within your control. Muscle tissue is metabolically active — it burns calories at rest. More muscle = higher BMR. This is the main mechanism through which exercise (particularly resistance training) raises metabolic rate — not directly through calorie burn during exercise, but through building muscle that raises baseline burn permanently.
Age — BMR naturally declines with age, primarily due to age-related muscle loss (sarcopenia). This is not inevitable — resistance training largely prevents it.
Sex — men generally have higher BMRs than women of the same height and weight, primarily because men tend to carry more lean muscle mass.
Body size — larger bodies (regardless of composition) burn more calories at rest simply because there's more tissue to maintain.
Thyroid function — the thyroid gland produces hormones that directly regulate metabolic rate. Hypothyroidism (underactive thyroid) meaningfully reduces BMR; hyperthyroidism raises it.
Genetics — some heritable variation in metabolic rate exists, but it's smaller than most people assume. Twin studies suggest genetic variation in resting metabolic rate is approximately 40-70% heritable.
The "Slow Metabolism" Myth
Many people attribute weight gain or difficulty losing weight to a slow metabolism. In most cases, the evidence doesn't support this.
A 2016 study in Obesity Reviews found that clinically significant metabolic differences between individuals of similar body composition account for roughly 200-400 calories per day at most — not the several hundred to thousand calorie difference needed to explain dramatic weight discrepancies.
What does explain most variation in weight: differences in calorie intake (often underestimated), differences in non-exercise activity (NEAT — fidgeting, walking, standing), and differences in appetite regulation.
That said, genuine metabolic disorders exist. Thyroid conditions, insulin resistance, polycystic ovary syndrome (PCOS), and Cushing's syndrome all affect metabolism meaningfully. If you genuinely can't lose weight on a reasonable calorie deficit after 3+ months, a GP blood panel to check thyroid and metabolic markers is worthwhile.
What Actually Affects Metabolic Rate
Raises BMR:
- Building lean muscle through resistance training
- Eating adequate protein (protein has a 20-30% thermic effect — your body burns more calories digesting it than carbs or fat)
- Not chronically under-eating (severe calorie restriction causes adaptive thermogenesis — the body downregulates BMR to conserve energy)
Lowers BMR:
- Significant muscle loss (from aging, injury, or very low calorie diets)
- Hypothyroidism
- Extreme calorie restriction sustained over time
Does not meaningfully change BMR: Cold showers, green tea, cayenne pepper, eating 6 small meals vs 3 large ones. These are myth-level metabolism "hacks" with negligible real-world effects.
As covered in not losing weight eating healthy, the most common reasons for unexplained weight plateaus are not metabolic disorders — they're measurement and consistency issues.