What Is Metabolism? Why Your Body Burns Calories Differently Than Everyone Else's

What Metabolism Actually Means

Metabolism encompasses every chemical reaction occurring in your body's roughly 37 trillion cells, 24 hours a day. These reactions fall into two categories.

Anabolism is building up — constructing complex molecules from simpler ones. Building muscle from amino acids, synthesizing DNA for cell division, storing glucose as glycogen, forming fat from excess calories. Anabolic processes require energy input.

Catabolism is breaking down — dismantling complex molecules to release energy. Digesting food into nutrients, breaking down glucose for energy (cellular respiration), metabolizing fat stores during caloric deficit. Catabolic processes release energy.

Your total daily energy expenditure (TDEE) — the total number of calories your body burns per day — has four components. Basal Metabolic Rate (BMR) accounts for 60-75% of total calories burned. This is the energy required to keep you alive while doing absolutely nothing — breathing, circulating blood, maintaining body temperature, brain function, cell repair. Even in a coma, you'd burn these calories.

The thermic effect of food (TEF) accounts for about 10% of TDEE. Your body burns calories digesting and processing food. Protein has the highest TEF (20-30% of its calories are burned during digestion), followed by carbs (5-10%) and fat (0-3%).

Non-exercise activity thermogenesis (NEAT) accounts for 15-30% of TDEE. This includes all movement that isn't deliberate exercise — fidgeting, walking to the kitchen, typing, standing, gesturing while talking. NEAT varies enormously between individuals and is the biggest source of metabolic difference between people.

Exercise activity thermogenesis (EAT) — deliberate exercise — typically accounts for only 5-10% of TDEE in most people. Even serious gym-goers rarely burn more than 300-500 calories per workout.

What Determines Your Metabolic Rate

Several factors influence your BMR, but most are not under your control.

Body size and composition are the strongest predictors. Larger bodies burn more calories simply because there's more tissue to maintain. Muscle tissue burns roughly 6 calories per pound per day at rest, while fat tissue burns about 2 calories per pound per day. This is why people with more muscle mass have higher BMRs — but the difference is smaller than fitness marketing suggests. Adding 10 pounds of muscle increases BMR by roughly 60 calories per day — less than an apple.

Age reduces metabolism — but the pattern is surprising. The 2021 Science study found that metabolism is highest (adjusted for body size) in infancy, gradually declines through childhood and adolescence, remains remarkably stable from age 20 to 60, then declines by about 0.7% per year after 60. The common belief that metabolism plummets in your 30s or 40s is not supported by the data. Middle-age weight gain is primarily due to reduced physical activity and increased calorie intake, not metabolic slowdown.

Sex matters modestly. Men typically have 5-10% higher BMRs than women of the same size and age, primarily due to differences in muscle mass and body fat percentage. However, when studies control for body composition, the sex difference largely disappears.

Genetics play a role but a modest one. Studies of identical twins show that BMR has a heritability of about 40-70%. However, most of this genetic influence operates through body composition (genetics affects how much muscle vs. fat you carry) rather than metabolic efficiency per se.

Hormones — particularly thyroid hormones (T3 and T4) — directly regulate metabolic rate. Hypothyroidism (underactive thyroid) can reduce BMR by 10-15%, while hyperthyroidism can increase it by a similar amount. However, clinical thyroid disorders affect only about 5% of the population.

Metabolic Adaptation: Why Diets Plateau

One of metabolism's most frustrating features is metabolic adaptation (or adaptive thermogenesis) — your body's tendency to resist weight change by adjusting its energy expenditure.

When you eat fewer calories than you burn, your body doesn't simply burn stored fat at a constant rate. It fights back. BMR decreases beyond what would be expected from weight loss alone. Your body becomes more efficient — muscles perform the same work using fewer calories. NEAT drops subconsciously — you fidget less, move less, take fewer steps without noticing. Hunger hormones (ghrelin) increase while satiety hormones (leptin) decrease. This ensemble of adaptations can reduce TDEE by 15-20% beyond what weight loss alone would predict.

The 'Biggest Loser' study (Fothergill et al., 2016) dramatically illustrated this. Contestants who lost massive amounts of weight on the show were studied six years later. Their metabolisms were burning roughly 500 calories per day LESS than expected for people of their size — their bodies were still fighting to regain lost weight years later. Most had regained significant weight.

This doesn't mean weight loss is impossible — it means crash diets are counterproductive. Severe caloric restriction triggers more aggressive metabolic adaptation. A moderate deficit (300-500 calories per day), combined with adequate protein (to preserve muscle mass) and resistance training, produces more sustainable results with less metabolic pushback.

The reverse is also true: when you overeat, your body increases NEAT, TEF, and heat production to resist weight gain. This is why some people seem to 'eat whatever they want' — their bodies compensate by burning more through NEAT and heat. However, this compensatory mechanism has limits and varies substantially between individuals.

Sources: Pontzer et al., Science (2021), Fothergill et al., Obesity (2016), Hall et al., Cell Metabolism (2016), Institute of Medicine Dietary Reference Intakes, Levine, Mayo Clinic Proceedings (2000).